TRS-80 DOS - NEWDOS/80 v2.0 for the Model III - SYS1/SYS Disassembled

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SYS1/SYS is the command interpreter module in NEWDOS/80 v2.0 for the TRS-80 Model III. It handles SVC (Supervisor Call) dispatching, command parsing, MINI-DOS entry/exit, filename processing, and system prompts. This module is loaded into memory at 4D00H and interacts with other SYS modules (e.g., SYS0 for boot, SYS6 for errors). Key features include the command name table at 4F6FH, flag-based protection for sensitive commands, and integration with ROM routines for display.

• 4D00H: SVC Dispatcher – Routes SVC calls (e.g., 23H filename scan, 63H command exec).
• 4D48H: Error Display – Calls external handler at 4467H with table at 51DCH.
• 4D5BH: MINI-DOS Entry – Saves state, sets flags, jumps to command executor.
• 4D8BH: SVC Exit Routine – Disables interrupts, restores state, displays prompt.
• 4E4BH: Command Parser/Executor – Copies line, checks "R" repeat, matches names.
• 4E97H: Command Name Matching – Compares input to table at 4F6FH.
• 4F41H: Drive Prefix Insertion – Adds drive spec to filenames.
• 50CEH: Display Command List – Prints all commands with padding.
• 50F2H: NMI/Break Handler – Saves stack, enters MINI-DOS.
• 5111H: NMI Return – Restores registers and exits.
• 5154H: Filename Parsing – Scans and validates names/extensions.
• 5167H: Filename Parser – Copies and checks chars (alphanumeric, dots).
• 51ACH: Display Utilities – Outputs spaces/CR/chars via ROM 0033H.

4D00H - SVC DISPATCHER: COMMAND/SERVICE ROUTER

SVC (Supervisor Call) Dispatch Table — Routes incoming service requests by comparing the SVC function code in Register A against known service numbers, then jumping to the appropriate handler. Register C contains a sub-function counter used for extended SVC routing.

4D48H - SUB-FUNCTION 9: DISPLAY ERROR TABLE

Points HL to the error/status data table at 51DCH, then jumps to the system display routine at 4467H to output the message.

4D4EH - SVC SETUP: PUSH RETURN CONTEXT VIA ALTERNATE REGISTERS

Sets up SVC return context by loading alternate register set BC' with SVC dispatch code and DE' with return address, pushing them onto the stack, then executing RST 28H to invoke the SVC handler.

4D59H - SUB-FUNCTION 1: POP AND RETURN

Simple SVC exit: pops the saved AF from the stack and returns to the caller.

4D5BH - SVC C3H HANDLER: ENTER MINI-DOS COMMAND INTERPRETER

Entry point for the MINI-DOS interactive command interpreter. Saves all CPU registers, manages the MINI-DOS stack frame (supporting nested invocations via 42BDH), and enters the command input loop. The system control block at 428AH tracks MINI-DOS state via bit 6.

4D78H - SUB-FUNCTION 7: DOUBLE POP AND EXIT

Pops two values from the stack (discarding nested return addresses) and falls through to the SVC exit routine.

4D7CH - SVC 43H HANDLER: DOS COMMAND CHARACTER

Returns the DOS command prompt character. Clears A and sets the Carry flag to indicate the character is available.

4D80H - SUB-FUNCTION 5/12: CLOSE/RESET FILE STATE FLAGS

Clears file and system status flags in the DOS System Control Block. Clears bits 7 (NMI/Break), 6 (MINI-DOS), and 4 (special mode) of 428AH, and clears bit 5 of 4289H. Falls through to 4D8AH to clear A and enter SVC exit.

4D8AH - SVC 23H / COMMON EXIT: CLEAR A AND ENTER SVC EXIT

Sets Register A to zero (success) and falls through to the SVC exit routine at 4D8BH. Also serves as the handler for SVC 23H (Filename Scanner).

4D8BH - SVC EXIT ROUTINE: STATUS PROCESSING AND RETURN DISPATCH

Central SVC exit handler. Disables interrupts, reads the DOS system status flags from 428AH/4289H, and determines whether to: return normally, dispatch to the MINI-DOS command loop, signal an error via RST 28H, or restore the system to the NEWDOS/80 READY prompt. This is the convergence point for all SVC completions in SYS1.

4DC4H - SYSTEM STATE RESTORATION AND NEWDOS/80 READY PROMPT

Checks the NMI/Break flag to determine the exit path. Normal mode: resets the stack to 41E0H, patches the BASIC ENTER key handler at 4478H (self-modifying code: JP or RET), displays "NEWDOS/80 READY", sets up the command input buffer, and calls the ROM line input routine at 0040H. NMI/Break mode: restores the NMI-saved stack, displays "MINI-NEWDOS/80 READY" instead. Both paths end by enabling interrupts, displaying the prompt, setting the DOS Ready flag (bit 5 of 428AH), and entering ROM line input with the SVC context E308H stacked so the entered command routes to sub-function 8 at 4E4BH.

4E27H - MINI-DOS EXIT: RESTORE STACK AND RETURN TO CALLER

Handles exit from a MINI-DOS command or session. Restores the MINI-DOS stack frame from 42BDH, checks auxiliary flags, manages nested MINI-DOS invocations (clearing the MINI-DOS active flag on the outermost exit), restores the previous stack frame pointer, and returns to the caller through the register restoration routine at 5132H.

