TRS-80 DOS - NEWDOS/80 v2.0 for the Model III - SYS6/SYS Disassembled

SYS6/SYS

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Introduction / Summary

SYS6/SYS — NEWDOS/80 v2.0 Disk Utility Overlay (Model III)

SYS6/SYS is the largest overlay module in NEWDOS/80 v2.0 for the TRS-80 Model III, occupying addresses 4D00H through 6FF8H (approximately 8,953 bytes). It implements four major DOS commands: FORMAT, COPY, APPEND, and HEX DUMP/COMPARE. The module is loaded into memory by the DOS command interpreter (SYS1) whenever one of these commands is invoked.

Entry Point and Command Dispatch: Execution begins at 4D00H, where the IY register is set to 4280H (the DOS work area base pointer used throughout all SYS overlays), and the overlay ID at IY+08H is set to 08H. The code then examines Register A to determine which command was requested:

• A = 68H — Jump to 6F4AH: HEX DUMP/COMPARE entry point
• A = 28H — Jump to 65FAH: APPEND entry point
• A = 48H — Jump to 4D1EH: COPY/FORMAT main entry (shared entry; bit 6 of 428AH distinguishes FORMAT from COPY)
• Any other value — Jump to 5208H: Error handler (invalid command code)

FORMAT Command: Formats floppy disks in either single-density or double-density mode. The FORMAT path initializes drive geometry, builds boot sector templates from configuration tables at 6CC1H-6CF5H, writes track-by-track using the WD1793 FDC Write Track command (port F0H), and installs NEWDOS/80 boot sectors. Self-modifying code throughout the format loop patches sector header bytes, gap lengths, and timing parameters for the target density and geometry. The format routine at 695AH is the core FDC track-write engine, directly controlling the WD1793 through port-based I/O at F0H-F4H.

COPY Command: Copies files between disks with full directory management. COPY supports extensive options parsed from the command line via the option keyword table at 505CH-5185H. The copy engine reads source directory entries, matches them against destination directory entries, handles density mismatches between source and destination disks (single-density ↔ double-density conversion), patches directory fields as needed, manages GAT (Granule Allocation Table) bitmaps, and performs sector-by-sector data transfer using buffered I/O through SYS0 routines. A linked-list structure starting at 5B57H manages up to three drive parameter blocks (at 5AE5H, 5B17H, and 5B49H) for multi-drive operations.

APPEND Command: Adds files to an existing disk without full copy. APPEND enters at 65FAH and shares much of the COPY infrastructure for filespec parsing, drive initialization, and directory management, but uses a simplified transfer path that appends new entries to the destination directory without overwriting existing files.

HEX DUMP/COMPARE: Provides hexadecimal dump and sector-by-sector comparison of disk contents. This command enters at 6F4AH and uses a simpler code path that opens source and (optionally) destination files, then reads sectors through the EXX-based dual-register-set technique to alternate between source and destination contexts during comparison.

Key Architectural Features:

• Extensive Self-Modifying Code: Over 50 self-modifying locations patch operands at runtime to configure drive geometry, sector numbers, track counts, gap lengths, FDC timing parameters, and control flow flags. This is characteristic of memory-constrained Z80 programming where RAM variables within instruction operands save both space and execution time.
• PDRIVE Linked-List Management: Three drive parameter blocks are managed through a linked list (sentinel at 5B53H, list start at 5B57H) that enables dynamic drive swapping during multi-disk copy operations with disk swap prompts.
• Double-Density Support: Full MFM double-density formatting and conversion capability, with density flag checks, compatibility validation, and field-patching routines that adjust directory entries when copying between SD and DD disks.
• WD1793 FDC Direct Control: The format engine directly programs the Western Digital 1793 FDC through Model III ports F0H-F4H, including Write Track commands with precise timing for gap bytes, sync marks, ID fields, and data fields.
• Track Buffer System: A track buffer system uses 258-byte slots (1 marker byte + 1 status byte + 256 data bytes) starting at 5D14H and extending to 593CH, with the number of available slots computed dynamically based on available memory.

Complete list of all RAM variables, self-modifying code locations, and DOS work area references used by SYS6/SYS. Variables are grouped by function: DOS work area (IY-relative), SYS0 system pointers, SYS6 work area, option flags, drive parameter blocks, self-modifying code, and runtime buffers.

DOS Work Area (IY = 4280H)

Address	IY+	Description
427FH	—	FDC control/status byte (bit 7: force-head-load; bit 5: head-loaded; bit 4: buffer-valid)
4282H	02H	DOS mode byte (bit 5: FORMAT mode flag — 1=FORMAT, 0=COPY/APPEND)
4283H	03H	FDC sector register shadow
4287H	07H	Drive type flags byte (bit 2: head-step enable; bit 4: density adjust; bit 6: double-sided)
4288H	08H	Active overlay ID (set to 08H for SYS6)
4289H	09H	DOS work area flags (bit 3: set for non-hex commands — COPY/FORMAT/APPEND)
428AH	0AH	DOS command status byte (bit 6: 0=FORMAT, 1=COPY)
428BH	0BH	DOS flags byte (bit 1: write ops active; bit 2: write-in-progress)
428CH	0CH	DOS status flags (bit 7: special mode; bit 1: alternate)
42B9H	39H	PDRIVE table base pointer (16-bit)

SYS0 System Pointers and External References

Address	Description
4030H	SYS0 DOS re-entry point
402DH	SYS0 normal completion return address
4409H	SYS0 error handler entry point (pushed as return address)
4411H	SYS0 high memory pointer (upper limit of available RAM)
421AH	DOS work area: high byte of DD sector adjustment offset
421CH	DOS work area: low byte of DD sector adjustment offset
4296H	Date/version stamp in DOS work area
431FH	GAT configuration byte (packed sectors-per-track encoding)
43CEH	GAT date/version stamp field in DOS work area
43D0H	GAT disk name field (first 8 bytes)
43D8H	GAT disk name field (second 8 bytes)
447FH	One byte before SYS0 work buffer (INC DE targets 4480H)
4480H	SYS0 work buffer for drive-qualified filespec strings
48D5H	Drive cache invalidation flag (FFH = force re-read)
4F56H	SYS0 parsed directory entry index (zero-based)

SYS6 Work Area — Drive Numbers, Geometry, and Operation State

Address	Description
593BH	Disk operation state flags (bit 7: drive param blocks initialized)
593CH	Saved high memory limit (copied from 4411H during FORMAT init)
593EH	Current drive parameter pointer
5940H-41H	Operation mode flags word (2 bytes)
5942H	Primary drive number (0-3)
594CH	Source drive number (0-3)
5956H	Destination drive number (0-3)
5978H	Source disk geometry cache (16-bit, track/size count)
597AH	Source total track count (used for date-stamp validation)
597CH	Source drive number stored by 5D8FH
597DH	Destination geometry / directory pointer (stored at 5EEDH)
5981H	Date/version stamp value (16-bit)
5983H	Source disk name buffer (16 bytes)
598BH	New disk name buffer (8 bytes, set by NDN= option)

Option Flags (4 bytes at 5994H-5997H)

Address	Bit	Description
5994H	1	Source ≠ destination check
5994H	2	Suppress geometry copy
5994H	4	NDN= option active
5995H	3	Copy sub-mode
5995H	4	NDN option
5995H	5	Suppress date copy
5995H	6	Per-entry prompt mode
5996H	2	Suppress geometry copy (set at 4DE5H)
5996H	3	Filespec loop active
5996H	5	FORMAT mode flag (set at 4D29H)
5996H	7	Source/dest parsed flag
5997H	3	Dest filespec present
5997H	4	Source filespec present
5997H	5	Dest filespec parsed

Source Drive Geometry Block (59B7H)

Address	Description
59B7H	Source drive geometry parameter block base
59B9H	Source density flag byte (bit 5: MFM enable = double density)
59BCH	GAT configuration parameter (sectors per granule multiplier)
59C0H	Source geometry byte (sectors-per-track) stored by 5D93H

Destination Drive Geometry Block (59C1H)

Address	Description
59C1H	Destination drive geometry/config pointer (16-bit)
59C3H	Destination disk total track/size count (16-bit)
59C5H	Drive geometry copy destination (10+6 bytes)
59C6H	Destination primary geometry byte (track boundary limit for GAT)
59C7H	Destination secondary geometry byte (density flags, bit 5=MFM)
59C8H	Destination side count
59C9H	Destination geometry: head-load/step parameter
59CAH	Sectors-per-GAT-byte (bits per GAT byte for disk geometry)
59CBH	Destination geometry: track count byte
59CCH	Destination geometry control byte
59CDH	Boot sector patch byte
59CEH	Destination geometry count byte (stored by 5EF1H)
59CFH	Total destination disk capacity (sector count, 16-bit)
59D1H	Source disk total track/size count (16-bit)

Drive Parameter Blocks (Linked List)

Address	Description
5A1DH	Drive number digit position within swap prompt template
5A1FH	Directory data buffer
5AE5H	Source drive parameter block base (50 bytes)
5AEBH	Source drive number field (within source param block)
5AEFH	Source sector count / progress field (16-bit, zeroed at 636EH)
5AF1H	Destination capacity pointer (stored at 6352H)
5B17H	Destination drive parameter block base (50 bytes)
5B1DH	Destination drive number field in dest param block
5B1EH	Destination buffer-cleared flag (FFH = cleared)
5B21H	Destination sector count / progress field (16-bit, zeroed at 6371H)
5B49H	Third drive parameter block (partial, 7 bytes)
5B53H	PDRIVE linked list sentinel (set to 0002H at 6424H)
5B57H	Drive parameter linked list start

Directory Buffer Pointers

Address	Description
5D12H	Directory buffer start pointer (reset to 6491H at 5E12H)
5D14H	Directory buffer end pointer (reset to 6491H at 5E15H; advanced at 5EA7H)

Self-Modifying Code Locations

Address	Instruction	Description
4DFAH	LD A,00H	Operand patched with current drive number
4F85H	CALL NZ,0000H	Operand patched with keyword handler address
4FB8H	(instruction byte)	Cleared by 60ADH
5314H	(byte)	Track counter in read loop
531AH	(byte)	Per-pass track limit
53D7H	(byte)	Destination write success flag
53ECH	(byte)	Destination file write counter
544FH	(byte)	Buffer position pointer
5401H	(byte)	Stored destination sector count
570EH	(byte)	Drive cache comparison byte
5F1EH	LD HL,nnnn	Patched to dest directory entry pointer (at 5F04H)
6028H	CP nn	Sector assignment byte (patched by 5FE1H)
6077H	LD (nnnn),HL	Sector slot data start (patched by 5FF6H)
60C3H	LD A,00H	Unmatched-entry flag (patched by 608CH)
612DH	JR nn	Displacement patched to 18H by 5CE9H to enable branch
61D8H	LD HL,nnnn	Directory buffer work pointer (init from 5D14H; updated with DE+18H)
61DBH	(word)	Tracks-per-pass sentinel (computed at 5D9DH as 593CH − 24)
64ACH	(byte)	Head settle time field
64AFH	(byte)	Stepping pulse width field
64BCH	(byte)	Boot sector patch byte (written by 5C88H and 6799H)
65B7H	(byte)	Dest geometry byte (patched by 675CH)
65B8H	(byte)	Dest geometry byte (patched by 67A8H)
65B9H	(byte)	Dest geometry byte (patched by 6762H)
65EEH	(byte)	Computed byte (patched by 66DEH)
65F0H	(word)	Track count word (patched by 67A2H)
670CH	(byte)	Computed byte (patched by 66E1H)
69D6H-69D8H	LD BC,nnnn	Operand patched by 69B5H
69DBH-69DDH	LD BC,nnnn	Operand patched by 69A5H
69EFH-69F1H	LD HL,nnnn	Operand (HL working address)
69F2H-69F4H	LD DE,nnnn	Operand patched by 69ADH
6A01H	(byte)	Starting sector number (patched by 69BBH)
6A03H	(byte)	Sector index counter (updated throughout format loop)
6A34H	(byte)	Sectors-per-track value (patched by 69C0H)
6B04H, 6B10H	(bytes)	Additional sector header bytes
6B24H	(byte)	Current track number for format
6B2AH	(byte)	FDC motor/head control byte
6B42H, 6B48H, 6B5AH, 6B66H	(bytes)	Sector header bytes (patched by 695AH)
6B7CH, 6B82H, 6B90H, 6B96H	(bytes)	Gap/sync bytes (patched by 695AH)
6BA9H	(word)	Stack pointer save for write-track interrupt handler
6BC9H-6BCCH	SBC HL,BC	Operand for track count comparison
6BD2H-6BD5H	LD BC,007DH	Operand (125 tracks)
6C62H	(byte)	Disk side flag (0=side 0, 1=side 1)
6C74H	(byte)	Track number save for 695AH
6C94H	(byte)	Write-track retry counter (initialized to 0AH)
6F1EH	LD HL,nnnn	Working HL storage in hex string parser

Address layout of the SYS6/SYS module, showing code sections, data tables, message strings, work buffers, and runtime data areas. Total module size: 8,953 bytes (4D00H-6FF8H).

