TRS-80 DOS - NEWDOS/80 v2.0 for the Model III - SYS16/SYS Disassembled

SYS16/SYS

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SYS16/SYS is the NEWDOS/80 overlay module that implements the PDRIVE (Physical Drive) command. PDRIVE is one of the most powerful and complex commands in NEWDOS/80, allowing the user to view and modify the physical drive configuration table that controls how DOS communicates with each of the four physical floppy disk drives (drives 0–3) and six pseudo-drives (drives 4–9).

Each PDRIVE entry is a 16-byte record stored in a 10-entry table at 4300H in RAM. The entry contains all the parameters DOS needs to know about a drive: its interface type (TI), drive type (TD), track count (TC), sectors per track (SPT), track stepping rate (TSR), granules per lump (GPL), directory start lump (DDSL), and directory granule allocation count (DDGA). These parameters are read from the system diskette's boot sector during each computer RESET.

The PDRIVE command syntax is:

PDRIVE,dn1,dn2,TI=type,TD=type,TC=tracks,SPT=sectors,TSR=rate,GPL=granules,DDSL=lump,DDGA=count

where dn1 is the drive whose PDRIVE entry is to be changed, and dn2 is an optional source drive whose current settings are copied first. All keyword=value parameters are optional and modify specific fields of the PDRIVE entry.

Table at 4300H – Ten 16-byte entries (160 bytes total, 4300H–439FH)

Offset	Holds
00H	TI flags byte 0 (interface type bit flags, low byte)
01H	TI flags byte 1 (interface type bit flags, high byte)
02H	Configuration flags (TD drive type in bits 2–4, plus additional flags)
03H	Additional configuration
04H	TC – Track count
05H	SPT – Sectors per track
06H	Sign-extension byte for offset 07H
07H	TSR – Track stepping rate code (bits 0–2)
08H	GPL – Granules per lump
09H	DDSL – Directory start lump number
0AH–0FH	Additional configuration and computed values

Address	Holds
4300H	PDRIVE working table buffer (160 bytes, 10 entries × 16 bytes)
4302H	Flag byte: non-zero means changes were made to the PDRIVE table
43A0H	Current number of configured drives (used as upper bound for copy-back)
4EA2H	Current drive/entry number being processed (0–9) [SELF-MODIFYING CODE at 4EA1H]
4DF5H–4DF6H	Self-modifying flag: LD A,00H modified to non-zero to skip re-display [SELF-MODIFYING CODE]
5114H	Flag: non-zero means a second drive number (dn2) was specified
5118H	Counter/flag for command table scanning
510EH	Pointer into command table strings at 511CH
5106H–510DH	Maximum sectors-per-track table (8 bytes, one per TD drive type A–H)
511CH–5176H	Command keyword strings (TI=, TD=, TC=, SPT=, TSR=, GPL=, DDSL=, DDGA=, DSL=)
50CBH–5105H	Text messages (error and status strings)

Address	Routine
4D00H	PDRIVE Command Entry Point: parses the command line for drive numbers and parameters
4D5EH	Hex Digit Parser: validates and converts a hex digit character
4D69H	TI Flag Letter Processor: converts letter flags (A–P) into bit positions
4DA4H	Main Processing Loop: iterates through all 10 PDRIVE entries
4DB5H	Sign Extension and Write-Back: extends 8-bit TSR values to 16-bit
4DDAH	Display All Entries: loops through 10 entries displaying each one
4DE8H	Copy Table to System Area: copies working buffer to DOS system memory at 4291H
4E16H	Display Single Entry: formats and prints one PDRIVE entry's parameters
4E97H	Calculate IX Pointer: computes pointer to a specific entry in the PDRIVE table
4EB0H	Parameter Validation: validates and constructs PDRIVE parameters from parsed values
4FC3H	Set Up Command Table Pointer: initializes for parameter keyword scanning
4FCCH	Parse Next Command Token: gets next token from command line input
4FECH	Cross-Drive Compatibility Check: verifies no incompatible settings across drives
500FH	Character Output: wrapper for ROM character display at 0033H
5017H	Command Table Scanner: matches input against keyword=value command strings
502EH	Decimal Number Parser: parses a decimal number from the command line
503FH	Hex Number Parser Entry: parses a hex number from the command line
5062H	General Number Parser: core numeric conversion (decimal or hex) routine
5099H	16-Bit Decimal Output: displays a 16-bit value as a decimal number
51CBH	Command Line Scanner with Table Reset: scans characters and resets PDRIVE table on specific input

