TRS-80 DOS - LDOS 5.3.1 for the Model I - SYS7/SYS Disassembled

5200H - DATE and TIME Command Entry Vectors and Data Tables (Commands 15H/16H)

This overlay serves two commands from the SYS7 directory: DATE (command 15H) and TIME (command 16H). It opens with two jump vectors; the resident overlay loader enters the vector for the requested command. 5200H (JP 52CEH) is the TIME command (display or set the time, and install or remove the on-screen clock). 5203H (JP 524CH) is the DATE command. Following the vectors are three read-only data tables: the three-letter day-of-week names, the three-letter month names, and a days-per-month table used for date validation and the day-of-week calculation.

524CH - DATE Command

Display or set the system date. With no argument the current date in DATE$ (4044H-4046H) is displayed. With an "MM/DD/YY" argument the fields are parsed, range-checked against the days-per-month table (with leap-year adjustment), and stored into DATE$ and the derived resident date words (SVDAT1$ 4306H, SVDAT2$ 4307H, 4048H, 4466H). The error tail displays "Date not in system" or "Bad format".

52CEH - TIME Command (and On-Screen Clock)

Display or set the time, and install or remove the interrupt-driven on-screen clock. With no argument the current time is displayed via @TIME. With an "HH:MM:SS" argument the fields are parsed, range-checked, and stored into TIME$ (4041H-4043H). A "(CLOCK)" parameter installs or removes the recurring clock-display task (the SYS0 routine at 4D3AH) through @ADTSK/@RMTSK.

533BH - Day-of-Week Computation and Date-String Builder

Compute the day of the week for the date in DATE$ by accumulating the day-of-year from the days-per-month table (adjusted for a leap year), adding the year and its leap days, and taking the total modulo 7 with @DIV. The weekday is stored into the DATE$ flags byte and used to index the day-name table; the routine then builds the "Day, Mon dd, yyyy" string in the display buffer and shows it with @LOGOT.

53EBH - Helper Routines (Name Copy, Field Parser, Decimal Writer)

The support routines shared by the DATE and TIME paths: 53EBH copies a three-letter name from a table indexed by Register B; 53FAH parses up to three colon- or slash-separated decimal fields; 541FH and 5435H read a two-digit decimal number; and 543CH writes a value as right-justified decimal digits into a buffer.

5455H - Parameter Table, Display Buffer, and Messages

The data at the end of the overlay: the @PARAM keyword descriptor for the "CLOCK" parameter, the display buffer pre-initialized with the date template, and the two error messages. These are data, not code; the disassembler renders the bytes as spurious instructions.

5200H - CREATE Command (Command 13H)

Pre-allocate a disk file to a fixed size and set its record length and fill pattern, so the file occupies a contiguous, known amount of space. The command line is a file specification followed by parameters: SIZE (or S) sets the size in K, REC (or R) sets the size in records, LRL (or L) sets the logical record length, FILL (or F) sets the fill byte, and SHRINK reduces an over-large file. The file is opened or created via @INIT, extended to the requested length by writing fill records, then closed.

5358H - CREATE Parameter Table and Messages

The @PARAM keyword descriptor table and the message strings for CREATE. These are data, not code. Each 8-byte table entry is a six-character space-padded keyword followed by the two-byte address of the value cell @PARAM fills.

5200H - DEBUG and VERIFY Command Entry Vectors; DEBUG Command (Commands 14H/1BH)

This overlay serves two related commands: DEBUG (command 14H, entered at 5203H) and VERIFY (command 1BH, entered at 5200H). DEBUG arms or disarms the resident breakpoint re-entry: when turned on it writes a "JP 400FH" into the RST 30H hook area at 4315H-4317H and sets SFLAG$ bit 7, and an EXT option loads the extended debugger via RST 28H code 8BH; when turned off it writes a RET there and reclaims the debugger's high memory. VERIFY (5271H) patches the resident @WRITE vector so disk writes are read-verified. Both parse an ON/OFF/Y/N option through @PARAM (DEBUG also accepts EXT).

525EH - Extended Debugger Loader

Load the extended debugger, refusing if the debugger is already active or the system is busy. Issues RST 28H request code 8BH to bring in the extended module.