4E47H - SVC 63H HANDLER: DOS COMMAND LINE EXECUTION ENTRY

Entry point for SVC 63H — executes a DOS command from a command line string. Resets the stack to the DOS base and saves AF before falling through to the command parser.

4E4BH - COMMAND LINE PARSER AND EXECUTOR

Core command line processing routine. Receives input from either the ROM line input (via stacked SVC context E308H sub-function 8) or from SVC 63H. Copies the command line from the source to the working buffer at (4225H) while converting to uppercase via CALL 4548H. Handles the "R" (Repeat) command to re-execute the previous command. Then matches the first word against the command name table at 4F6FH, comparing character-by-character with support for command abbreviations.

4EBEH - COMMAND DISPATCH: READ TABLE DATA AND EXECUTE

After a command name has been matched in the command table, reads the 2-byte dispatch data following the name, combines it with the first byte of the next table entry, and routes to the appropriate command handler. The dispatch data encodes the command's flags, SVC number, and execution parameters.

4F41H - SVC A3H / DRIVE PREFIX INSERTION

Inserts a drive prefix (e.g., ":0/") into a filename string. Examines the filename pointed to by HL and DE, checks if it already has a drive specification (colon or slash), and if not, shifts the filename right to make room and inserts the default drive prefix. This is used to ensure all filenames have an explicit drive reference.

4F6FH - NEWDOS/80 COMMAND NAME TABLE

This table contains the command names for NEWDOS/80, each followed by a separate flag byte (with bit 7 set to mark the end of the name), and two parameter bytes used to compute the SVC (Supervisor Call) number in Register A and sub-function in Register C. The table is scanned sequentially during command parsing (starting at 4E8DH). No abbreviations are supported—full command names must match exactly. The table ends with a 00H byte.

Flag byte: Bit 7 always set. Bit 6 set indicates special handling (checked at 4EC7H, may lead to permission check or early return). Low 5 bits (&1FH) become part of parameters.

Parameter bytes: Byte2 and Byte3 are used to construct SVC details. SVC A = 43H if bit 7 of Byte3 set, else 4DH. Sub-function C = Byte3 & 0FH. Byte2 is OR'd with (Byte3 & F0H) for additional parameters pushed to stack.

50CEH - SUB-FUNCTION 2: DISPLAY COMMAND LIST

Displays the list of all available NEWDOS/80 commands by walking through the command name table at 4F6FH. Prints each command name (stripping the bit-7 marker from the last character), formatting them in columns with spaces between entries and carriage returns between rows.

50F2H - SUB-FUNCTION 4: NMI/BREAK HANDLER

Handles the NMI (Non-Maskable Interrupt) break condition. Saves the complete CPU state, checks the current DOS status, saves the stack pointer, enables cursor visibility, restores the original screen character, and enters the MINI-DOS command loop with the "MINI-NEWDOS/80 READY" prompt.

5111H - SUB-FUNCTION 6: NMI RETURN / RESUME INTERRUPTED PROGRAM

Returns from the MINI-DOS break mode to the interrupted program. Checks that a break was actually active, restores the stack pointer from the NMI save area, re-enables the cursor character, restores all saved registers, and returns to the interrupted code.

5142H - REGISTER SAVE SUBROUTINE

Saves the complete CPU register state (all main and alternate registers, plus IX and IY) onto the stack. Used by the MINI-DOS entry (4D5BH) and NMI break handler (50F2H) to preserve the caller's context. Uses a clever stack manipulation technique to save the return address while pushing all registers.

5154H - SVC 83H HANDLER: FILENAME PARSING

Handles SVC 83H — parses a filename from the command line. Calls the filename skip routine to advance past the filename, then checks the delimiter character to determine the filename type.

5164H - FILENAME PARSE TO DEFAULT BUFFER

Parses a filename from the command line into the default file parameter block at 4480H. Sets up DE to point to 4480H, then falls through to the general filename parser at 5167H.

5167H - FILENAME PARSER: PARSE NAME FROM COMMAND LINE

Parses a filename from the text pointed to by HL into the buffer pointed to by DE. Handles the asterisk (*) wildcard prefix and copies filename characters (letters, digits, and special characters) into the destination buffer. Returns with B=0 on success.

5171H - FILENAME COPY ENGINE

Core filename copying routine. First checks for the asterisk (*) wildcard prefix, then copies valid filename characters (digits and certain special characters) from the command line at (HL) to the destination buffer at (DE). Uses the character class checker at 51A0H to validate characters.

51A0H - CHARACTER CLASS CHECKER: ALPHANUMERIC TEST

Tests whether a character is alphabetic (A-Z or a-z). If the Carry flag is already set on entry (indicating the character was already classified as a digit), returns immediately. Otherwise, checks for uppercase (41H-5AH) and lowercase (61H-7AH) letters.

51ACH - DISPLAY PADDING SPACES

Outputs B space characters (20H) for column alignment in the command list display. Called by the command list routine at 50CEH to pad between command names.

51B4H - DISPLAY CARRIAGE RETURN

Outputs a carriage return (0DH) character. Used to end display lines in the command list and other output routines.

51B6H - DISPLAY CHARACTER SUBROUTINE

Displays the character in Register A on the screen by calling the ROM display routine at 0033H. Preserves DE and AF across the call.

51BEH - STRING DATA TABLES

Static string data used by the command interpreter and MINI-DOS prompt. These bytes are ASCII text, not executable code — the disassembler has interpreted them as Z80 instructions.