Address Range	Size	Content
4D00H-4D1DH	30	Entry point and command dispatch (IY init, A-register dispatch to FORMAT/COPY/APPEND/HEX)
4D1EH-4DF0H	211	COPY/FORMAT main entry: source drive parse, filespec processing, option setup
4DF3H-505BH	617	FORMAT init, boot sector builder, option keyword parser
505CH-5185H	298	[DATA] Option keyword table (not executable code)
5200H-5321H	290	Error handlers, cleanup routines, track batch write setup
5324H-578AH	1,127	Track batch write, drive setup, PDRIVE swap, buffer flush, seek/cache
578AH-5963H	474	Destination write setup, status display, BREAK check, disk label display, string output, menu prompts
5982H-5AE4H	355	[DATA] Message strings (ASCII with 03H/0DH/00H terminators)
5AE5H-5B63H	127	[DATA] Drive parameter blocks (3 blocks × ~50 bytes) + linked list
5B64H-5C0AH	167	[DATA] Swap data blocks (3 structured records for overlays 02H/04H/05H)
5C0BH-5D15H	267	Sector read processor, GAT boundary check, seek error handler, utility routines, filespec input
5D16H-5EB0H	411	COPY/APPEND filespec input and validation, directory scan loop, extension compare, menu prompt
5EB3H-60C5H	531	Source/dest directory copy, entry matching, field swap, density mismatch patching, GAT allocation
60C8H-625BH	404	Unmatched entry copy, dest field-patching, GAT init, geometry init, directory utilities
625CH-6290H	53	[DATA] ASCII string area ("RUNNY", error messages, comparison utility)
6291H-6390H	256	[BUFFER] Directory bit-map work buffer (runtime data, 256 bytes)
6391H-6490H	256	[BUFFER] Secondary GAT work buffer (dest GAT copy)
6491H+	var	[BUFFER] Directory work buffer / filespec string buffer base
64A3H-64BFH	29	[DATA] Drive geometry data table (4 entries)
64C0H-5591H	—	Double-density write track routine (major FDC I/O)
65A2H-65B5H	20	[DATA] Error message strings ("\x1C\x1FERROR\x03", "\x1C\x1FNO SYS\x03")
65B7H-65BEH	8	[DATA/SELF-MOD] Drive geometry parameter bytes (self-modified at runtime)
65BFH-65ECH	46	[DATA] Double-density boot sector templates (two templates)
65EDH-65F9H	13	[DATA] FDC timing/format parameter bytes
65FAH-6663H	106	APPEND entry point, drive geometry loader, GAT validation, drive init
6664H-670FH	172	Write NEWDOS boot sectors, format track write loop (10 tracks)
6710H-674CH	61	FORMAT drive initialization, error display ("EITHER BAD PARAMETER...")
674DH-67C4H	120	DD capability check, drive geometry patch, write boot sector
67C5H-695AH	406	Main drive initialization routine
695AH-6CA2H	841	Write formatted track (main FDC format routine with extensive self-mod patches)
6CA3H-6CC0H	30	Load sector count, conditional head step, head step with delay
6CC1H-6D5CH	156	[DATA] Drive configuration tables (SD/SS, DD/SS, DD/DS)
6D5DH-6EB3H	343	[DATA] ASCII message strings (FORMAT/COPY prompts, error messages)
6EB4H-6EC7H	20	[DATA] Drive configuration / boot sector patch source data
6ECAH-6F49H	128	Filespec parsing: drive number, drive designator, hex parameter, hex string to binary
6F4AH-6F8DH	68	HEX DUMP/COMPARE entry point and main loop
6F8EH-6FB6H	41	Filename/extension validation (8 chars from HL→DE)
6FB7H-6FE3H	45	Parse destination extension for APPEND, copy extension string
6FE4H-6FF5H	18	Parse source filespec parameters
6FF6H-6FF8H	3	[DATA] "TO" + NUL (final 3 bytes of module)

Complete list of all internal SYS6 routines and external SYS0/ROM routines called by this module. Internal routine addresses serve as hyperlink targets within the disassembly. External routines are listed for reference; their code resides in other overlays or ROM.

Internal SYS6 Routines

Address	Description
Address	Routine Description
4D00H	Module entry point and command dispatch (A=68H→HEX, 28H→APPEND, 48H→COPY/FORMAT)
4D1EH	COPY/FORMAT main entry (shared; bit 6 of 428AH distinguishes)
4DF3H	FORMAT initialization (PDRIVE sentinel, boot sector builder, geometry setup)
4E1AH	Filespec processor continuation (tail-called from 60BCH)
4E3FH	Filespec parser entry point
4E72H	Post-filespec-parse handler
4E8AH	Filespec input parser entry
4E8DH	Filespec input parser (alternate entry, DE=param block)
4EA7H	Option parse with B=mode byte
502CH	Option processing loop re-entry
5025H	Checksum/encryption handler
50CFH	Internal directory lookup routine
50FEH	Null-terminated name processor
521AH	Main error handler entry
5222H	Final cleanup
5230H	Flush all drive buffers
5240H	Boundary check / capacity error helper
5243H	Same-disk / parameter-conflict warning display and prompt
524EH	Directory buffer relocation (jumped to from 6157H "all done" exit)
5324H	Write track batch to destination
552DH	Write directory and exit (when buffer empty)
5535H	Set up destination drive parameter block
5578H	Source drive setup (HL=594CH)
557DH	Destination drive setup (HL=5956H)
5582H	Primary drive setup (HL=5942H)
5585H	Common drive setup with PDRIVE swap
55C1H	Disk swap prompt
55ECH	Write PDRIVE entry and validate
5608H	Alternate dispatch (jumped to from 5F67H when overlay ≠ 05H)
5646H	Flush all drive buffers (iterate param blocks)
5658H	PDRIVE parameter swap via linked list
568BH	Drive setup dispatcher alternate entry (A=04H)
568DH	Drive setup dispatcher (A=function code)
5699H	Advance HL by 3 and compare vs directory end (5D14H)
56A2H	Drive configuration scan — find entry with bit 5 set
56D4H	Track seek dispatch
56DDH	Drive utility wrapper (CALL 48F0H)
56E2H	Track seek with cache validation
56EFH	Entry into 56E2H track-seek group (A=track number)
56F4H	Drive initialization utility (calls 48DBH)
56F9H	Drive init with XOR A (calls 48AFH)
56FAH	Drive init alternate entry (A=mode)
56FDH	Common drive error check
5701H	Drive utility wrapper (CALL 48C7H)
5706H	Save drive cache flag
570DH	Flush GAT and reset cache
5711H	Flush GAT (calls 570DH)
571BH	Activity indicator toggle
572BH	Check for BREAK key and read character
5733H	Batch mode and BREAK key scan
5751H	GAT bit clear exit check
5762H	Zero out GAT bits for excess tracks
5784H	Post-copy initialization placeholder (6 NOPs)
578AH	Destination write setup
57C8H	Read sector stub (IX→DE, JP 4436H)
57CEH	Sector operation stub (IX→DE, JP 5CEDH)
57D4H	Write sector stub (IX→DE, JP 443CH)
57DAH	FORMAT/COPY status display
584AH	Batch mode abort check
585AH	Read error with track increment
5868H	Conditional source disk label display
587EH	Display string and carriage return
5881H	Clear / output CR
5886H	Display buffer with space padding
5898H	Display B spaces
58A0H	Display filespec name (filename/extension)
58B3H	Display non-space chars from buffer
58BDH	Single character output
58C5H	Display string and confirm prompt
58C8H	Operator confirmation prompt
58DEH	Alternate menu prompt
58E4H	Menu response scanner
5909H	Decimal number formatter
5C0BH	Sector read return code processor
5C1EH	GAT track boundary check and conditional drive init
5C3EH	Seek/restore error handler and retry counter
5C43H	Seek retry exhausted — launch format or abort
5C5BH	Scan name string for null terminator
5C69H	Filespec parse: decrement entry count and advance
5C71H	Store date/version stamp into buffer
5C80H	Boot sector patch callback
5C8EH	Set destination buffer pointer and mark drive active
5C96H	Set disk operation mode
5C9FH	Double-density compatibility check and linked-list reconfig
5CEDH	Sector write mode flag propagation
5CFBH	Directory flag check and dispatch
5D16H	COPY/APPEND filespec input and validation
5D71H	Keyboard character input subroutine
5D86H	Drive setup and GAT initialization calls
60A9H	Sector write via SYS0 + clear 4FB8H + tail JP 4E1AH
60BFH	Post-destination-copy return dispatcher (self-mod check at 60C3H)
60C8H	Source-to-destination copy loop (unmatched entries)
60E1H	Append unmatched entry to destination directory
60F1H	Source exhausted: push 6157H, fall to 60F5H
60F5H	Destination directory copy and field-patching routine
614FH	Flush GAT, mark entry processed, loop
6157H	"All done" exit (FFH→5B1EH/586AH, JP 524EH)
6162H	GAT sector state init (CALL 56FAH + LDIR copy)
616FH	Copy 256 bytes from HL to 4300H
6172H	LDIR 256 bytes (HL→DE utility)
6178H	Disk geometry init (FORMAT: A=10H, C=06H; COPY: divides for sectors-per-track)
6190H	Reload directory buffer control pointers
619DH	Advance directory buffer pointer (C-indexed from 6291H)
61BAH	Advance to next directory slot
61CBH	Decimal digit extraction (divide A by B, store ASCII quotient)
61D6H	Fetch next directory entry address (via RST 18H, self-mod HL at 61D8H)
61EAH	Check if either source or dest drive supports MFM
61F6H	Fetch FORMAT mode flag (4282H AND 20H)
61FCH	XOR source/dest density flags — non-zero if they differ
6209H	Conditional directory date pointer update
621CH	GAT bit manipulation — mark sectors allocated in bitmap
6254H	B-byte comparison loop (DE vs HL)
6377H	Build source/dest parameter blocks, check compatibility, start COPY
6392H	Parse drive designator from filespec
63A0H	Parse "=" parameter from filespec (calls 6EE6H)
63B3H	Drive geometry loader (flush + SET 7,(593BH) + copy geometry)
63ECH	Conditional geometry copy with drive-number flag update
6424H	Init PDRIVE linked list sentinel (write 0002H to 5B53H)
6437H	Build drive parameter block from filespec drive designator
6592H	FDC ready wait (IN A,(F0H); RRCA; loop)
659CH	Short delay (LD A,08H; DEC; JR NZ)
6643H	GAT validation / drive init (COPY pre-flight entry)
6664H	Write NEWDOS boot sectors to destination
666AH-670FH	Format track write loop (10 tracks, self-mod boot data)
6710H	FORMAT drive initialization (geometry setup → 4C37H via IX)
6747H	Error display: "EITHER BAD PARAMETER OR CONFLICT WITH DRIVE DATA"
674DH	Double-density capability check
6759H	Drive geometry patch (self-modifying bytes in 64xx/65xx range)
67ACH	Write boot sector (0 retries path, BC=0)
67B3H	Write boot sector (BC=retry count, DE=template pointer)
67C5H	Main drive initialization (CALL 6759H + 557DH + mode-dependent init)
695AH	Write formatted track (main FDC format routine with self-mod patch)
6CA3H	Load sector count from IY+04H (halved if double-sided)
6CAEH	Conditional head step
6CB6H	Head step with delay
6ECAH	Parse drive number from filespec (look for digit after optional colon)
6ED6H	Parse and validate drive designator (with CALL 4723H drive select)
6EE6H	Parse hex parameter from filespec (HL→DE 16-bit result)
6EF0H	Drive letter validator (A-H range check)
6F0EH	Hex string parser setup (push 6F08H return, call 6F13H)
6F13H	Hex string to binary (16-bit, updates HL position)
6F4AH	HEX DUMP/COMPARE entry point
6F8EH	Filename/extension validation (8 chars from HL→DE)
6FB7H	Parse destination extension for APPEND
6FDDH	Copy extension string via SYS0 4470H
6FE4H	Parse source filespec parameters (CALL 6EC3H + 4C6AH + 4C7EH)

External SYS0 / ROM Routines Called by SYS6

Address Range	Source	Content
0013H	ROM	Keyboard scan (waits for key)
001BH	ROM	Get character from keyboard
0033H	ROM	Output character in A at cursor
002BH	ROM	Wait for key
RST 18H	DOS	NEWDOS/80 directory traversal system call
4420H	SYS0	Drive init (standard PDRIVE entry)
4424H	SYS0	Drive init (function code A, param block DE)
4428H	SYS0	Disk write (GAT/directory, param block DE)
4436H	SYS0	Read sector (DE=parameter block)
4439H	SYS0	Sector write dispatch
443CH	SYS0	Write sector (DE=parameter block)
4448H	SYS0	Drive-ready check
4467H	SYS0	Display null-terminated string (HL=pointer)
4470H	SYS0	Copy string to buffer
4548H	SYS0	Keyboard processing / BREAK check
466FH	SYS0	Drive operation utility
46F5H	SYS0	ROM call (conditional)
46F7H	SYS0	Motor-on / output to C port
46FFH	SYS0	Head load
4715H	SYS0	Motor-off / head unload
471BH	SYS0	Printer init or screen clear
4720H	SYS0	Deselect drive
4723H	SYS0	Select drive (A=drive number 0-3)
4789H	SYS0	Wait for FDC DRQ
4791H	SYS0	Write PDRIVE entry for drive A
4925H	SYS0	Sector write via parameter block
48AFH	SYS0	Drive initialization (basic init, A=0)
48C4H	SYS0	Write GAT sector to disk
48C7H	SYS0	Drive utility
48D4H	SYS0	Track seek with head configuration
48DBH	SYS0	Drive initialization utility
48ECH	SYS0	Disk operation continuation (seek→read/write dispatch)
48F0H	SYS0	Drive utility
4C37H	SYS0	Track seek (A=mode byte, usually 30H)
4C39H	SYS0	Track/sector arithmetic
4C57H	SYS0	Sector position/CHS calculation (normal data sector)
4C59H	SYS0	GAT byte/bit offset calculation (A=function code)
4C6AH	SYS0	String search (BC=pattern, HL=input)
4C7EH	SYS0	Scan string character
4C92H	SYS0	Wait for keyboard input (BC=params)

4D00H - SYS6/SYS ENTRY POINT AND COMMAND DISPATCH

SYS6/SYS is the NEWDOS/80 disk utility overlay module, loaded by SYS1 into memory at 4D00H when the user enters the FORMAT, COPY, or APPEND command. Register A contains a command code set by SYS1: 68H for hex dump/compare mode, 28H for APPEND, 48H for COPY/FORMAT. IY is loaded with 4280H (the DOS work area base pointer) and remains set throughout SYS6 execution.

4D1EH - COPY/FORMAT COMMAND INITIALIZATION

This section begins the COPY/FORMAT command processing. It first checks whether this is a COPY or FORMAT operation by examining bit 6 of the DOS status byte at 428AH. For FORMAT commands (bit 6 clear), it sets the FORMAT flag and saves the high memory pointer. It then parses the drive number from the command line, stores it as both source and destination drive, and sets up the option parameter blocks.