4D00H - PDRIVE COMMAND ENTRY POINT

Entry point for the PDRIVE command. On entry, Register A holds the SVC function code and Register B holds command-line flags. The code first validates that SVC function code 32H (PDRIVE) was requested.

4D19H - PARSE FIRST DRIVE NUMBER (dn1)

The command line has parameters. Parse the first drive number (dn1) which specifies the target drive whose configuration will be viewed or modified. The syntax is PDRIVE,dn1 or PDRIVE,dn1,keyword=value or PDRIVE,dn1=dn2.

4D24H - COPY DRIVE CONFIGURATION (dn1=dn2)

The user entered PDRIVE,dn1=dn2 syntax to copy the entire PDRIVE configuration from drive dn2 to drive dn1. Parse the source drive number, calculate the source entry address, then block-copy the 16-byte PDRIVE entry.

4D3FH - KEYWORD PARAMETER PARSING ENTRY

The command line did not use the dn1=dn2 copy syntax. Check for more tokens, then enter the keyword parsing loop to process keyword=value parameters such as TI=A, TD=E, TC=40, etc.

4D5BH - PARAMETER ERROR EXIT

Issue a "parameter not valid" error (error code 2FH) when a parsed value is out of range.

4D5EH - PARSE SINGLE-DIGIT DRIVE NUMBER SUBROUTINE

Subroutine to parse a single decimal digit (0-9) from the command line as a drive number. Called from 4D19H (parse dn1) and 4D25H (parse dn2). Returns the digit value in Register A. Issues error 20H if the character is not a valid digit.

4D69H - FLAG-LETTER PARAMETER PARSER (TI= TYPE)

Bit 7 of the dispatch byte was clear. Now test Bit 6 to determine if this keyword uses flag-letter parsing (like TI=ADHK where each letter sets a bit in a 16-bit field) or single-letter parsing (like TD=E where one letter selects a drive type).

4D93H - SINGLE-LETTER PARAMETER PARSER (TD= TYPE)

Parse a single letter value for keywords like TD=E (drive type). The letter A-Z is converted to a zero-based index (0-25) and stored directly in the PDRIVE entry field.

4D9BH - STORE PARSED VALUE INTO PDRIVE ENTRY

Common convergence point for both numeric parameters (from 4D59H) and single-letter parameters (from 4D99H). Store the parsed value (in Register A) into the PDRIVE entry field at IX+00H, then continue to check for more parameters.

4D9EH - PARAMETER LOOP CONTINUATION

After processing one keyword=value parameter (or completing the dn1=dn2 copy), check if there are more parameters on the command line. If so, loop back to parse the next keyword. If not, fall through to display or save the configuration.

4DA4H - DISPLAY ALL 10 DRIVE CONFIGURATIONS

End of command line reached after setting parameters. Now display all 10 drive configurations (drives 0-9) by calling the single-drive display routine (4EB0H) in a loop. Register A is used as the drive counter, cycling from 0 to 9.

4DB5H - SIGN-EXTEND BYTE 07H ACROSS ALL 10 DRIVES

After displaying all drives and performing the compatibility check, this loop iterates through all 10 PDRIVE entries and sign-extends byte 07H (the SPT low byte or sector configuration byte) into byte 06H. If bit 3 of Register C (set by the compatibility check at 4FECH) is set, the sign extension propagates the high bit of byte 07H into byte 06H. Otherwise byte 06H is cleared to 00H.

4DD3H - WRITE PDRIVE TABLE TO DISK

All 10 drives have been sign-extended. Now write the modified PDRIVE configuration table back to disk. This calls the SYS0 disk write routines to persist the changes.