5271H - VERIFY Command

Turn disk write-verification on or off by patching the address field of the resident @WRITE vector at 443AH to point at the write routine used when verification is on (48E6H) or off (48CBH).

527FH - Common Exit and Parameter Parser

The shared exit that restores the caller's stack and returns (or aborts if not at the command level after an error), and the parameter-parse helper used by both commands: it selects the keyword table (DEBUG's includes EXT), saves the stack pointer, calls @PARAM, and reports whether an OFF/N keyword was seen.

52AFH - Error Message Display

Display one of the two refusal messages and set the failure return code. The two entry points share a single display call through a DD-prefix overlap.

52C6H - Messages and Parameter Table

The two refusal messages and the @PARAM keyword table. These are data, not code. DEBUG points @PARAM at the full table (5319H); VERIFY points it at 5329H so the EXT keyword is not offered.

SYSGEN is Library B command 1CH. It creates or deletes the invisible system configuration file CONFIG/SYS.CCC:d on the drive requested by the caller. On a build it snapshots the entire live system state - protected high memory, all Device Control Blocks, the DOS SVC vectors that filters and drivers have patched, the interrupt/task workspace, the keyboard-task hook, the flag bytes and the SPOOL control block - and writes it out as a standard LDOS load module so the boot loader can reload and re-apply it automatically at every power-up. On a delete it kills the file and clears the boot-time auto-configuration flag. The overlay is loaded at 5200H by the resident overlay loader (4BF5H) and entered at its first byte with the drive number in H and the ON/OFF switch in L.

5200H - Entry, Environment Check, Drive and Switch Setup

On entry H holds the target drive number (0-7, from the DRIVE= parameter parsed by SYS1) and L holds the switch (non-zero = ON = build, zero = OFF = delete). SYSGEN first suspends any active Job Control Language, then refuses to run if another module has hooked @EXIT or if SYS1 is busy, then patches the drive digit into its filename and message strings.

5229H - Build: Create CONFIG/SYS and Write the Module Header

The build path opens (creating) CONFIG/SYS on the target drive, then writes the load-module name record: a type-05 record carrying the six-character module name "CONFIG".

524CH - Build: Write the Screen Banner and Interrupt-Vector Records

Two type-01 load records are written next. The first paints "*** CONFIGURING ***" into the video line at 3F96H when the file is reloaded at boot; the second forces the RST 38H interrupt vector at 4012H to a RET so interrupts are inert while the rest of the configuration loads.

5262H - Build: Size RAM and Save Protected High Memory

SYSGEN walks down from FFFFH in 16K steps to find the physical top of RAM, computes the span from HIGH$ to that ceiling, and writes the whole protected high-memory region (utility modules and user programs held above HIGH$) into the configuration file.

5284H - Build: Save the Resident Configuration Regions

A driver byte in the KI Device Control Block is cleared, then a table at 54CEH is walked. Each four-byte entry is a byte count followed by a resident address; SYSGEN saves that region in place (source address equals reload address) so every DCB, vector, flag byte and workspace listed in the table is captured. The table lists HIGH$, the KI/DO/PR DCBs, the @WRITE vector, the task control blocks, the clock/task workspace, the keyboard-task and configuration hooks, the flag bytes, the overlay-dispatcher patch cell, the module-chain head and the SPOOL control block.

529AH - Build: Save the Device Tables, Finish the Module, Enable Auto-Config

The drive-table image is written, the banner record is completed, the type-02 transfer record ends the module and the file is closed. Finally the boot-time configuration control sector is read, its flag byte is set to enable auto-configuration, written back, and the success message is issued through @CMNDI.

52EBH - Helper: Write a Large Region as 254-Byte Load Records

This routine writes an arbitrary-length block to CONFIG/SYS as one or more type-01 load records, splitting the block into chunks of at most 254 (0FEH) data bytes so each record length fits in one byte. On entry Register Pair DE is the source/reload address and Register Pair BC is the byte count.

5324H - Helper: Write One Region as a Load Record

Writes a single type-01 load record of up to 255 bytes. On entry Register B is the data count, Register Pair DE is the reload address written into the record, and Register Pair HL is the source of the data bytes (source and reload address may differ, as they do for the banner and drive-table records).