4D8BH - SINGLE-DRIVE / NO DESTINATION DEFAULT SETUP

When no destination drive was specified on the command line, this code sets default operation parameters. It configures the mode flags at 5940H-5941H for a single-drive operation (FORMAT or single-disk COPY), then re-parses the command line for additional parameters.

4D98H - COMMAND LINE OPTION PREFIX AND PARAMETER COPY

This section handles the optional "$" prefix on the command line (which triggers special copy modes), copies the first 80 bytes of the command line into a parameter buffer, and then proceeds to parse source and destination filespecs for file-by-file copy operations.

4DF3H - DIRECTORY BUFFER INITIALIZATION

This subroutine initializes the directory tracking data structures for the COPY operation. It reads the GAT (Granule Allocation Table) from the source disk, then sets up the file entry table with initial values. The routine computes block offsets using the source drive number and a fixed multiplier of 10 (000AH).

4E14H - DRIVE NUMBER VALIDATION AND FLAG SETUP

This section validates the source and destination drive numbers and builds a drive usage flags word in Register Pair DE. The flags indicate which drives are involved in the operation and whether special handling (such as single-drive swap mode or error conditions) is needed. The flags are stored at 5940H for use throughout the copy operation.

4E68H - SYSTEM DISK CONFIRMATION PROMPT

This subroutine displays an "ARE SYSTEM AND THE SAME DISKETTE?" prompt when one of the drives involved in the COPY operation is drive 0 (the system disk). It checks whether the prompt should be suppressed based on option flags, displays the appropriate message (SOURCE or DESTINATION), and if the user answers "No", clears the drive flags to cancel that drive's involvement.

4E86H - FLAG MERGE HELPER

A small helper routine that merges the contents of Registers D and C into Register D by ORing them together. Used to combine operation mode flags.

4E8AH - FILESPEC PARSER (DESTINATION) / 4E8DH - FILESPEC PARSER (SOURCE)

These two entry points parse filespecs from the command line. The 4E8AH entry sets DE to the destination parameter block at 5B17H, while 4E8DH is the common entry that accepts DE as a parameter. Both call SYS0's filespec parser at 4C7AH to extract drive, filename, extension, and password from the command line text at (HL).

4E93H - FILESPEC PARSER LOOP (CONTINUATION FROM 4E8DH)

This is the body of the filespec parsing loop that began at 4E8DH. After each call to SYS0's filespec parser at 4C7AH, this code checks the result: if parsing succeeded (CARRY set), it copies the next command-line byte into the parameter block and loops back. If parsing ended (CARRY clear), it terminates the block with an ETX marker (03H). If the loop counter expires without a clean end, error 30H is reported.

4EA7H - OPTION KEYWORD PARSING ENGINE

This is the main option keyword parsing engine for the COPY command. It is called with B = maximum keywords to parse and HL pointing to the remaining command line text. The engine first checks if the command line has ended (via 6EBFH). If text remains, it jumps to the keyword table search at 4F1DH. If no text remains, it performs a multi-step option compatibility validation, checking that the combination of option flags in 5994H-5997H represents a valid configuration. If any incompatibility is detected, it jumps to error handlers at 4F49H (option conflict) or 5218H (parse error). If valid, the final option flags are stored back to 5994H-5997H.

4F1DH - KEYWORD TABLE SEARCH ENGINE

This routine searches the option keyword table at 505CH for a match against the user's option text pointed to by HL (passed in via DE after the exchange). The table contains encoded entries: each entry starts with ASCII keyword characters, followed by a byte with bit 7 set (marking end of keyword name), then flag and parameter bytes. The routine compares the user's text character-by-character against each table entry. On match, it extracts the option's flag byte, OR-mask value, conflict-check bytes, handler address, and optionally calls the handler. On no match after exhausting the table, it reports a parse error.

4F49H - OPTION CONFLICT / PARSE ERROR DISPATCHERS

Two error dispatch stubs. 4F49H handles option conflict errors (incompatible option combinations); 4F4CH is the "mismatch during keyword comparison" handler that checks if the mismatch occurred at the end of the keyword name.

4F8BH - NAME/PASSWORD PARSING HANDLERS

Three entry points that parse name/password strings from the command line into specific work area buffers. Each sets DE to the destination buffer and falls through to the common parser at 4F98H. The buffers are: 5970H (diskette name, 8 bytes), 5968H (diskette password, 8 bytes), and 5983H (alternate name/date field).

4F9EH - STEPPING RATE HANDLER

Handler for the SPW= (Stepping Pulse Width) option. Parses a single digit (2-6) from the command line, converts it to a stepping rate code (2+offset), and stores it in the drive geometry table at 64AFH.

4FACH - COMMAND LINE FILESPEC BUFFER HANDLER

Handler for the CFWO (Copy File With Options) keyword. This routine copies the remaining command line text into the DOS command buffer at 4480H, parsing it as a series of filespecs. It pre-fills the buffer with 0DH (end-of-line markers), then repeatedly calls the filespec parser.

4FC3H - NUMERIC PARAMETER HANDLERS

A group of small handler routines for numeric option parameters. Each calls 6EE6H or 6EE1H to parse a numeric value from the command line, then stores the result into a specific location in the drive geometry or FDC configuration tables. The error exit at 4FCBH catches invalid values.

4FDDH - BUFFER ADDRESS OPTION HANDLERS

Three option handler routines that parse hex addresses from the command line (via 5025H) and store them as 16-bit buffer pointers in the SYS6 work area. These support the OPFST= (Output First Sector), PFTC= (Post-Format Track Count), and similar buffer/address options.

4FF5H - DRIVE NUMBER PARAMETER HANDLERS

Two entry points for parsing drive numbers (0-9) from option parameters. Both call the common routine at 5002H which parses a digit, validates it is 0-9, and stores it at a handler-specific address. 4FF5H stores into 63D6H (copy tracking) and also writes the drive number into the self-modifying location at 4DFAH; 4FFFH stores into 63CAH.

500DH - HEX CODE PARSER (3-DIGIT)

Parses up to 3 hexadecimal characters from the command line and copies them into the buffer at 624CH. Used for the hex identification code options. Validates that each character is a valid hex digit (0-9 or A-Z).

5025H - CHECKSUM/ENCRYPTION COMPUTATION

This routine parses a name string from the command line (via 6F8EH) and computes a 16-bit checksum/encryption value from it using a CRC-like polynomial algorithm. The input is the 8-character name at 5960H; the output is a 16-bit value in DE that serves as a disk verification code. The algorithm processes each character through bit rotations and XOR operations to produce a pseudo-random hash. It also overwrites the name buffer with spaces (20H) after processing each character.

505CH - OPTION KEYWORD TABLE (DATA — 298 BYTES)

This is the encoded option keyword table for the COPY and FORMAT commands. The table contains 32 encoded keyword entries used by the option parser engine at 4F1DH to match command-line options, set flags, check for conflicts, and dispatch to handler routines. The table covers all options for the COPY command (6 formats), FORMAT command, and related operations.

Each entry consists of: (1) ASCII keyword characters, (2) a control byte with bit 7 set marking end-of-name, (3) an OR-mask byte, (4) optionally 4 conflict-check bytes, and (5) optionally a 2-byte handler address. The table is terminated by a 00H byte at 5185H.

Control byte encoding:
Bits 0-1: Index into option flags array (0=5994H, 1=5995H, 2=5996H, 3=5997H)
Bit 2: If set, 4 conflict-check bytes follow the OR-mask
Bit 4: If set, 2-byte handler address (lo/hi) follows
Bits 5-6: Mode qualifier — AND'd with the command mode register B to validate applicability:
  Mode 1 = COPY-only options
  Mode 2 = FORMAT-only options
  Mode 3 = options valid for both COPY and FORMAT

Conflict checking: When the 4 conflict bytes are present, each is AND'd against the corresponding flags byte (5994H-5997H). If any result is non-zero, the option conflicts with a previously-set option and error 2FH is raised via 4F49H → 5218H.

5200H - ERROR DISPATCH ENTRY POINTS AND MAIN ERROR HANDLER

Multiple entry points load specific NEWDOS/80 error codes into Register A, then fall through or jump to the main error handler at 521AH. The main handler at 521AH is the single most-referenced routine in SYS6, with 25 CALL references and 6 JP references throughout the module. It outputs a carriage return, sets up the SYS0 error handler return address on the stack, optionally flushes drive parameter blocks, clears the non-hex command flag in the DOS work area, and returns with the error code in Register A so the SYS0 error dispatcher at 4409H can process it.

Entry points and their error codes:

5200H	Error 20H (space/buffer error)
5204H	Error 3CH (disk geometry mismatch)
5208H	Error 2AH (general error — referenced from the entry dispatcher at 4D00H)
520CH	Error 2CH (directory full)
5210H	Command line termination check (returns if at end, else error 34H)
5214H	Error 34H (parse error)
5218H	Error 2FH (option conflict — referenced from option parser at 4F49H)

524EH - DIRECTORY BUFFER RELOCATION AND TRACK-PER-PASS CALCULATION

This routine performs two critical setup tasks for FORMAT/COPY operations. First, it relocates the directory entry buffer from its default position to a new base at 5D16H by copying its contents, freeing space for track data buffers. Second, it calculates how many disk tracks can be processed in a single pass given the available memory between the buffer end pointer (5D14H) and the saved high memory limit (593CH). Each track requires 0102H (258) bytes of buffer space — the routine divides available memory by this track size to determine the per-pass track count, then stores the result via self-modifying code into the comparison operand at 531AH. Finally, it initializes the source drive parameters and jumps into the main track processing loop at 5316H with the track counter initialized to zero.

529DH - SOURCE TRACK READ LOOP (READ TRACKS INTO BUFFER)

This is the inner loop that reads tracks from the source disk into the track data buffer. For each track, it: (1) advances HL past the 2-byte header to set the data buffer address at 5AE8H, (2) calls the disk read routine at 57C8H, (3) handles read errors with retry or skip logic, (4) stores a marker byte (FFH) and the error/success status in the header, (5) optionally patches boot sector data for small-track-count disks, and (6) increments the track counter via self-modifying code at 5314H until it reaches the per-pass limit stored at 531AH. When the limit is reached, the routine calls the track write-back routine at 5324H then loops back to 5291H for the next pass.

5324H - DESTINATION TRACK WRITE-BACK ROUTINE

This routine writes buffered track data from memory to the destination disk. It is called from 531EH after the source track read loop fills all available buffer slots, and also recursively from 534DH when a destination track mismatch requires mid-write re-initialization. The routine: (1) selects the destination drive via 5535H, (2) checks if any tracks were actually read (track counter at 5314H), (3) initializes the buffer pointer (544FH) and sector counter (5401H), (4) examines option flags to handle destination filespec mode vs. whole-disk mode, (5) for each sector in the buffer, calls the disk write routine 57CEH, (6) handles write errors with retry/skip logic, (7) optionally patches the boot sector via 546DH, and (8) decrements the track counter at 5314H until all buffered tracks are written. At the end, 53FBH flushes any remaining sectors and resets the disk I/O control byte to write mode (20H) for the next pass.

5360H - DESTINATION SECTOR COUNT CHECK AND WRITE DISPATCH

Entered from 5349H (no destination filespec) or 534FH (track match). Checks whether the destination sector count has been exhausted. If sectors remain, falls through to 53D5H to write the current buffer slot. If the sector count is zero, this is the start of a new destination track — it initializes write tracking, performs the disk write via 57CEH, handles errors, and manages the write success flag at 53D7H.

53FBH - POST-WRITE SECTOR FLUSH ROUTINE

Called from 5352H when a track mismatch is detected during file-level copy, and as the fall-through path when the write loop at 533EH-53F8H completes all buffered tracks. Checks whether any sectors were actually written during this pass (via the write counter at 53ECH). If sectors were written, resets the destination sector count, switches the disk I/O mode to write (20H), and calls the sector write engine to flush all pending data to the destination disk. This routine also handles end-of-data detection (status 06H), data verification against the directory template, and write error retry logic.

5461H - DIRECTORY DATA MISMATCH HANDLER

Entered from 544AH when a byte comparison between the written sector and the directory template fails during post-write verification. Decrements the destination sector count, displays a "sector mismatch" error (code 3AH), and depending on the operator's response, either skips the sector or retries by re-entering the flush loop.

546DH - WRITE SECTOR WITH BOOT PATCH AND VERIFY

Called from 53DFH (conditional on Z FLAG) and 5436H (retry path) to write a single sector to the destination disk with optional boot sector patching and write-with-verify mode. The routine: (1) checks the status byte for end-of-data (06H) and sets bit 0 of IX+00H to signal the caller, (2) calls 5C8EH to configure the destination drive's buffer pointer, (3) sets the I/O mode to write-with-verify (40H), (4) writes via 57CEH, and (5) retries on error with operator prompting. Returns Z FLAG on success, NZ FLAG on unrecoverable failure.

5492H - DECREMENT DESTINATION SECTOR COUNT

Utility subroutine that decrements the 16-bit destination sector count at 5B21H. Called from 542BH (status mismatch), 543BH (post-verified write), and 5461H (directory mismatch) to account for sectors that have been processed (whether successfully or with errors).

549CH — Post-Copy Completion: Write-Back and Option Dispatch

54D4H — Whole-Disk Copy: Return to Option Processing or Post-Copy GAT/Directory Write

54D7H — Post-Copy GAT/Directory Update for Destination Disk

550AH — GAT Free Space Adjustment and Directory Write-Back

552DH — Write Destination Directory and Exit

5535H — Set Up Destination Drive Parameter Block

5578H — Drive Setup Entry Points (Source, Destination, Primary)

5585H — Common Drive Setup: Select, Initialize, and Configure PDRIVE Access

55C1H — Disk Swap Prompt: Ask Operator to Insert Correct Diskette

55ECH — Write PDRIVE Entry and Validate Drive Configuration

55FBH — Linked List Traversal: Process Drive Parameter Block Entry

561EH — Swap Block Processing: Exchange PDRIVE Parameters

5637H — Advance to Next Linked List Entry and Loop Control

5646H - FLUSH ALL DRIVE BUFFERS

Iterates through all three drive parameter block base addresses (primary at 5942H, source at 594CH, destination at 5956H), each 10 bytes apart. For each drive that has a valid drive number (not FFH), calls the PDRIVE parameter swap routine at 5658H to flush that drive's cached data back to disk. This is the main cleanup routine called from 5230H before returning to DOS.