4DDAH - DISPLAY SINGLE DRIVE OR ALL DRIVES (DISPLAY-ONLY ENTRY)

This is the entry point reached when the command line contains only PDRIVE (no parameters) or PDRIVE,dn1 (display only). It is also reached from the JP Z at 4D16H and 4D43H. Display all 10 drive configurations by looping through drives 0-9.

4DE8H - DISPLAY INCOMPATIBILITY WARNING IF NEEDED

After displaying all drives and running the compatibility check, test whether any incompatible TI flag combinations were found. If so, display a warning message. Then check whether any drives are actually configured before proceeding.

4DF5H - COPY PDRIVE TABLE TO SYSTEM AREA

After displaying and validating all drives, copy a portion of the in-memory PDRIVE table to the system work area at 4291H. This copies selected bytes from each of the first 4 (or fewer) PDRIVE entries. The variable at 4DF6H is a [SELF-MODIFYING CODE] target — its value controls whether this copy operation executes.

4E16H - DISPLAY SINGLE DRIVE CONFIGURATION

Entry point to display the full configuration for one drive. On entry, Register A holds the drive number (0-9). This routine prints the drive number, then iterates through the keyword table to print each parameter label and its current value. Called from the display-all loop at 4DDCH and from the display-after-modify path at 4DA9H (via 4EB0H).

4E35H - PRINT KEYWORD LABELS AND VALUES FROM TABLE

Walk through the keyword table at 511CH, printing each keyword's label string and then its corresponding value from the current drive's PDRIVE entry. The keyword table entries are null-terminated strings followed by a dispatch byte that encodes the field offset and value type.

4E54H - DISPLAY SINGLE-LETTER VALUE (TD= TYPE)

Display a single-letter parameter value (e.g., for TD=E). The value in Register E is a zero-based index (0-7) which is converted back to a letter (A-H) and printed.

4E5CH - DISPLAY FLAG-LETTER VALUE (TI= TYPE)

Display a flag-letter parameter value (e.g., TI=ADHK). The 16-bit value in DE contains individual flag bits, where each set bit corresponds to a letter A-P. Rotate through all 16 bits and print the letter for each set bit.

4E6DH - DISPLAY NUMERIC VALUE

Display a numeric parameter value (e.g., TC=40, SPT=10) as a decimal number. The 16-bit value is in DE. Clear the high byte D to 00H (these parameters are all single-byte values despite being loaded as 16-bit), then call the decimal display routine at 5099H.

4E72H - CHECK FOR NEXT KEYWORD IN TABLE

After printing one parameter's value, restore BC (the keyword table pointer) and check whether there are more keywords in the table. If so, print a comma separator and loop back. If the table is exhausted, finish the line and check for drive errors.

4E7EH - FINISH DISPLAY LINE AND CHECK DRIVE ERRORS

The keyword table is exhausted — all parameters for this drive have been displayed. Call the drive validation routine to check for configuration errors, then print a carriage return to end the display line.

4E92H - PRINT CARRIAGE RETURN AND EXIT DISPLAY

Print a carriage return (0DH) to end the current drive's display line, then return to the caller.

4E97H - SET UP IX POINTER TO PDRIVE ENTRY FIELD

Subroutine to calculate the IX pointer to a specific field within the current drive's PDRIVE entry. The dispatch byte at (BC) encodes the field offset in its low nibble. The drive number is read from 4EA2H. IX is set to 4300H + (drive × 16) + offset. Called from 4D4EH (keyword parsing), 4E40H (display), and 4EB4H (validation).

4E9AH - CALCULATE IX = PDRIVE BASE + (DRIVE × 16) + OFFSET

Core IX pointer calculation subroutine. On entry, Register A holds the field offset (0-15). The drive number is read from 4EA2H. Calculates IX = 4300H + (drive × 16) + offset. Also called directly from 4D31H (copy path) and 4EB1H (validation) with A pre-loaded.

4EB0H - VALIDATE SINGLE DRIVE PDRIVE ENTRY

Validation routine for a single drive's PDRIVE configuration. Called from 4DA9H (display-after-modify loop) and 4E7EH (display line finish). First clears unused bits in the configuration flags byte (IX+02H), then checks the drive type (IX+04H) against the interface type (IX+0FH) to verify compatibility. If the drive type is 00H (unconfigured), returns immediately.