5346H - Delete: Disable Auto-Config and Kill CONFIG/SYS

The SYSGEN OFF path reads the boot control sector, sets its flag byte to C9H so the boot loader skips configuration loading, writes it back, then opens and kills CONFIG/SYS. If the file is not present it reports "No user configuration found".

5369H - Common Exit and Abort Handler

The shared exit restores the Job Control context and returns to the resident RST 28H dispatcher when running at LDOS command level, or transfers to @ABORT when a JCL/DO file is active so the batch stream is terminated.

Self-Modifying Code
The entry Stack Pointer saved at 5200H is planted into the LD HL operand at 5366H; the exit reloads it into Register Pair HL and tests it, then returns to the resident RST 28H dispatcher (RET) when at LDOS command level or transfers to @ABORT at 4030H when a JCL/DO file is active (SFLAG$ 430FH bit 5). The LD SP,0000H at 5369H is the label the error paths (5394H, 53A0H) branch to before the same return sequence.

537BH - Helpers: Read the Boot Control Sector and Write a File Byte

Two small helpers. The first reads the boot-time configuration control sector on the SYSGEN drive into the buffer at 4200H. The second writes one byte to CONFIG/SYS through the ROM output vector, and also serves as the shared disk-error entry point.

5396H - Error Messages: No Configuration and Command-Level Refusal

A shared display tail with a deliberate DD-prefix overlap. Reached at 5396H it reports "No user configuration found"; reached at 539AH (from the entry environment check) it reports "Can't -- Valid only at LDOS command level".

Deliberate DD-Prefix Overlap
The entry environment check (520DH and 5215H) jumps to 539AH, one byte inside the four-byte LD IX,547EH at 5399H. Landing on the 21 7E 54 tail runs LD HL,547EH instead, so the two refusal messages "No user configuration found" (5462H) and "Can't -- Valid only at LDOS command level" (547EH) share the single @LOGOT and exit tail.

53A2H - Suspend Job Control and Enter Command-Level Context

Called at entry. If a JCL/DO file is active (SFLAG$ 430FH bit 5) this routine clears that flag, swaps an alternate @EXIT/@ABORT vector block into place, redirects the KI keyboard driver vector and resets the keyboard device so SYSGEN behaves as though it were entered at the command line. If no JCL is active it does nothing. The saved flag state is used later by 53D6H to decide whether to restore.

53CDH - Helper: Copy Six Bytes into the @EXIT Vector Area

Copies six bytes from the address in Register Pair HL to the @EXIT/@ABORT vector block at 402DH. Used by 53A2H to install the alternate vectors and by 53D6H to restore the originals.

53D6H - Restore Job Control Context

Undoes what 53A2H did. If a JCL/DO file was active on entry (recorded by the self-modified test byte), this restores SFLAG$, swaps the original @EXIT/@ABORT vectors back and re-enables the JCL keyboard path; otherwise it returns immediately.

53EDH - Data: CONFIG/SYS Filename and File Control Block

The configuration file specification, whose drive digit at 53FCH is patched at entry. It doubles as the File Control Block passed to @INIT, @OPEN, @CLOSE, @KILL and the byte-output driver. A short unloaded gap follows the string.

540DH - Data: Message Strings

The five carriage-return-terminated (or command-line) strings SYSGEN displays, plus the ATTRIB command line it hands to @CMNDI. Each drive digit is patched to match the target drive.

54A8H - Data: Configuring Banner

The 38-byte banner written into video RAM (reload address 3F96H) as two type-01 records, so "*** CONFIGURING ***" appears on screen while CONFIG/SYS reloads at boot.

54CEH - Data: Configuration Save-Region Table

The list walked at 5284H. Each four-byte entry is a byte count (DEFW) followed by a resident address (DEFW); the block at that address is saved into CONFIG/SYS to reload in place. A zero count terminates the table. Together these entries capture HIGH$, the KI/DO/PR and auxiliary DCBs, the @WRITE vector, the task control blocks, the clock/task workspace, the keyboard-task and configuration hooks, the flag bytes, the overlay-dispatcher patch cell, the module-chain head and the SPOOL control block.