5658H - PDRIVE PARAMETER SWAP AND FLUSH

Selects the drive indicated by the first byte of the parameter block at (HL), then copies 10 bytes of drive geometry data either TO or FROM the DOS PDRIVE table at (42B9H). The direction of the copy is controlled by bit 1 of Register B as returned by 4723H: bit 1 set = copy FROM the SYS6 block TO the PDRIVE table; bit 1 clear = copy FROM the PDRIVE table TO the SYS6 block. After copying, deselects the drive. On error, jumps to the error handler at 521AH unless bit 2 of B is set (suppressed error mode).

568BH - DRIVE SETUP DISPATCHER

Entry point for drive setup operations controlled by the function code in Register A. Saves the function code, calls 5582H to set up the primary drive parameter block, then dispatches based on whether the function code is zero or non-zero. Non-zero codes trigger a SYS0 error dispatch via RST 28H with E0H OR'd into the code. Also compares the source directory buffer end pointer (5D14H) against the current position using RST 18H.

56A2H - DRIVE CONFIGURATION SCAN AND SOURCE SETUP

Scans through drive configuration entries looking for one with bit 5 set (FORMAT mode flag), then uses the found entry to initialize the source drive parameter block at IX=5AE5H. Calls the option keyword table parser at 4E21H and the filespec validator at 4F2EH to process drive-specific options.

56D4H - TRACK SEEK DISPATCH

Converts a track number in Register A to a 16-bit value in HL, fetches the DOS position byte from (IY+04H), and jumps to SYS0 routine 4C39H to perform a track seek via the PDRIVE table.

56DDH - DRIVE UTILITY CALL (48F0H WRAPPER)

Calls the SYS0 drive utility routine at 48F0H, then falls through to the common error check at 56FDH.

56E2H - TRACK SEEK WITH CACHE VALIDATION

Performs a track seek via SYS0 routine 48D4H, but first checks whether the drive cache invalidation flag at 570EH (self-modifying code) needs to be updated. If the new track's adjusted value differs from the cached value, calls 570DH to flush the GAT and reset the cache before seeking.

56F4H - DRIVE INITIALIZATION (48DBH WRAPPER)

Calls the SYS0 drive initialization utility at 48DBH with the current drive parameters, then falls through to the common error check.

56F9H - DRIVE INITIALIZATION WITH BASIC INIT (48AFH WRAPPER)

Clears Register A to 00H (basic initialization mode) and calls SYS0 routine 48AFH, then falls through to the common error check.

56FDH - COMMON DRIVE OPERATION ERROR CHECK

Shared return point for all drive utility wrappers (56DDH, 56E2H, 56F4H, 56F9H, 5701H). If the Z FLAG is set (success), returns to caller. If NZ (error), jumps to the main error handler at 521AH.

5701H - DRIVE UTILITY CALL (48C7H WRAPPER)

Calls SYS0 drive utility routine 48C7H and jumps to the common error check at 56FDH.

5706H - SAVE DRIVE CACHE FLAG

Copies the current drive cache invalidation flag from 48D5H into the self-modifying code location at 570EH for later comparison by the track seek routine at 56E2H.

570DH - FLUSH GAT AND RESET CACHE

Called when the track seek routine at 56E2H detects that the target track differs from the cached track. Writes the GAT sector to disk via 48C4H and resets the cache comparison byte to FFH (invalidated). The first two bytes at 570DH-570EH contain a self-modifying instruction whose operand is the cached track key.

571BH - ACTIVITY INDICATOR TOGGLE

Toggles an asterisk (*) character at screen position 3C3FH (top-right corner of the video RAM, column 63 of row 0) as a visual activity indicator. If the current character is an asterisk, replaces it with a space; if anything else, writes an asterisk. This provides feedback to the user that the copy/format operation is actively running.

572BH - CHECK FOR BREAK KEY AND READ CHARACTER

Reads a character from the keyboard via ROM routine 002BH, then calls SYS0 keyboard processing at 4548H for BREAK key detection. Falls through to the batch mode and BREAK key scan logic at 5733H. This routine is used by interactive prompts (e.g., the menu response scanner at 58E4H).

5733H - BATCH MODE AND BREAK KEY SCAN

Checks whether the operation is in batch mode (5995H bit 7). If not in batch mode, directly scans the keyboard hardware at 3840H for the BREAK key combination. The BREAK key on the TRS-80 Model III requires SHIFT+@ to be held simultaneously. This routine detects the key combo and, if BREAK is pressed while SHIFT is also held, reports error 39H (BREAK key abort) to the error handler.

5751H - GAT BIT CLEAR EXIT CHECK

Exit routine for the GAT bit zeroing loop at 5762H. Checks whether the track count in L has reached 60H (decimal 96, the maximum track number for standard disks). Also validates against the destination disk's total track count at 59C6H. If either limit is reached or exceeded, returns to the caller that invoked the GAT zeroing loop (popping 3 register pairs to unwind the stack).

5762H - ZERO OUT GAT BITS FOR EXCESS TRACKS

Clears individual bits in the GAT (Granule Allocation Table) for tracks that exceed the destination disk's capacity. When copying from a larger disk to a smaller one, the source GAT may have bits set for tracks that don't exist on the destination. This routine walks through the GAT byte-by-byte, clearing one bit at a time for each excess track, using a rotating bitmask. Register DE holds the count of tracks to clear, HL points to the current GAT byte, and C tracks the bit position within the current byte.

5784H - POST-COPY INITIALIZATION PLACEHOLDER

Six NOP instructions occupying addresses 5784H-5789H. This is placeholder/reserved space that can be patched at runtime or in a future version. The code was identified in the continuation notes as "3 NOPs" but the disassembly shows six consecutive NOP bytes. Execution falls straight through to 578AH.

578AH - DESTINATION WRITE SETUP AND SECTOR COUNT LOOP

Initializes the destination drive for writing by storing the write-sector command code (20H) to 57EBH (the disk operation type control byte), saving the sector count from DE to 5B21H, and setting the destination buffer pointer at 5B1AH to 4300H. Then compares the total destination disk capacity (from 59CFH) against the sector count. If the destination has enough room, displays the "COPYING" message and enters a read-sector loop that reads sectors via 57C8H (which calls SYS0 4436H), handles errors via 585AH, toggles the activity indicator, and decrements the sector count until HL reaches 0000H.

57C8H - READ SECTOR VIA SYS0 (IX→DE, CALL 4436H)

Copies the source drive parameter block pointer from IX to DE, then jumps to SYS0 routine 4436H to read a sector. This is a thin wrapper that converts the IX-based parameter passing convention to the DE-based convention expected by SYS0.

57CEH - SECTOR OPERATION VIA SYS0 (IX→DE, CALL 5CEDH)

Copies IX to DE and jumps to the SYS6 internal routine at 5CEDH for a sector-level disk operation.

57D4H - WRITE SECTOR VIA SYS0 (IX→DE, CALL 443CH)

Copies IX to DE and jumps to SYS0 routine 443CH to write a sector to disk.

57DAH - FORMAT/COPY STATUS DISPLAY

Displays a multi-line status message showing the current FORMAT or COPY operation progress. Saves all registers, outputs a carriage return, displays the operation label from 59D3H (e.g., "FORMAT" or "COPY"), then selects a mode-specific label based on bits 5 and 6 of Register B (loaded from the self-modifying byte at 57EBH). Displays the track count (either destination from 5B21H or source from 5AEFH depending on bit 7 of B), a separator string, and the formatted decimal sector count via 5909H. Then checks if a source filespec was parsed (5996H bit 2) and either returns via 521EH or displays the source disk label and restores the drive configuration.

584AH - BATCH MODE ABORT CHECK

Checks the batch mode flag (5995H bit 7). If batch mode is active, jumps to the error handler at 521AH — batch mode cannot tolerate interactive prompts. If not in batch mode, OR's C0H into Register A and calls the SYS0 error handler entry at 4409H, then jumps to the post-prompt continuation at 58C8H.

585AH - READ ERROR WITH TRACK INCREMENT

Called when a sector read returns an error (not 00H and not 06H). Displays the error status via 57DAH. If the display routine indicates the user chose to continue (returns NZ), increments the source track count at (IX+0AH)/(IX+0BH) as a 16-bit value, clears A to return Z FLAG (signal to retry), and returns.

5868H - CONDITIONAL SOURCE DISK LABEL DISPLAY

Checks a flag byte at 5869H (self-modifying operand). If the flag is zero, returns immediately. If non-zero, displays the source drive information message from 59E0H, calls the drive utility at 56DDH, displays the filespec name via 58A0H, outputs a carriage return, and returns with NZ flag set. The operand at 5869H is written by other routines to enable or disable this display.

587EH - DISPLAY STRING AND CARRIAGE RETURN

Displays the null-terminated string at (HL) via SYS0 4467H, then falls through to 5881H to output a carriage return character.

5881H - OUTPUT CARRIAGE RETURN

Loads Register A with 0DH (carriage return) and jumps to the single-character output routine at 58BDH.

5886H - DISPLAY BUFFER WITH SPACE PADDING

Displays B bytes from the buffer at (HL), replacing any non-printable characters (below 20H or at/above 80H) with spaces. This is used for displaying filenames and disk names where non-printable characters would corrupt the display. Advances HL past each byte.

5898H - DISPLAY B SPACES

Outputs B space characters to the screen. Used for padding and alignment in status displays.

58A0H - DISPLAY FILESPEC NAME (FILENAME/EXTENSION)

Displays a filespec in "FILENAME/EXT" format. Advances HL by 5 to skip the directory entry header, displays 8 characters (the filename) via 58B3H, then checks whether the extension is blank (first byte is 20H). If the extension is non-blank, displays a slash separator followed by 3 extension characters. The routine at 58B3H displays characters from (HL) but skips spaces (only displays non-space characters).

58B3H - DISPLAY NON-SPACE CHARACTERS FROM BUFFER

Displays B characters from the buffer at (HL), but only outputs characters that are not spaces (20H). Space characters are silently skipped. This produces a compressed display of filenames/extensions with trailing spaces removed.

58BDH - SINGLE CHARACTER OUTPUT

Core character output routine used by all display functions in SYS6. Saves DE and AF, calls the ROM character output routine at 0033H to display the character in Register A at the current cursor position, then restores the registers.

58C5H - DISPLAY STRING AND PROMPT FOR CONFIRMATION

Displays the null-terminated string at (HL) via 4467H, then falls through to the operator confirmation prompt at 58C8H. This is the entry point used when a message must be shown before prompting.

58C8H - OPERATOR CONFIRMATION PROMPT

Checks the batch mode flag (5995H bit 7). If batch mode is active, jumps to 5249H (the directory buffer relocation routine that bypasses operator prompts). If not batch mode, displays a response menu from 5A84H via the menu scanner at 58E4H. If the user's selection is >= 01H (an affirmative response), returns with NO CARRY. If selection is 00H (abort/cancel), loads error code 39H (BREAK abort) and jumps to the error handler.

58DEH - ALTERNATE MENU PROMPT (5AB3H MENU)

Displays the string at (HL) via 4467H, then loads HL with 5AB3H (an alternate response menu) and falls through to the menu scanner at 58E4H. This provides a different set of menu options compared to the 58C8H prompt.

58E4H - MENU RESPONSE SCANNER

Interactive menu display and response scanner. The menu string at (HL) contains one or more null-terminated option entries. The routine first displays all option strings (scanning past null terminators to find and display a trailing prompt string), then waits for a keypress via 572BH. It matches the pressed key against the first character of each option string, returning the 0-based index of the matched option in Register A. If no option matches, re-prompts. The matched character is echoed to the screen followed by a carriage return.

5909H - DECIMAL NUMBER FORMATTER

Converts the 16-bit value in DE into a decimal ASCII string and displays it. Uses a table of four 16-bit divisor constants at 5933H (64,000; 6,400; 256; -1) for successive division. The algorithm repeatedly subtracts each divisor from the value, counting iterations to determine each digit. Leading zeros are suppressed except for the final (ones) digit. Register B controls the number of divisor iterations (4 digits plus the ones remainder), and Register C acts as the leading-zero suppression flag (0 = still suppressing, non-zero = digit has been output).

5933H - DECIMAL DIVISOR CONSTANT TABLE

Four 16-bit values used as negative divisors by the decimal number formatter at 5909H. The values are stored in two's complement (negative) form so the division loop can use ADD HL,BC instead of SBC HL,BC. The four divisors correspond to the ten-thousands, thousands, hundreds, and tens places of a 5-digit decimal number (max 65535).

593BH - SYS6 WORK AREA VARIABLES (593BH-5960H+)

Data area containing the SYS6 runtime work variables. These bytes are not executable code — the disassembler incorrectly interprets them as instructions. This area includes the disk operation state flags, the saved high memory limit, the current drive parameter pointer, the mode flags, and the three 10-byte drive parameter blocks (primary at 5942H, source at 594CH, destination at 5956H). The contents shown reflect their state at the time of the disassembly dump and are overwritten at runtime.

5960H - WORK AREA: BUFFER SPACE AND OPTION FLAGS (5960H-5993H)

Continuation of the SYS6 runtime work area. Addresses 5960H-5977H contain a 24-byte buffer pre-filled with spaces (20H) that holds the disk name entered by the user or read from the source disk. This is followed by zero-filled work bytes and the date/version stamp at 5981H. The ASCII text "BNOTNAMED2:/4:/:" starting at 5982H is the default disk name template with embedded date separators. The option flags bytes at 5994H-5997H control all FORMAT/COPY operational modes. All data shown reflects the initial state captured in the disassembly dump.