4ED3H - EXTENDED DRIVE VALIDATION — SPT AND TC CHECKS

The interface supports this drive type (or the table lookup was skipped). Now perform extended validation: check the SPT value (IX+03H) against the drive type, and verify that the drive's track and sector configuration is internally consistent. This calls SYS0 routines at 4C37H and 4C59H for detailed range checking.

4F1DH - DDGA CROSS-VALIDATION

Continue the extended validation. The boundary was an exact match (Z was set at 4F16H and RET NZ at 4F1BH did not return). Now verify that the directory allocation (DDGA) does not exceed the available disk space by comparing the computed boundary in HL against the directory sector offset in DE.

4F21H - 16-BIT MULTIPLICATION VALIDATION

DDGA exceeds available space. Perform a 16-bit multiplication check to verify the total sector count. Load the DDGA and related parameters from the PDRIVE entry, then multiply to check that sectors_per_granule × granules_per_lump × lumps does not overflow a 16-bit value.

4F42H - TI FLAG BIT DISPATCH — DECODE INTERFACE FLAGS

All basic range checks have passed. Now decode the TI flag bits (stored in IX+00H/IX+01H as set by the TI= parameter) and set the appropriate configuration bits in the drive's config bytes (IX+02H and IX+07H). This section tests individual TI flag bits (corresponding to flags I, J, K, L, M as documented in the PDRIVE manual) and dispatches to the appropriate encoding logic.

4F6EH - ENCODE TI FLAGS INTO CONFIGURATION BYTES

A TI flag (K, J, I, L, or M) has been identified as active and compatible. Now encode the flag's effects into the drive's configuration bytes IX+02H and IX+07H. Register D holds the config bits to OR into IX+02H. Register E will accumulate additional flag bits for IX+07H based on secondary TI flag tests (flags H, A/D, B/C/E).

4FBCH - STORE COMPUTED CONFIGURATION BYTES AND RETURN

All TI flag bits have been tested and encoded. Register D holds the primary configuration byte (to be stored at IX+07H) and Register E holds the secondary flag byte (to be stored at IX+02H). Write both values to the PDRIVE entry and return.

4FC3H - INITIALIZE DISPLAY FORMAT SUBROUTINE

Short subroutine to initialize the PDRIVE display format variables. Sets the display column count to 2 and points DE to the default parameter template buffer. Called from 4D0BH (entry setup) and 4DD3H (pre-disk-write setup).

4FCCH - COMMAND-LINE SCANNER WRAPPER

Wrapper around the DOS command-line scanner at 4C7AH that adds PDRIVE-specific logic. After scanning, checks whether the PDRIVE invocation is display-only (by testing the flags at 5114H and the self-modifying byte at 4DF6H). If display-only and the user typed a suffix character after the drive number, this routine dispatches to the suffix handler at 51CBH. Returns: CARRY = error/BREAK, Z = end of line, NZ = token found.

4FE0H - SUFFIX HANDLER RETURN POINT

Return point from the suffix handler at 51CBH. After the A suffix is processed, the suffix handler jumps here to continue scanning the command line. If the suffix was valid, HL is advanced past it and scanning resumes. If not valid, the write-enable byte at (DE) is cleared and the routine returns with NZ and A=1.

4FECH - COMPATIBILITY CHECK ACROSS ALL 10 DRIVES

Scan all 10 PDRIVE entries to check for cross-drive TI flag incompatibilities. Specifically, certain TI flags (B, C, E on Model I) are inter-drive mutually exclusive — if one drive uses flag B, no other drive may use flag C or E, and vice versa. This routine ORs together the relevant flag bits from all drives and checks whether incompatible combinations are present. Called from 4DB2H and 4DE5H after displaying all drives. Returns with CARRY set = compatible (no conflicts), NO CARRY = incompatible (conflicts found). Register C holds the accumulated flag bits.