5520H - Data: Module Pad

The remaining bytes of the overlay slot are zero fill.

FREE is Library B command 22H. With no drive it lists, for every ready drive, a one-line summary matching the DIR header: drive number, disk name and date, geometry (cylinders / density / heads), free and total space in K, and free and total directory entries. With a drivespec (FREE :d) it draws a granule allocation map of that one floppy - six cylinders per line, one character per granule (period unused, X used, asterisk locked out, D directory). The (P) option echoes the output to the printer. The overlay is loaded at 5200H by the resident overlay loader (4BF5H) and entered at its first byte with Register Pair HL pointing at the command tail.

DCT$ Field Reference
All disk geometry is read from the Drive Code Table entry for the selected drive (IY = 4700H + drive x 10, returned by GETDCT 478FH): (IY+03) drive-spec flags (bit 6 double density, bit 5 8-inch, bit 3 rigid/hard, bit 2 fixed); (IY+04) more spec (bit 5 double-sided); (IY+05) current cylinder; (IY+06) highest cylinder number; (IY+07) highest sector / head count; (IY+08) allocation (bits 7-5 granules per track, bits 4-0 sectors per granule); (IY+09) directory cylinder. The GAT sector (directory-cylinder sector 0, read to 4200H) holds one byte per cylinder, a set bit meaning an allocated granule; the disk name and date sit at 42D0H and 42D8H within it. The HIT sector (sector 1) holds one hash byte per directory record, zero meaning a free slot.

5200H - Entry and Command-Tail Parse

The entry stack pointer is saved for the exit, then the command tail at Register Pair HL is scanned. A ":d" drivespec selects single-drive map mode; the option list is parsed by @PARAM (only the P option exists). A colon routes to the granule map, otherwise to the global multi-drive listing.

5228H - Global FREE: Loop All Ready Drives

With no drivespec, FREE steps drive numbers 0 through 7, and for each ready drive displays one summary line. The P option prefixes a blank line to the printer.

524BH - Single-Drive Granule Map: Validate and Set Up

The FREE :d path validates the drive number and readiness, then clears the screen and prints the drive header before drawing the granule grid.

5261H - Granule Map: Build and Display the Grid

The map is drawn six cylinders per line. For each cylinder the GAT byte is decoded into one character per granule (X used, period free, asterisk locked out) or the whole cylinder is marked D when it is the directory cylinder. Lines are displayed as they fill, with a cylinder-range label at the left.

52C5H - Free-Space Line: Read the GAT and Count Free Granules

This routine builds one drive's summary line. Entered at 52C5H for the single-drive map header (drive number from the patched operand) or at 52C7H from the global loop (drive already in Register C). It reads the GAT, records the sectors-per-granule figure, and counts the free granules across all cylinders.

52FCH - Free-Space Line: Free K, Cylinders, Density and Heads

The free-granule count is converted to K and placed in the Free field, then the cylinder count and the density/heads characters of the "cyldh" field are formed from the DCT geometry.

533BH - Free-Space Line: Total K and Directory-Entry Counts

Granules per cylinder are computed (and saved for the map), multiplied by the cylinder count to give total granules, and converted to total K. The disk name and date are copied from the GAT into the header, then the HIT sector is read to count free and total directory entries. Finally the assembled line is displayed.

Known FREE Patch
The byte at 53C3H is C3H (JP 53F8H) in this image, which is the corrected form. The originally shipped overlay carried CDH (CALL 53F8H) here, which re-entered the pause/printer tail with the header pointer and mis-displayed the line as " :/ ". The official correction is PATCH SYS7/SYS.SYSTEM (D0C,59=C3:F0C,59=CD), changing the CDH CALL to a C3H JP. This copy already has the patch applied.

53C6H - Screen-Pause Helper

Counts displayed lines and, every fifteen lines, pauses for ENTER (unless a JCL/DO file is active) so the header does not scroll away.

53DBH - Clear Screen and Prepare the Line Buffer

Clears the display, prints the drive header line and a row of dashes, then blanks the line buffer to spaces. Also provides the buffer-fill helper used elsewhere.