5998H - WORK AREA: DRIVE GEOMETRY BUFFERS (5998H-59D2H)

Two sets of drive geometry parameter buffers, each holding 10 bytes of drive configuration data copied from the PDRIVE table via the swap routines at 5658H. The source geometry block runs from 59A3H-59ACH and the destination geometry block from 59ADH-59B6H, with additional configuration fields and pointers preceding and following them. All bytes are initialized to 00H at load time and filled at runtime when the drives are selected.

59D3H - MESSAGE STRINGS: OPERATION LABELS AND PROMPTS (59D3H-5AE4H)

All ASCII message strings used by SYS6 for screen display during FORMAT, COPY, and APPEND operations. Strings are terminated with either 03H (ETX, used by the 4467H display routine as an alternate terminator), 0DH (carriage return), or 00H (null). The disassembler misinterprets all of these as Z80 instructions. Each entry is documented with its address, ASCII text, and purpose.

5AE5H - SOURCE DRIVE PARAMETER BLOCK (5AE5H-5B16H)

The 50-byte source drive parameter block referenced by IX throughout SYS6. This block is initialized at runtime when the source drive is selected via 56A2H/56BCH. The first few bytes contain status flags and buffer pointers; the remainder holds drive geometry data, track counts, and sector buffer addresses. Fields at offsets +0AH/+0BH are the track count (incremented by 585EH on read errors). Most bytes are FFH (uninitialized/unused) in the dump.

5B17H - DESTINATION DRIVE PARAMETER BLOCK (5B17H-5B48H)

The 50-byte destination drive parameter block. Same layout as the source block at 5AE5H but used for the destination drive. Key fields: offset +06H (5B1DH) holds the destination drive number, offset +04H/03H (5B1AH) holds the buffer pointer (set to 4300H by 5796H), offset +08H/09H (5B1FH/5B20H) are parameter bytes, and offset +0AH/0BH (5B21H/5B22H) hold the destination sector count (stored by 578FH).

5B49H - THIRD DRIVE PARAMETER BLOCK (5B49H-5B56H)

A partial third drive parameter block. Only the first 14 bytes are visible before the linked list area begins at 5B57H. This block is used for the alternate/swap drive during three-drive operations or as scratch space during drive configuration.

5B57H - DRIVE PARAMETER LINKED LIST (5B57H-5B63H)

The linked list of drive parameter swap entries used by the PDRIVE swap routine at 5585H. Each entry is a 4-byte record: byte 0 = entry type (00H-01H = basic, >= 02H = overlay-specific with overlay ID), byte 1 = mode flags, bytes 2-3 = pointer to the next entry or swap data block. The list is terminated when byte 0 of an entry is 00H. Three entries are visible here.

5B64H - SWAP DATA BLOCK 1 (OVERLAY 02H — SOURCE DRIVE)

Structured swap data block for overlay 02H. This is a DATA area — the bytes here are not Z80 instructions but a compact record format used by the PDRIVE parameter swap routine at 5658H. The linked list entry at 5B57H (overlay 02H) points here. The format is: one byte = byte count to copy, followed by that many data bytes representing the source PDRIVE parameters, followed by a 16-bit pointer to the next swap block (00 00H = end of chain). When the swap routine processes this block it exchanges these bytes with the live PDRIVE table in the DOS work area, so that the source drive geometry is correctly reflected for FORMAT/COPY operations involving overlay 02H.

5BA3H - SWAP DATA BLOCK 2 (OVERLAY 04H — DESTINATION DRIVE)

Structured swap data block for overlay 04H. The linked list entry at 5B5BH (overlay 04H) points here. This block encodes the PDRIVE geometry parameters for the destination drive. Same format as Block 1: byte count, data bytes, 16-bit next-pointer. The three NOP bytes at 5BAA–5BACH (00H 00H 00H) end the chain (00H 00H) with one pad byte (00H). All bytes from 5BA3H through 5BDFH are DATA.

5BE1H - SWAP DATA BLOCK 3 (OVERLAY 05H — THIRD DRIVE)

Structured swap data block for overlay 05H (used when a third drive or alternate configuration is involved in the COPY operation). The linked list entry at 5B5FH (overlay 05H) points here. The block runs from 5BE1H through 5C09H. The sentinel at 5C09H–5C0AH (00H 00H) marks the end of the linked-list data area. Code resumes at 5C0BH.

5C0BH - SECTOR READ RETURN CODE PROCESSOR

This routine processes the return code after a sector read or write operation. On entry, Register A holds the FDC status/error code from the completed operation. The routine determines whether the result is a FORMAT-mode success, an end-of-data marker (06H), or a true error, and returns accordingly.

5C1EH - GAT TRACK BOUNDARY CHECK AND CONDITIONAL DRIVE INIT

This routine checks whether a given track number (passed in Register A) falls within a valid range relative to the current GAT configuration, and conditionally calls the SYS0 drive initialization routine at 48AFH if the track is beyond the currently initialized boundary. On entry, Register A = track number to check; BC = preserved caller context.

5C3EH - SEEK/RESTORE ERROR HANDLER — RETRY COUNTER

This entry point handles seek or restore errors during disk operations. Register A' (alternate accumulator, accessed via EX AF,AF') holds the current retry count. The routine decrements the retry count and either retries the seek or falls through to the format-launch path at 5C43H.

5C43H - SEEK RETRY EXHAUSTED — LAUNCH FORMAT OR ABORT

Reached when all seek retries have been exhausted (falling through from 5C3EH above). The routine cleans up the stack by discarding three saved frames, checks whether a previous head-load was cached at 4860H, and either launches the SYS0 FORMAT completion routine or performs a final drive-position correction before returning to the FORMAT track writer.

5C5BH - SCAN NAME STRING FOR VALID NULL TERMINATOR

This routine scans a name/label string buffer (pointed to by HL) for up to 80 (50H) characters, looking for a null byte (00H) that marks the end of a valid string. If a null is found, it jumps to 50FEH to handle the null-terminated name. If no null is found within 80 bytes, it returns A=1AH with NZ set.

5C69H - FILESPEC PARSE: DECREMENT ENTRY COUNT AND ADVANCE

This small utility routine decrements a filespec entry counter (in Register E) and either loops back to the parser at 4E3FH (if more entries remain) or advances HL and jumps to the post-parse handler at 4E72H (if all entries processed).

5C71H - STORE DATE/VERSION STAMP FROM DOS WORK AREA INTO BUFFER

This routine stores two bytes — the DOS date/version stamp values from the SYS0 work area at 421CH and 421AH — into a buffer pointed to by HL. Used to copy the current disk date/version stamp into the destination disk's GAT header during a COPY or FORMAT operation.

5C80H - BOOT SECTOR PATCH CALLBACK

This routine is called as a callback during boot sector configuration. It checks Register A for an error and, if successful, copies the boot sector configuration byte from offset +2 of DE (the drive parameter block) into the boot sector data buffer at 64BCH. On success it restores DE to its original base before returning.

5C8EH - SET DESTINATION BUFFER POINTER AND MARK DRIVE ACTIVE

Two short utility routines. The first (5C8EH) stores Register HL as the new destination buffer pointer at 5B1AH and sets bit 6 of IX+01H, marking the destination drive buffer as active. The second (5C96H) stores Register A into the disk operation mode field at 46A6H and sets the disk-operation-active flag at 468FH to 02H.

5C9FH - DOUBLE-DENSITY COMPATIBILITY CHECK AND COPY PARAMETER SETUP

This major routine checks whether the source and destination drives are compatible for the intended COPY operation — specifically whether both drives support double density (MFM), whether they share compatible sector geometry, and whether the copy direction is valid. If incompatible parameters are detected it jumps to the error handler at 6747H. If compatible, it configures the linked-list pointers used during the COPY to route parameter swaps correctly between overlays.

5CEDH - SECTOR WRITE MODE FLAG PROPAGATION

This routine is the target of the sector operation stub at 57CEH (JP 5CEDH via IX→DE). It checks the FORMAT mode flag and, if FORMAT mode is active, sets bit 5 of the sector parameter block's byte at IX+02H before forwarding to the SYS0 sector write routine at 4439H. This ensures that FORMAT-mode sector writes use the correct FDC write-with-format-mark command rather than a normal data write.

5CFBH - SECTOR PARAMETER BLOCK DIRECTORY FLAG CHECK AND DISPATCH

This short routine checks whether the current sector parameter block (IX) has bit 5 of IX+02H set (indicating a directory/system sector) and whether bit 0 of IX+07H is set (primary directory sector flag). Based on these conditions it dispatches to either the SYS0 sector position routine at 4C57H or the GAT sector calculation at 4C59H with function code 03H.

5D0DH - DIRECTORY BUFFER POINTER DATA WORDS

This is a DATA area, not code. Four NOP bytes (00H) followed by two 16-bit data words constitute the directory buffer control pointers used by SYS6's COPY routines. The disassembler has misread these as Z80 instructions (NOPs, LD B,E, LD D,5DH). The actual structure is described below.

5D16H - COPY/APPEND FILESPEC INPUT AND VALIDATION ROUTINE

This major routine handles the interactive input of filespecs (source and/or destination file specifications) for the COPY and APPEND commands. It: (1) calls the drive setup dispatcher to initialize drives, (2) reads the PDRIVE-option-flags byte (5997H) to check whether source and/or destination filespecs are still needed, (3) if filespecs are needed, prompts for input and reads from the keyboard (up to 13 valid characters per filespec), validates each character as printable/alphanumeric, and stores the result into a filespec buffer at 6491H, (4) handles BREAK/1CH abort and error conditions, and (5) calls into the source/destination drive setup after filespec entry.

5D71H - KEYBOARD CHARACTER INPUT SUBROUTINE

This subroutine reads one character from the keyboard for the filespec input routine above. It calls the ROM keyboard scan routine at 0013H, filters out null or high-byte-only presses, strips the high bit, and checks for CR (0DH). If a 1CH (cursor function code) is received it substitutes a CR to terminate input and writes a double-CR to the filespec buffer to signal that no filename was entered. On return: A = character (0DH=CR with Z FLAG set; other = character with NZ FLAG set).

5D86H - DRIVE SETUP AND GAT INITIALIZATION CALLS

This short sequence is the tail end of the filespec input routine and is also the jump target from 5D28H when no filespec prompts are needed. It calls three routines in sequence: (1) 5578H to set up source drive parameters, (2) 6162H to initialize or reset the GAT sector allocation state, and (3) 6178H to perform additional disk geometry initialization. On return from 6178H, the source drive number (in A) and geometry (in C) are stored to the work area at 5D8FH onward.

5D8FH - Store Geometry Results and Compute Tracks-Per-Pass Sentinel

Save the drive number and sectors-per-track returned by disk geometry init, then compute the tracks-per-pass sentinel from the high memory limit minus 24 bytes.

5DA0H - Check for Outstanding Filespec Prompts

Examine option flags byte 4 at 5997H to determine whether the user specified source and/or destination filespecs on the command line. If neither bit 4 (source filespec) nor bit 5 (destination filespec) is set, skip ahead to the directory scan at 5E0FH. If a source filespec was given, clear the 256-byte directory bitmap at 6291H and enter the filespec copy loop. If only a destination filespec was given, skip the bitmap clear.

5DB5H - Filespec Copy Loop: Build Drive-Qualified Filespec at 4480H

Process each filespec stored in the buffer at 6491H. For each entry: copy the filespec string into the SYS0 work buffer at 4480H, append a colon separator and the source drive number as a decimal ASCII string, then call SYS0 drive init at 4424H. On success, record the parsed drive index into the directory bitmap at 6291H. Loop until a 0DH (carriage return) terminator is encountered.

5DE6H - Drive Init Error/Retry Dispatch

Handle the return code from SYS0 drive init at 4424H. On success (Z), proceed to record the parsed drive index. Error code 18H means the filespec was re-directed (retry from the top of the filespec). Error code 19H means the filespec was re-assigned (also retry). Any other non-zero error is fatal.

5DF1H - Record Parsed Drive Index in Directory Bitmap

After a successful drive init (or error 19H), fetch the parsed directory entry index from SYS0 variable 4F56H and use it as an offset into the 256-byte directory bitmap at 6291H. Store either the source filespec flag (if C is non-zero) or a default option code (from 4D6EH) at that bitmap position. Then loop back to process the next filespec.

5E07H - No-Source-Filespec Path: Set Drive Mode and Re-Init Source

Reached when the filespec buffer at 6491H starts with a 0DH terminator, meaning no more filespecs remain to be processed. Set the drive setup mode to 05H and re-initialize the source drive before falling through to the directory scan initialization at 5E0FH.

5E0FH - Initialize Directory Buffer Pointers and Begin Directory Scan

Reset the directory buffer start and end pointers at 5D12H and 5D14H to both point to 6491H (the beginning of the directory work buffer area). Then enter the main directory scan loop, which reads directory entries from the source disk, applies multi-criteria filtering based on option flags, and either copies entries to the directory buffer or skips them.

5E20H - Multi-Criteria Directory Entry Filter

Apply a cascade of flag-based tests to the current directory entry. The filters check FORMAT mode, option flags at 5996H, density and in-use flags in the directory entry, and file extension matches. Entries that fail any test are skipped by jumping to 5EAAH. Entries that pass all tests proceed to the operator menu at 5E63H or directly to the directory buffer write at 5E87H.

5E5BH - Density Flag Test and Double-Density Compatibility Check

Load the first byte of the directory entry into Register D and test bit 6 (the density/allocation flag). If set, call the double-density compatibility check at 61EAH. If the drives are incompatible for double-density, skip the entry.

5E63H - Date-Stamp Update Flag and Operator Menu Prompt

Check bit 6 of option flags byte 2 (5995H) to see if the operator should be prompted for each directory entry. If set, display the filespec name and present a menu with options: 0=skip/display FORMAT message, 1=format single entry, 2=skip this entry, or 3+=add to directory buffer.

5E87H - Write Directory Entry to Directory Buffer

Add the current directory entry to the directory buffer. Write an 80H marker (busy flag) at the current end-of-buffer position, then conditionally set bit 6 (density flag) based on a series of flag checks. Finally, append the BC register pair (additional entry data) and advance the buffer end pointer.