500FH - CHARACTER OUTPUT WRAPPER SUBROUTINE

Wrapper subroutine that prints the character in Register A to the screen via the ROM routine at 0033H, preserving Register Pairs DE and AF. Called extensively throughout the display routines.

5017H - KEYWORD TABLE SCANNER SUBROUTINE

Subroutine to compare the command-line token (pointed to by HL) against keyword entries in the keyword table (pointed to by BC). Each keyword entry is a null-terminated string followed by a dispatch byte. The scanner walks through the table, calling the SYS0 string comparison routine at 4C6AH for each entry. If a match is found, returns NZ with BC pointing to the dispatch byte. If all entries are exhausted without a match, issues error code 34H (parse error). Register D is the skip counter, used to advance past entries in the table. Called from 4D4BH.

502EH - PARSE NUMBER FROM COMMAND LINE (DECIMAL ENTRY)

Entry point for parsing a decimal number from the command line. Calls the core digit-accumulation routine at 503FH (which starts in decimal mode, B=00H), then checks whether the result is valid (non-zero high byte in D means overflow). Called from 4D56H (numeric parameter) and 4D5EH (drive number).

5033H - PARSE NUMBER FROM COMMAND LINE (HEX ENTRY)

Entry point for parsing a hexadecimal number from the command line. Calls the core digit-accumulation routine at 505DH (which starts in hex mode, B=01H), then checks the result validity.

5036H - VALIDATE PARSED NUMBER

Check whether the parsed number's high byte (D) is zero. For PDRIVE parameters, all values must fit in a single byte (0-255). If D is non-zero, the value overflowed. Returns with A = E (the low byte of the result).

503AH - ERROR EXIT: PARAMETER NOT VALID (2FH)

Issue error code 2FH ("parameter not valid") via the DOS error handler. Reached when a parsed value overflows or is out of range.

503CH - COMMON ERROR EXIT VIA DOS ERROR HANDLER

Common error exit point. Register A holds the error code. Jump to the SYS0 DOS error handler at 4409H which displays the error and aborts the command. This routine does not return.

503FH - DECIMAL NUMBER PARSER ENTRY

Parse a decimal number from the command line. First check if the number starts with a hex digit indicator. If the first digit is followed by characters A-H (indicating hexadecimal), redirect to the hex parser. Otherwise, accumulate decimal digits. On return, DE holds the 16-bit result.

505DH - HEXADECIMAL NUMBER PARSER ENTRY

Direct entry point for parsing a hexadecimal number. Sets B=01H (hex mode) and calls the digit accumulation routine. Returns via the result check at 5057H.

5062H - DIGIT ACCUMULATION INITIALIZATION (DECIMAL MODE)

Initialize for decimal digit accumulation. Set B=00H (decimal mode flag) and fall through to initialize DE and start the accumulation loop.

5064H - DIGIT ACCUMULATION INITIALIZATION (COMMON)

Common entry point for both decimal and hex parsing. Initialize the 16-bit accumulator DE to zero, then enter the digit processing loop.

5067H - DIGIT ACCUMULATION LOOP

Main digit processing loop. For each character on the command line: convert from ASCII to binary, verify it is a valid digit (0-9 for decimal, 0-F for hex), then multiply the accumulator (DE) by the base (10 or 16) and add the new digit. Exits when a non-digit character is encountered.

5078H - MULTIPLY ACCUMULATOR BY BASE AND ADD DIGIT

A valid digit has been parsed (value in Register A, range 0-15). Multiply the current accumulator (DE) by the base (10 for decimal, 16 for hex), then add the new digit. The multiplication is performed using shifts and adds: ×16 = 4 left shifts; ×10 = (×8 + ×2) via shifts and an intermediate ADD.

5099H - DECIMAL NUMBER DISPLAY SUBROUTINE

Convert the 16-bit unsigned value in DE to decimal digits and print them to the screen. Uses a divisor table at 50C3H containing the values 10000, 1000, 100, and 10 (as 16-bit big-endian pairs). For each divisor, performs repeated subtraction to extract the digit, then prints it. Leading zeros are suppressed. Called from 4E6FH to display numeric PDRIVE parameters.