53F5H - Display the Line Buffer and Optional Printer Echo

Displays the line buffer through @DSPLY, then either accounts for a screen line (pause check) or, when the P option is active, echoes the line to the printer.

5409H - Helper: Convert Granules to K (with Tenths)

Converts a granule count to kilobytes. Each granule is (sectors-per-granule) sectors of 256 bytes, so K = granules x sectors-per-granule / 4. The integer part is written as a decimal field and the tenths digit (0, 2, 5 or 8, from the two low bits) is returned.

5435H - Helpers: Right-Justified Decimal Writers

A set of staged entry points that write Register Pair HL as a blank-padded decimal number of one to five digits into the buffer at Register Pair DE, sharing a common per-place digit extractor.

545DH - Helper: One Decimal Digit by Repeated Subtraction

Extracts one decimal digit for the current place value (Register Pair BC) from Register Pair HL, writing it (or a leading blank held in Register A) to the buffer at Register Pair DE. Leading zeros are shown as blanks until the first significant digit.

5478H - Helper: Plot One Cylinder's Granules

For an ordinary cylinder, writes one character per granule from the GAT byte at Register Pair HL: X for an allocated granule, period for a free one. A second pass over the lock-out map marks locked-out granules with an asterisk on floppies.

54A9H - Helper: Plot a Directory Cylinder

Fills the current cylinder's granule positions with 'D' to mark the directory track.

54B5H - Helper: Write the Cylinder-Range Label

Writes the left-margin "start- end" cylinder range for the next map line into the line buffer, advancing the range by the six cylinders per line.

54E1H - Helper: Read a Directory-Cylinder Sector

A general read of directory-cylinder sector 0 into 4200H, returning error code 14H on failure. (This helper is present in the overlay for the read used by the summary path.)

54F4H - Errors and Common Exit

The parameter-error and disk-error entries report through @ERROR and set a failure code, then the shared exit restores the stack and returns to the resident dispatcher, or aborts to @ABORT when a JCL/DO file is active.

5512H - Data: Header Template and Cylinder Label

The summary/header line template, whose fields are filled in place by the code above, followed by the first cylinder-range label copied into the line buffer.

555AH - Data: Display Line Buffer (unloaded)

The 64-byte working buffer where each output line (summary or map row) is assembled. It carries no data in the load module.

559AH - Data: @PARAM Option Table and Pad

The keyword descriptor table passed to @PARAM, defining the single P option.

The SYSTEM library command selects DOS features and configures the user-definable areas of the operating system. It is the largest overlay in SYS7/SYS (3303 bytes, 5200H-5EE6H). On entry from the resident overlay loader (4BF5H) HL points to the command line just past the command name, and register B held the command number (A1H) the loader used to select this overlay. The command line is expected in the form SYSTEM (parm,parm,...); a single call to the resident @PARAM service (4476H) parses every keyword against the descriptor table at 5DF6H, depositing each supplied value into a two-byte cell that is the operand of a LD BC,nnnn test instruction further down. Each sub-command handler then examines its cell and acts. This section documents the first 35% of the overlay, the group of "flag / task / clock" sub-commands, ending at the RET of the ALIVE task template at 5591H, immediately before the SVC handler at 5592H.

SYSTEM (Command A1H) - Parameter Value Cells (@PARAM table 5DF6H)

Each cell below is the two-byte operand of a LD BC,nnnn instruction. @PARAM (4476H) overwrites the cell with the value the user supplied for that keyword; if the keyword is absent the assembled default remains. The handlers read a cell into BC, form A = B OR C, then DEC A: an assembled default of 0001H means "not specified" (skip), 0000H means the OFF form, and any other value (FFFFH for a bare switch, or a number) means the ON form. Only the cells used by this first-35% section are listed.

SYSTEM (Command A1H) - Shared Helper Routines (later in the overlay)

The handlers in this section call four shared helpers that live in the last third of the overlay (beyond this first 35%). Their behaviour is summarised here so the calls can be read in context.

5200H - Entry and @PARAM Parse

Save the caller's stack pointer, point @PARAM at the keyword descriptor table, and parse the whole parameter list in one call. A parse error aborts through the @PARAM error exit; otherwise every value cell has been loaded and control falls into the first sub-command test.