5EAAH - Advance to Next Directory Slot and Loop

Advance the source directory position counter by calling 61BAH, then loop back to process the next directory entry if more remain. This is the common exit/continuation point for all paths through the directory filter — both entries that passed (after being added to the buffer at 5E87H) and entries that were skipped.

5EB3H - Directory Buffer Dispatch: Check for Entries and Begin Processing

After the directory scan loop (ending at 5EB0H) has populated the directory buffer at 6491H, this code reloads the buffer control pointers and checks whether any entries were collected. If the buffer is empty, jump to the finalize/exit routine at 552DH. Otherwise, begin iterating through the buffer entries, setting up destination and source drives for each one.

5ECEH - Append-Entry Path: Copy 24-Byte Directory Entry from Disk

When the directory entry index is past the current directory end (CARRY was set from 61D6H), this code reads the track number from the directory buffer entry, seeks to that track, and copies 24 bytes of directory entry data from the source disk into the destination buffer area using LDIR. Then it marks the directory buffer slot as used (SET bit 4) and continues scanning.

5EE0H - Push Post-Copy Return Address and Set Up Destination Drive

Push the address 60BFH onto the stack as a return address for the destination directory matching phase. Then fall through into the destination drive setup routine at 5EE4H. When the destination matching loop completes and returns via RET, execution will resume at 60BFH (the post-directory-copy dispatcher).

5EE4H - Destination Drive Setup: Init Drive, GAT, and Geometry

Set up the destination drive by calling 557DH (destination drive setup), 6162H (GAT sector state init), and 6178H (disk geometry init). Store the returned drive number at 597DH and sectors-per-track at 59CEH. Then check whether a destination filespec was specified — if so, enter the destination directory matching loop at 5EFCH; if not, jump to the no-dest-filespec path at 5F92H.

5EFCH - Destination Directory Matching Loop: Find Matching Entries

When a destination filespec was given (bit 3 of 5997H set), scan the destination disk's directory to find entries that match files in the source directory buffer. For each match, swap the track/sector assignment fields between source and destination entries and update the density/date pointers. This is the core of the file-level COPY operation when a destination filespec is specified.

5F32H - Directory Entry Copy: Swap Track/Sector Fields Between Source and Destination

A filename match was found between a source directory buffer entry and a destination directory entry. Copy the track/sector assignment fields from the source to the destination, preserving key fields. This swaps the data allocation so the destination file now points to the source file's data sectors. Then call 6209H to update the density/date pointer, flush the GAT, and check if additional drive writes are needed.

5F6AH - Post-Match: Check Density and Conditional Drive Write

After a successful filename match and directory entry swap, check the destination entry's density flag. If the destination entry does not have bit 6 set (not double-density), perform a sector write of the destination directory to disk via 60A9H. Then restore the overlay ID to 05H.

5F89H - Advance Destination Directory Slot and Loop

Advance the destination directory position by calling 61BAH, then loop back to scan the next destination directory entry. This is the common continuation/exit point for the destination matching loop.

5F92H - No-Destination-Filespec Path: Check Option and Rebuild GAT

When no destination filespec was specified (bit 3 of 5997H clear), or after the destination matching loop completes, check bit 3 of option flags byte 2 (5995H). If set, return immediately. Otherwise, re-initialize the source drive, copy the GAT to a backup buffer at 6391H, re-read the GAT and directory, and enter the GAT sector allocation loop to rebuild the destination disk's sector allocation from the source directory entries.

5FD7H - Mark Entry and Copy Directory Data to Destination

The entry passed all validation checks. Mark it as processed (SET bit 5), swap the sector assignment value between the GAT work buffer and the directory buffer, seek to the appropriate track, copy 22 bytes of directory entry data using LDIR, load the file's granule/sector count, fill 10 bytes with FFH markers in the sector slot buffer, and save the drive cache flag.

6004H - Density Flag Check and GAT Sector Allocation Calculation

Check the directory buffer entry's density flag (bit 6). If clear, skip to the next entry. If set and the track/sector value C is less than FEH, perform a complex geometry calculation to determine the GAT byte and bit positions for the sectors used by this file, then mark those sectors as allocated in the GAT bitmap at 6391H via 621CH.

6030H - Complex Geometry Calculation for Dual-Density GAT Allocation

When the sector assignment requires the dual-density path, perform a multi-step geometry calculation using the source and destination drive parameters. This computes the correct starting sector position for the GAT bit allocation by factoring in the sectors-per-track differences between source and destination geometries.

6064H - GAT Bit Allocation: Mark Sectors in Destination GAT Bitmap

Using the computed starting sector position in HL (from either the simple or complex path), calculate the GAT byte and bit offset via SYS0 4C59H, check for boundary overflow, build the allocation mask, and call 621CH to mark the file's sectors as allocated in the backup GAT buffer at 6391H.

6083H - Advance to Next Source Entry and Loop

Common continuation point for the GAT allocation loop. Fetch the next directory entry address, check its status flags, and optionally patch the self-modifying byte at 60C3H. Loop back to 5FA7H to continue processing.

6092H - Finalize GAT: Flush and Copy Bitmap to Disk

Reached when all directory buffer entries have been processed (Z flag from 5FA7H→5699H→5FAAH). Flush the current GAT to disk, copy the directory bitmap from 6291H to the 4300H work area, write it to the drive, then copy the backup GAT from 6391H to 4300H and write that as well.

60A9H - Sector Write Utility: Write via SYS0 and Extract Filespec Data

Write a sector to disk using SYS0 4925H, then clear the instruction byte at 4FB8H, extract additional fields from the parameter block, and tail-call to the filespec processor at 4E1AH. This is a utility routine called from multiple locations including the destination directory write at 5F7BH.

60BFH - Post-Directory-Copy Dispatcher

This is the return address pushed at 5EE0H. After the destination directory matching and GAT allocation phases complete and return, this dispatcher checks whether any unmatched entries remain (via the self-modifying byte at 60C3H). If the patched value is 00H, all entries were matched — jump to the "all done" exit at 6157H. Otherwise, continue to the second-pass copy loop at 60C8H.

60C8H — SOURCE-TO-DESTINATION COPY LOOP (UNMATCHED ENTRIES)

This section handles the second pass of the COPY operation: copying directory entries from the source disk that were NOT matched to existing entries on the destination disk. The preceding code at 60BFH checked the self-modifying flag at 60C3H; if it was non-zero, at least one source entry had no match on the destination, so execution falls through to here. The outer loop at 60C8H iterates through all source directory entries; entries with bit 5 set in the status byte are skipped (they were already matched and copied in the first pass). For each unmatched entry, the code either overwrites an existing destination directory slot (via 60F5H) or appends to the destination directory (via 60E1H).

60E1H — APPEND UNMATCHED ENTRY TO DESTINATION DIRECTORY

When the destination directory has no more existing slots (CARRY set from 61D6H), this code appends the unmatched source entry at the end of the destination directory. It sets bit 4 in the entry status byte to mark it as newly appended, seeks to the track indicated in the entry, and copies 22 bytes of directory data to the destination via LDIR.

60F1H — SOURCE SCAN COMPLETE: PUSH EXIT ADDRESS AND ENTER DESTINATION FINALIZATION

After all source directory entries have been scanned, this code pushes the "all done" exit address 6157H onto the stack, then falls through to 60F5H to begin the destination directory finalization pass. When the destination scan at 60F5H completes (via its RET Z at 60FEH), the stacked 6157H address causes execution to jump to the "all done" cleanup code.

60F5H — DESTINATION DIRECTORY SCAN AND COPY LOOP

This subroutine scans the destination directory for entries marked with bit 4 set (entries that were flagged during the source-to-destination copy). For each marked entry, it performs a directory lookup via 50CFH, copies 22 bytes of directory data, and then applies density-dependent field patching if the source and destination disks have different densities. This routine is called both from 60DCH (individual copy) and as a fall-through from 60F1H (batch finalization).

6124H — DENSITY-DEPENDENT DIRECTORY ENTRY FIELD PATCHING

When the source and destination disks have different densities (one single-density, one double-density), this section patches the date/version stamp fields in the copied directory entry. It checks bit 5 of the density flag: if set (destination is double-density), it loads the date stamp from the DOS work area at 421AH/421CH and uses the date callback address 5CEFH; if clear (destination is single-density), it uses zero for the date stamp and the standard date address 4296H. The patching writes the date stamp and callback pointers into the directory entry at specific offsets.

614FH — FLUSH GAT, MARK ENTRY PROCESSED, AND LOOP

After copying directory data (and optionally patching density-dependent fields), this section flushes the GAT to disk, marks the entry as processed (bit 5 set), and loops back to check the next destination directory entry.

6157H — "ALL DONE" EXIT: CLEAR FLAGS AND RETURN TO MAIN COMMAND LOOP

This is the final exit point for the COPY/FORMAT operation. It marks both the destination buffer and the conditional source display flag as FFH (cleared/complete), then jumps to 524EH for the directory buffer relocation and return to the main command processing loop.

6162H — GAT SECTOR STATE INITIALIZATION

This routine initializes the GAT (Granule Allocation Table) sector state by calling the drive initialization utility, then copying 256 bytes from the GAT buffer to the 6291H directory bitmap work buffer (and the previous 256 bytes to 4300H). The DEC H at 616DH adjusts the high byte of HL to point one page back, effectively rewinding after the LDIR advanced HL by 256 bytes.

616FH — COPY 256 BYTES TO 4300H (DOS WORK AREA)

Copies 256 bytes from the address in HL to the DOS work area at 4300H. Falls through to the generic 256-byte LDIR at 6172H.

6172H — COPY 256 BYTES FROM HL TO DE (LDIR UTILITY)

Generic utility that copies exactly 256 bytes from the source address in HL to the destination address in DE.

6178H — DISK GEOMETRY INITIALIZATION

This routine determines the disk geometry (tracks, sectors-per-track) for the currently selected drive. It first checks if FORMAT mode is active (bit 5 of 4282H); if so, it returns a fixed geometry of 10H (16) tracks and 06H sectors-per-track. Otherwise, it reads the disk geometry from the GAT sector data at 431FH (total sectors/granules), adds 8, then divides by 5 to compute the sectors-per-track count. The remainder becomes the total track count in Register A.

6190H — RELOAD DIRECTORY BUFFER CONTROL POINTERS

Reloads the directory buffer control pointers from the stored values at 5D12H and 5D14H. This resets the directory scan position so subsequent calls to 5699H will start from the beginning of the directory buffer. The end pointer from 5D14H is stored to the working end-of-directory pointer at 61D8H, and HL is positioned 3 bytes before the start pointer (5D12H minus 3) so the first call to 5699H advances HL to the first entry.

619DH — ADVANCE DIRECTORY BUFFER POINTER (INDEXED)

This routine uses the value in Register C as an index into the 256-byte directory bitmap at 6291H to fetch a directory entry count. If the count is less than 01H (zero entries), it returns with the CARRY FLAG set. Otherwise, it checks if the drive is in FORMAT mode and, depending on mode, performs a track seek to load the directory sector, then validates the sector status bits.

61BAH — ADVANCE TO NEXT DIRECTORY SLOT

This routine advances to the next directory slot by computing an offset based on the current disk geometry. In FORMAT mode, the offset is fixed at 50H (80 decimal). In COPY mode, it takes the value from Register C (sectors-per-track), adds 20H (32), and uses that as the increment. It then checks if the new position has reached or passed the total directory slot count.

61CBH — DECIMAL DIGIT EXTRACTION (DIVIDE A BY B)

This is a general-purpose routine that extracts a decimal digit by dividing A by B. It stores the ASCII quotient digit (30H + quotient) at (HL), then returns with the remainder in A and HL advanced by 1. The initial CP B check returns with CARRY set if A < B (quotient is zero), which the caller can use to suppress leading zeros.

61D6H — FETCH NEXT DIRECTORY ENTRY ADDRESS (VIA RST 18H)

This routine fetches the next directory entry address from the directory buffer. It uses self-modifying code at 61D8H (HL operand) and 61DBH (DE operand) which are set by 6190H. The RST 18H instruction performs a 16-bit comparison of HL vs DE (directory current position vs end position). If HL >= DE, the CARRY FLAG is clear (entries remaining); if HL < DE, CARRY is set (past the end). After the comparison, the new position is stored back to 61D8H for the next call.

61EAH — CHECK IF EITHER SOURCE OR DEST DRIVE SUPPORTS MFM

This routine checks the MFM (double-density) capability of both the source and destination drives by ORing their density flag bytes. The source density flag is at 59B9H and the destination density flag is at 59C7H. The result is masked with 20H (bit 5) to isolate the MFM capability bit. Returns NZ if at least one drive supports MFM.

61F6H — FETCH FORMAT MODE FLAG

Simple utility that reads the DOS mode byte at 4282H and tests bit 5 (FORMAT mode). Returns NZ if FORMAT mode is active, Z if COPY mode.

61FCH — XOR SOURCE/DEST DENSITY FLAGS

This routine XORs the source and destination density flag bytes to determine if they differ. The source density flag is at 59B9H and the destination is at 59C7H. The result is masked with 20H (bit 5) to check only the MFM capability. Returns Z if densities match, NZ if they differ. Register A is loaded with the destination density byte (59C7H) before returning.

6209H — CONDITIONAL DIRECTORY DATE POINTER UPDATE

This routine conditionally updates a date pointer field in a directory entry based on whether the source and destination disks have different densities. If Register A is zero on entry (OR A sets Z), no update is needed. Otherwise, it calls 61FCH to check density differences, and adjusts the two-byte date pointer at (HL) by incrementing (if destination is single-density) or decrementing (if destination is double-density).

621CH — GAT BIT MANIPULATION: MARK SECTORS ALLOCATED IN BITMAP

This routine marks sectors as allocated in the GAT (Granule Allocation Table) bitmap. It takes a track number in E and a sector count in C, then sets the corresponding bits in the GAT bitmap at (HL). The bitmap is organized with one bit per sector within each byte, and multiple bytes per track. The routine uses the sectors-per-GAT-byte value from 59CAH to handle bit/byte boundaries, and calls 5240H (boundary check) when all available sectors on a track are exhausted.