50C3H - DIVISOR TABLE FOR DECIMAL DISPLAY

Table of four 16-bit divisor values used by the decimal display routine at 5099H. Each entry is stored in low-byte/high-byte order. The values are stored as NEGATIVE numbers (two's complement) because the repeated-subtraction loop at 50A7H uses ADD HL,BC instead of SBC HL,BC.

50CBH - ERROR MESSAGE STRING

ASCII error message displayed when a drive's configuration flags (byte 02H, bits 4,3,2) are all clear, indicating an invalid or incomplete configuration. Referenced by 4E88H.

50D9H - PADDING BYTES

Three padding bytes between the error message terminator and the incompatibility warning message.

50DCH - INCOMPATIBILITY WARNING MESSAGE STRING

ASCII warning message displayed when inter-drive TI flag incompatibilities are detected by the compatibility check routine at 4FECH. Referenced by 4DE8H.

5106H - INTERFACE-TO-MAX-DRIVE-TYPE LOOKUP TABLE

Eight-byte lookup table mapping each interface type index (0-7) to the maximum drive type code supported by that interface. Referenced by 4ECCH during validation. Each byte is the maximum TD= value (0=A through 7=H) that the corresponding TI= interface can handle.

510EH - DEFAULT PDRIVE PARAMETER TEMPLATE

Six-byte default parameter template buffer. Pointed to by DE after the initialization routine at 4FC3H. These values serve as default parameters for new or uninitialized drive entries.

5114H - PDRIVE VARIABLE AREA

Local variable storage area used by the PDRIVE command. These bytes are read and written at runtime by the various PDRIVE routines.

511CH - PDRIVE KEYWORD TABLE

Keyword table for the PDRIVE command. Each entry consists of a null-terminated ASCII keyword string followed by a dispatch byte. The dispatch byte encodes: Bit 7 = numeric value expected, Bit 6 = flag-letter value expected, Bits 3-0 = field offset within the 16-byte PDRIVE entry. The table is terminated by a 00H byte. Referenced by 4D46H (keyword matching), 4E35H (display), and 5017H (scanner). Per the NEWDOS/80 manual, the keywords are: TI= (interface type flags), TD= (drive type), TC= (track count), SPT= (sectors per track), TSR= (track stepping rate), GPL= (granules per lump), DDSL= (directory start lump), DDGA= (directory granule allocation), DSL= (alias for DDSL), NLD= (alias for a numeric parameter), and LLD= (alias).

5178H - SECONDARY VALIDATION SUBROUTINE

Short utility subroutine called from 4EE3H and 4F98H during TI flag validation. Sets bit 4 of Register E, then compares the value at IX+05H (the TC parameter) against the value already in Register A. Returns with the flags set from the CP instruction. Called conditionally (CALL NZ) when special mode or interface capability flags are active.

517EH - INTERFACE CAPABILITY CHECK SUBROUTINE

Short utility subroutine called from 4EE3H during validation. Tests Bit 2 of the interface/control byte at IX+0FH. If set, returns A=03H with NZ. If clear, returns with Z set. This adjusts the granule size from the default 05H to 03H for interfaces that use a 3-sector granule instead of 5-sector.

5186H - UNUSED PADDING AREA

Block of 69 NOP bytes (5186H-51CAH). This is unused reserved space within the SYS16 overlay, available for future patches or expansions. The area sits between the utility subroutines and the suffix handler.

51CBH - PATCHED AREA - NOT NATIVE TO THE OS - SUFFIX HANDLER (,A AND ,B MODES)

Processes the optional suffix character that follows the drive number in a display-only PDRIVE invocation. The suffix controls whether changes are saved to disk. Entered from 4FDDH with Register A holding the suffix character from the command line and Register Pair DE pointing to 4DF6H (the self-modifying write-enable byte). Two valid suffixes are recognized:
,A — Enable disk writes: set the byte at 4DF6H to non-zero, then re-enter the command-line scanner to process remaining parameters.
,B — RAM-only mode: clear the 7-byte area at 4DD3H-4DD9H (which contains the disk-write CALL sequence) to NOPs, disabling the disk write. Then re-enter the scanner.
Any other character is rejected.