624CH — ASCII STRING DATA: "RUNNY" MARKER AND COPY PROMPTS

The addresses from 624CH through 6290H contain ASCII string data used by the COPY/FORMAT display routines. These are not executable code — the disassembler has produced Z80 mnemonics but they decode as ASCII text. The strings include the "RUNNY" verification marker, the "COPY IT? (Y/N/R/Q)" operator prompt, the "COPYING" status message, and the "ILF/XLF FILE" format indicator.

6254H — B-BYTE COMPARISON LOOP (DE VS HL)

A generic byte-by-byte comparison utility that compares B bytes starting at (DE) against (HL). Returns Z if all B bytes match, NZ at the first mismatch. Both DE and HL are advanced past the compared bytes.

625CH–6290H — ASCII STRING DATA AREA

This block contains ASCII string constants used by the COPY/FORMAT operator menus and status display routines. The disassembler interprets these bytes as Z80 instructions, but they are pure data — never executed. The strings are referenced by address from display routines throughout SYS6. All ETX (03H) bytes serve as NEWDOS/80 string terminators, equivalent to the standard NUL terminator in other systems.

6291H–6390H — DIRECTORY BITMAP WORK BUFFER (256 BYTES, RUNTIME DATA)

The 256-byte block from 6291H to 6390H is a runtime work buffer used during COPY operations to track which directory entries have been processed. Each byte in this buffer corresponds to a directory entry slot, and bits within each byte indicate processing state (e.g., bit 5 = matched, bit 4 = pending copy). The buffer is cleared to zero by the initialization routine at 5DACH before each COPY operation begins. Note that the first few bytes of this buffer overlap with the executable code below — the instruction at 6291H is reached via JP from 4D88H, meaning the first 8 bytes of the "buffer" are actually executed as code before the buffer is later cleared and repurposed for bitmap storage.

6291H — COPY/FORMAT/APPEND PRE-FLIGHT VALIDATION ENTRY

This is the main pre-flight entry point for COPY, FORMAT, and APPEND operations, reached via JP 6291H from 4D88H in the command dispatch table. The code performs drive initialization, validates option flags, copies disk name and date stamps from the GAT, checks source/destination disk label compatibility, and dispatches to the appropriate operation path. On exit, it either jumps to the filespec input loop at 5D16H (if option bit 3 of 5996H requests per-file mode), or falls through to the capacity comparison and parameter block builder at 6339H.

632CH — OPERATION DISPATCH: FILESPEC MODE OR FULL-DISK COPY

All pre-flight validation paths converge here. The code clears the label verification flag (bit 7 of 5996H) since it has been processed, then checks option bit 3 of 5996H to determine the operation mode. If bit 3 is set, this is a per-file COPY with filespecs, and control jumps to the filespec input/validation loop at 5D16H. Otherwise, this is a full-disk operation (COPY or FORMAT), and the code compares source vs. destination capacity to determine if the copy will fit, builds drive parameter blocks, and initiates the copy.

6377H — BUILD SOURCE/DESTINATION PARAMETER BLOCKS AND START COPY

This routine is called from 4DF0H (JP 6377H) during the COPY command's post-option-parse phase. It builds the source and destination drive parameter blocks by calling 6437H twice, runs the double-density compatibility reconfiguration at 5C9FH, sets up drive buffers, and jumps to the copy loop entry at 5279H.

6392H — PARSE DRIVE DESIGNATOR FROM FILESPEC

This short routine parses a drive number from the current filespec position and stores it in three locations: the destination drive number field (5B1DH), the destination drive variable (5956H), and the self-modifying current drive number at 4DFAH. Called from 4D4DH during command-line parsing.

63A0H — PARSE "=" PARAMETER FROM FILESPEC

This routine checks if the current filespec position contains an "=" character. If it does, it parses the hexadecimal value following the "=" sign using the hex parser at 6EE6H and stores the result into the geometry data table entry pointed to by DE. Called from 4D53H and also used at 63A3H as a shared entry point for the option keyword parser.

63B3H — DRIVE GEOMETRY LOADER: FLUSH BUFFERS AND LOAD SOURCE/DEST GEOMETRY

This routine is the top-level geometry initialization entry. It flushes all three drive buffers, sets the "drive parameter blocks initialized" flag at 593BH bit 7, flushes again with two parameter blocks, then loads geometry data for both the source and destination drives. Finally, it processes the geometry data table at 64A3H to apply any operator-specified overrides. Called from 5C9FH (DD compatibility check) and 6630H (APPEND geometry setup).

63ECH — CONDITIONAL GEOMETRY COPY WITH DRIVE-NUMBER FLAG UPDATE

Copies 10 bytes of drive geometry from the PDRIVE table into the work area at (HL). If Register A is FFH on entry, the routine returns immediately (the caller handles the copy separately). Otherwise, A contains a drive index that is rotated into the high nibble and used as an offset into the operation mode flags at 5940H. Called from 63CBH and 63D7H.

6424H — INIT PDRIVE LINKED LIST SENTINEL AND READ GAT

Writes sentinel value 0002H to the PDRIVE linked list start pointer at 5B53H, then reads the GAT sector for the drive specified in the third parameter block at 5B49H. Called from 6417H.

6437H — BUILD DRIVE PARAMETER BLOCK FROM FILESPEC DRIVE DESIGNATOR

Constructs a drive parameter block at (DE) using the drive number at (BC) and filespec data. Handles three cases: (1) colon delimiter (3AH) — parse explicit drive letter; (2) ETX (03H) — use default drive; (3) other characters — continue scanning. Also detects same-drive operations and sets flags at 5940H. Called from 637DH and 6386H.

64A3H–64C0H — DRIVE GEOMETRY DATA TABLE AND BOOT SECTOR TEMPLATE [DATA]

Two data structures. First (64A3H–64AFH): geometry data table with count byte and 4 triplets of (target-addr-lo, target-addr-hi, value). Second (64BAH–64C0H): 7-byte boot sector config template patched at runtime by 5C88H, 6799H, 676CH, 6774H.

64B0H — CLEAR 256-BYTE BUFFER AT 4300H

Fills 256 bytes of the GAT sector buffer at 4300H with the value in Register A. Called with A=FFH from 6675H and A=00H from 66C1H.

64C1H — DOUBLE-DENSITY BOOT SECTOR WRITER (FDC DIRECT I/O)

Critical low-level FDC routine that writes boot sector data using direct port I/O with NMI-based interrupt handling. Operates with interrupts disabled in the alternate register set (EXX). Reads boot data byte-by-byte via CALL 4345H. Handles double-sided disks via sector comparison against 12H (18). Contains an NMI-driven halt loop at 655AH broken by the NMI handler restoring SP from 43A6H.

64FFH — SECTOR READ COMPLETION AND FDC MULTI-SECTOR WRITE DISPATCHER

Reached from within the boot sector writer's control flow. Verifies buffer readiness, switches to alternate registers, seeks to target track, writes sectors with NMI-based DRQ handling, and handles double-sided disk side switching. Contains halt loop at 655AH broken by NMI handler.

6591H — FDC READY WAIT (BUSY LOOP)

Polls FDC status at port F0H until Busy flag (bit 0) clears, returns final status in A.

659CH — SHORT DELAY LOOP

Timing delay counting down from 8. Approximately 40 T-states. Used for brief gaps between FDC register accesses.

65A2H–65F9H — DATA AREAS: ERROR MESSAGES, BOOT TEMPLATES, FDC PARAMETERS [DATA]

Multiple data blocks referenced by COPY, FORMAT, and boot sector routines. None of this is executable code — the disassembler interprets the bytes as Z80 instructions but they are pure data constants and self-modifying targets.

65FAH — APPEND COMMAND — MAIN ENTRY POINT

This is the main entry point for the APPEND command. APPEND adds data to a destination diskette that already contains formatted data. It parses the source drive, optional hex parameter, disk name, and date stamp from the command line, then sets operating mode flags before falling through to the shared COPY/FORMAT/APPEND initialization code. The dispatch at 4D00H reaches here when the command code in Register A is 28H.

6630H — APPEND/FORMAT/COPY — GEOMETRY LOAD AND DISPLAY

This section loads source and destination drive geometry, displays the "STARTING DISKETTE FORMAT" message, computes the destination diskette's capacity, sets up sector I/O, and pushes a cleanup return address before falling through to the format/copy pre-validation dispatcher. This code is shared by APPEND, FORMAT, and COPY commands.

6643H — FORMAT/COPY PRE-VALIDATION DISPATCHER

This routine performs the core pre-validation and initialization for a FORMAT or COPY operation. It initializes the destination drive, checks various option flags to determine if certain steps should be skipped, displays "INITIALIZING SYSTEM DATA", writes the boot sector, initializes the allocation bitmap (GAT), copies the disk label, computes the total sector count, and then enters the format track write loop to write system tracks 0–9.

66BFH — FORMAT TRACK WRITE LOOP (SYSTEM TRACKS 0–9)

This loop writes the system tracks (tracks 0 through 9) to the destination diskette during a FORMAT operation. For each track, the sector buffer at 4300H is prepared with the appropriate content: track 0 gets the GAT (already set up), track 1 gets the boot jump vector and interleave data, track 2 gets the BOOT/SYS directory entry template, track 3 gets the DIR/SYS directory entry template, and tracks 4–9 are written as zeroed sectors. After writing each track, the loop increments the track counter and repeats until all 10 system tracks are written.

6710H — COMPUTE DESTINATION CAPACITY AND VALIDATE

This routine computes the total sector capacity and usable sector capacity of the destination diskette based on its geometry parameters, then validates that the disk does not exceed the maximum supported size of 192 granules. IX is pointed to 59C5H (the destination geometry parameter block) and the computed values are stored at offsets +0AH/+0BH (total sectors = 59CFH/59D0H) and +0CH/+0DH (usable sectors = 59D1H/59D2H). If the granule count exceeds C0H (192), the routine jumps to the "DISK TOO LARGE" error handler.

674DH — DD (DOUBLE DENSITY) CAPABILITY CHECK

This routine checks whether the destination drive supports double density (DD) formatting. If the drive does not support DD and the option to suppress the check is not set, the routine exits with the "DISK TOO LARGE" error. If DD is supported (or the check is suppressed), the routine returns normally.

6759H — CONFIGURE SELF-MODIFYING GEOMETRY BYTES FOR FORMAT

This routine patches multiple self-modifying code locations throughout the FORMAT engine with geometry values for the destination diskette. It reads the source/destination track count, density/side configuration, sectors per track, boot sector count, and step rate from the geometry parameter block at 59xxH, and writes these values into specific instruction operand bytes in the 64xxH–65xxH range. This allows the FORMAT engine's inner loops to use the correct parameters for the target diskette geometry without needing to read from memory on every iteration. Self-modifying code is used extensively for performance — the Z80's limited register set makes it impractical to keep all geometry values in registers simultaneously.

67ACH — WRITE BOOT SECTOR HELPERS

These two short helper routines write sectors to the destination diskette using the boot configuration template at 64BAH and the destination drive parameter block at 5B17H. The first routine (67ACH) writes sector 0 (the boot sector), then increments the sector counter for the caller. The second routine (67B3H) is the shared sector write engine that sets up the parameter block and calls the SYS0 write routine.

67C5H — DRIVE INITIALIZATION FOR FORMAT

This routine initializes the destination drive in preparation for a FORMAT operation. It patches the self-modifying geometry bytes via 6759H, initializes the buffer, then reads and validates the existing GAT sector on the disk. Depending on option flags in Register C (copied from 5994H), it dispatches to various validation paths: checking whether the disk already has data, verifying disk name/date, handling unreadable disks, and prompting the operator before proceeding.

67F0H — GAT SECTOR VALIDATION AND HIT TABLE FILL

This section is reached when the GAT sector was successfully read from the destination disk. It validates the GAT contents using 48AFH, checks whether the disk already has data (triggering a warning), and fills the HIT (Hash Index Table) area of the GAT buffer with 3FH values for formatting purposes.

6808H — DISK NAME AND DATE VERIFICATION

This section checks whether the operator should be warned about existing data on the diskette, verifies that the disk name matches the expected name (if specified), verifies the date stamp, and conditionally copies the disk name and date from the existing GAT into the working parameter area. Multiple option flag bits in Register C control which checks are performed.

6842H — CONDITIONAL DISK NAME/DATE COPY AND DISPLAY

This section conditionally copies the disk name and date stamp from the existing GAT sector buffer into the working parameter area (for preservation during reformat), compares the disk name against the expected name, and optionally displays the diskette's name/date information to the operator.

6888H — FORMAT MOTOR SPEED CHECK AND DD AUTO-DETECT

This section checks whether the format operation should proceed with filespec loop mode, performs motor speed verification and seek validation via the FDC, and handles automatic double-density detection. It writes a test boot sector using a temporary drive configuration to determine if the destination disk can support the requested density, then restores the original drive parameters. If the test fails, an appropriate error is generated.

68D1H — NORMAL FORMAT SETUP AND DD AUTO-DETECT VIA PROBE

This code handles the normal format setup path (when DD auto-detect flag was not set in option byte 4) and an advanced DD auto-detect path that temporarily modifies the drive's PDRIVE parameters to test whether the disk can accept double-density writes. It writes a test boot sector with modified parameters, then restores the originals and checks the result.

6930H — DISPLAY HELPER: DISK NAME MISMATCH

This short helper routine displays the disk label string passed in HL (typically "DESTINATION" or "SOURCE") followed by the "DISKETTE NAME MISMATCH" message. It is called when the disk name on the target diskette does not match the user-specified name.

693CH — DISPLAY HELPER: DISK NAME/DATE INFORMATION

This helper routine displays the disk label string (e.g., "DESTINATION"), followed by the "DISKETTE OLD NAME/DATE = " header, then the actual disk name (8 bytes), date stamp (4 hex bytes), and additional configuration data (8 bytes) from the GAT sector buffer at 43D0H. It provides the operator with a complete view of the existing diskette's identity before proceeding with a destructive format operation.

695AH — FORMAT SETUP: CONFIGURE FDC TIMING PARAMETERS

This routine configures the FDC (Floppy Disk Controller) timing parameters for the FORMAT operation by reading values from the FDC timing parameter table at 6CC1H. The table is indexed by drive type (derived from the system configuration byte at 4287H). Each table entry is 26 (1AH) bytes containing timing constants, gap lengths, fill bytes, and other values that are patched into self-modifying code locations throughout the format track engine (6AxxH–6BxxH). After loading timing parameters, it initializes tracking variables and validates motor speed timing. Register A on entry contains the sector count or side indicator.

6A00H — INTERLEAVE TABLE CONSTRUCTION

This section computes the sector interleave offset, builds an interleave table at 6485H that determines the physical order of sectors on each track, and applies an optional rotation for double-sided disks. The table is built by placing logical sector numbers at positions separated by the interleave factor, with collision resolution for occupied slots. After construction, 1-based sector numbering is applied if bit 4 of the drive config flags requires it.

6A68H — FORMAT ENGINE INITIALIZATION

This section sets the retry counter, reinitializes the drive, updates the progress display, configures the drive select register for formatting (including side selection and write precompensation), loads FDC timing parameters into registers, installs the NMI handler for Write Track completion detection, disables interrupts, and issues the WD1793 Write Track (Format) command to begin physical track formatting.

6AE1H — FORMAT TRACK INNER LOOP (WRITE TRACK FDC DATA STREAM)

The core of the physical track format engine — a tightly-timed loop feeding the complete track image to the WD1793 FDC byte-by-byte. Register C = F3H (data port), Register D = current data byte, Register A = drive select value (port F4H), Register E = sector countdown, Register B = loop counter, HL = interleave table pointer. Each sector consists of: gap→sync→ID address mark→track/side/sector/length→CRC→gap→sync→data mark→256×E5H→CRC→gap. The loop runs with interrupts disabled and exits via NMI at the index hole. Self-modifying locations contain timing values patched from the FDC timing table.

6BA5H - NMI Handler: Format Track Complete

This is the NMI (Non-Maskable Interrupt) handler that fires when the FDC detects the index hole after the Write Track command has written the entire track. The NMI signals that the track format operation is complete. The handler restores the stack pointer (from a self-modified address), disables further NMIs, restores the NMI vector at 4049H to its standard RETN instruction, reads and saves the FDC completion status, issues a Force Interrupt to halt the FDC, and re-establishes the drive select latch. Execution then falls through to the format verification section.

6BBCH - Check Verify Pending Flag / FDC Status Analysis

After the NMI handler restores the system state, this section checks whether the verify-after-format step should be skipped (if bit 7 of option flags byte 3 at 5996H is set, indicating label-only verify is pending). If verify is not skipped, the FDC status from the format operation is analyzed for errors by masking with FCH (checking bits 7–2: Not Ready, Write Fault, Record Not Found, CRC Error, Lost Data, and ignoring DRQ and Busy).

6BCAH - Motor Speed Range Validation

After confirming the FDC reported no errors from the Write Track command, this section validates that the disk motor speed is within acceptable limits. The NMI handler's index-hole timing provides a measurement stored in HL. Two self-modifying thresholds are compared: the lower bound at 6BCBH (if HL is below this, the motor is too fast) and the upper bound at 6BD4H (if HL is above this, the motor is too slow). Both threshold values were patched by the format setup code at 69F5H–69FDH from the timing parameter tables.

6BDCH - Patch Read Sector Command for Verify

With the motor speed validated, this section patches the SYS0 sector read routine to use the Read Sector command (88H, with multi-record and settle delay options), restores the appropriate density setting, and then calls the sector read routine at 5C96H to perform the verify read-back of the just-formatted track.

6BEAH - Sector-by-Sector Verify Loop

This section implements the sector-by-sector verify loop. For each sector on the track, it reads the interleave table entry (from the table at 6485H) to determine which physical sector to verify next, sets the FDC sector register, patches the SYS0 seek routine to skip actual head movement (since we are already on the correct track), performs a seek to update the FDC track register, re-establishes drive select, restores the seek routine, and then checks the FDC status for verify errors.

6C15H - Verify Status Check

After each sector verify read, the FDC status (in Register B) is checked for error conditions. The mask 9CH (10011100 binary) tests for Not Ready (bit 7), Record Not Found (bit 4), CRC Error (bit 3), and Lost Data (bit 2), while ignoring Write Protect (bit 6), Deleted Data (bit 5), DRQ (bit 1), and Busy (bit 0). If no errors are found, execution branches to the multi-sector loop continuation.

6C1AH - Verify Error: Retry Logic

When a sector verify read fails, this section manages the retry logic. It decrements the retry counter at 6C94H (initialized to 0AH = 10 retries). If retries remain, execution jumps back to 6A6DH to re-do the entire format-and-verify cycle for this track. If all retries are exhausted, the code re-enables interrupts, builds an error message identifying the track and side where the failure occurred, displays it, and prompts the operator to retry or cancel.

6C50H - Multi-Sector Verify Loop Continuation

After a successful sector verify, this section decrements the remaining sector count and loops back to verify the next sector. When E reaches 1, the interleave table is indexed differently (using the first entry at 6485H directly). When all sectors are verified, execution falls through to the double-sided toggle logic.

6C5BH - Double-Sided Disk Toggle

After all sectors on the current side of the current track have been formatted and verified (or skipped), this section handles double-sided disks. It checks IY+07H bit 6 (the double-sided flag in the drive parameter block). If the disk is double-sided and we just finished side 0, the side indicator at 6C62H is toggled from 0 to 1 and execution jumps back to 6A02H to format side 1 of the same track. If we just finished side 1 (or the disk is single-sided), execution falls through to the track advance logic.

6C6EH - Track Advance Logic

After both sides (or the only side) of the current track have been formatted and verified, this section advances to the next track. The track counter at 6B24H is incremented and compared against 23H (35 decimal, the total number of tracks on a standard TRS-80 disk: tracks 0–34). If all tracks are done, the routine returns to the caller. Otherwise, the geometry is updated and the format process continues with the next track.

6C7CH - Error Exits: Motor Speed and FDC Errors

These are the error exit paths for motor speed violations and FDC errors detected during format or verify operations. Motor speed errors load a pointer to the appropriate error message ("MOTOR FAST" or "MOTOR SLOW") and set Register B to FFH (a special error code indicating a motor speed problem). FDC errors are decoded by rotating the status byte to find the highest set error bit and recording its position in Register B. All paths converge at 6C93H for retry or abort handling.

6C93H - Retry or Abort Decision

This is the common retry/abort entry point for all format errors (motor speed and FDC errors). The retry counter at 6C94H is decremented; if retries remain, execution jumps back to 6A6DH to re-attempt the format. If retries are exhausted, the error code in Register B is checked for specific conditions before returning with error status.

6CA3H - Helper: Get Sectors Per Track

This small helper routine retrieves the sectors-per-track value from the PDRIVE parameter block at IY+04H and halves it if the disk is double-sided (since each side contains half the total sectors per track for format purposes).

6CAEH - Helper: Update Geometry After Track Advance

This helper routine is called after advancing to a new track. It checks bit 2 of the PDRIVE flags at 4287H (which indicates double-density mode), conditionally calls itself at 6CB6H (a quirk of the code layout where 6CB6H is the LD BC,0100H instruction that follows), performs a motor delay via SYS0 routine 4C92H, and then seeks to the new track position via SYS0 routine 46F7H.

6CC1H - Format Timing Parameter Tables

These four 26-byte tables contain timing parameters used by the format track write routine. Each table is calibrated for a specific combination of recording density and CPU/drive speed. The format setup code at 6959H–69C3H selects the appropriate table and patches its values into self-modifying code locations throughout the format write routine. The tables control gap lengths, sync byte counts, data mark timing, and other critical values that must be precisely timed for the FDC to correctly write sector headers and data fields.

6D29H - Reserved/Padding Area

This area from 6D29H to 6D5CH is filled with 00H bytes. It serves as reserved padding between the timing parameter tables and the message string pool that follows. The last three bytes (6D5AH–6D5CH) are also 00H.

6D5DH - Format Parameter Bytes

Three bytes of format parameter data referenced by the format initialization code. These values provide default track and sector configuration information used during command parsing.

6D60H - Message String Pool

This is the main message string pool for SYS6. All user-visible messages displayed during FORMAT, COPY, and BACKUP operations are stored here as ASCII text terminated by 0DH (carriage return), 0AH (line feed + CR), or 03H (ETX — used when the message continues with additional output such as a filename or number). These strings are displayed by calling the SYS0 display routine at 4467H with HL pointing to the start of the string.

6E88H - Format Error Message Template

This is the composite format error message template used when a format verify operation fails after all retries are exhausted. The message is constructed from multiple components: the prefix "CAN'T FORMAT " at 6E88H, a side indicator area at 6E95H (5 bytes that are overwritten with either "FRONT" or " BACK"), followed by the fixed text "SIDE OF TRACK " and an ETX terminator (03H). The track number is appended by the display routine at 5909H. The two 5-byte side indicator source strings are stored at 6EAAH ("FRONT") and 6EAFH (" BACK").

6EB4H - Skip Whitespace and Validate Input

This utility routine skips leading whitespace characters in the input buffer pointed to by HL, then validates that the current character is not a carriage return (end of input). It calls 6EBFH to check for CR, and if not CR, calls the SYS0 whitespace-skip routine at 4C7EH. The routine returns with Z set if the input is at end-of-line (CR found), or with HL pointing past the whitespace and the first non-whitespace character in A.

6EBFH - Check for Carriage Return

A tiny helper that fetches the character at (HL) and compares it against 0DH (carriage return). Returns Z if the character is CR (end of input line).

6EC3H - Skip Whitespace with Error Exit

Similar to 6EB4H, this routine skips whitespace in the input buffer using SYS0 routine 4C7EH. If no valid input character is found (the skip routine returns No Carry and No Zero), the code jumps to the error handler at 521AH. This enforces that the next input token must be present.

6ECAH - Parse Drive Number from Input

This routine parses a drive number specification from the input buffer. It checks for an optional colon prefix, converts the ASCII digit to a binary drive number (0–9), validates it against the system maximum, and sets up the drive for subsequent operations. Returns with Carry set and drive number in A/E on success.

6EE1H - Parse Hex Number from Input

This routine parses a hexadecimal number from the input buffer. It calls 6EF0H to parse with hex digit support enabled (bit 0 of B set), then validates the result is not too large. Used for parsing hex addresses or values in command options.

6EE6H - Parse Decimal Number from Input

This routine parses a decimal number from the input buffer. It calls 6F0EH to parse with decimal-only mode (no hex letters), then validates the result. Used for parsing numeric parameters like drive numbers and track counts.

6EF0H - Parse Hex Number with "H" Suffix Detection

This routine parses a number from the input that may be hexadecimal. It saves HL, calls the number accumulator at 6F13H (which uses 6F15H internally), and then checks whether the number is followed by an "H" suffix (indicating hex). If the character after the digits is "H" (48H), the number is treated as hexadecimal; if not, it is treated as decimal. Register B bit 0 controls whether hex letters A–F are accepted: bit 0 = 1 means hex mode.

6F0EH - Parse Decimal Number Entry Point

Entry point for parsing a strictly decimal number. This pushes HL, sets up a return address at 6F08H (the digit-check exit), and falls through to 6F13H/6F15H with B bit 0 = 0 (decimal-only mode). The pushed return address at 6F08H ensures that after the number accumulator returns, the digit-found check is performed.

6F13H - Initialize Number Parser

Initializes the number parser by clearing Register B (which controls hex/decimal mode via bit 0, and tracks whether any digits were parsed via bit 1), then falls through to the main number accumulator loop at 6F15H.

6F15H - Number Accumulator Loop

This is the core number parsing engine used by both decimal and hexadecimal number routines. It reads digits from the input buffer pointed to by HL, accumulates them into Register Pair DE as a multi-digit number, and uses Register B bit 0 to control whether hex letters (A–F) are accepted. The accumulation multiplies DE by 10 (decimal mode) or 16 (hex mode) for each new digit and adds the digit value. Register B bit 1 is set when at least one valid digit has been parsed. Returns when a non-digit character is encountered.

6F4AH - "H" Command Handler (Hex Dump / Help)

This routine handles the "H" command (entered from the SYS6 command dispatcher when CP 68H matched at 4D08H). It parses a filename into the work buffer at 5AE5H, looks up the file using the "TO" keyword search, resets the input parser, calls 5210H for further processing, sets up parameter blocks at 65B0H and 64B0H, opens both source and destination files, and performs a sector-by-sector copy between them using SYS0 read/write routines.

6F8EH - Parse Filename from Input Buffer

This routine parses up to 8 characters of a filename from the input buffer at HL into the destination buffer at DE. Valid characters are alphanumeric (A–Z, 0–9). Non-alphanumeric characters are replaced with spaces (20H) to pad the filename to exactly 8 characters. Register C counts the number of valid characters parsed; the routine returns with NZ if at least one valid character was found.

6FB7H - Validate Disk Date/Name Against Input

This routine validates the disk name and date from the input against the stored disk information at 598BH. It uses the skip-whitespace routine at 6EB4H to find the input data, then compares up to 3 fields (name components) character by character against the stored values. If a mismatch is found, execution jumps to the error at 5218H. If the input matches, it calls 4470H to store the validated information.

6FE4H - Search for "TO" Keyword in Input

This utility routine searches for the "TO" keyword in the command input. It first skips whitespace via 6EC3H, then uses the SYS0 keyword comparison routine at 4C6AH to check whether the input at HL matches the "TO" keyword string stored at 6FF6H. If found, the input pointer is advanced past the keyword. If not found, the routine returns. If found but followed by an error condition, execution jumps to 521AH.

6FF6H - "TO" Keyword Data

The "TO" keyword string used by the keyword search routine at 6FE4H. This is a NUL-terminated ASCII string.