TRS-80 DOS - NEWDOS/80 v2.0 for the Model I - SYS9/SYS Disassembled

SYS8/SYS

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SYS9/SYS is a DOS overlay module that executes from the shared overlay area at 4D00H-51FFH. This module is the command executor for several important DOS commands:

• BASIC2 - Activate non-disk BASIC (Model I only)
• BOOT - Reset the computer
• CHAIN and DO - Shift keyboard input to come from a disk file
• CHNON - Alter chaining state (enable/disable chaining temporarily)
• MDCOPY - MINI-DOS single file copy command
• PAUSE - Wait for operator input
• STMT - Execute BASIC statements from DOS

The module also handles logical routine enqueuing/dequeuing and route invocations. This is a mandatory DOS module required for proper system operation.

Address	Bytes	Purpose
4280H	2	IY register initialization value (used by command dispatcher)
4289H	2	DOS system flags bytes (controls output modes, write protection, etc.)
42C5H	1	Saved character during chaining/input operations
428DH	1	System state flags (MINI-DOS active, special modes)
4058H	2	Pointer used by '+' command (stack manipulation)
44A0H	32	Filename parsing buffer / FCB template area
44A3H	2	Pointer to data area (initialized to 4FF7H)
44A5H	1	Character counter/position in filename buffer
44A9H	1	Calculated character count difference
3840H	1	Keyboard row 6 status (ENTER, CLEAR, BREAK, cursor keys, SPACE)
4978H	varies	Target area for EXX register swap operations
51E0H-51FFH	32	Temporary buffer for chaining file operations

4D00H - SYS9 COMMAND DISPATCHER ENTRY POINT

This is the main entry point for SYS9. The code initializes the IY register and then dispatches to various command handlers based on the command code passed in Register A. The dispatcher uses a series of comparisons and conditional jumps to route execution to the appropriate handler routine.

4D42H - ERROR HANDLER FOR UNRECOGNIZED COMMANDS

This section handles unrecognized command codes by setting up an error code and jumping to the error display routine. Two entry points exist: 4D42H for general errors (code 2AH) and 4D46H for specific error code 33H.

4D52H - SUBCOMMAND 8 HANDLER (PAUSE COMMAND)

This routine handles subcommand 8, which implements the PAUSE command. It displays a message prompting the user, then waits for the user to press ENTER before continuing.

4D63H - CHECK SYSTEM FLAGS AND DISPLAY "CHAINING"

This routine checks DOS system flags at 4289H to determine if output-related status messages should be displayed. Specifically, it checks if bit 6 of byte 4289H is set (output in progress) and bit 5 of byte 428AH is clear (prompt display needed), then calls the message display routine.

4D72H - SUBCOMMAND 5 HANDLER

This routine handles subcommand 5 of the EBH command group. It checks a flag at 44A0H, performs conditional operations based on bit 7, and then sets up either error code 26H or 34H before jumping to the error display routine.

After calling 4D82H to check flags, this tests the result and either loads error code 34H or jumps back to display the error.

4D82H - CHECK FILENAME BUFFER FOR Y/N/D FLAGS

This routine examines the filename buffer at HL, looking for specific control characters (N, Y, or D) that determine operational modes. It modifies DOS system flags at 4289H-428AH based on the character found, and returns with Z FLAG set if a valid character was found.

4DACH - SUBCOMMAND 11 HANDLER ("RUN ONLY" STOPPED MESSAGE)

This routine handles subcommand 11 (0BH) by displaying a message indicating that a "RUN ONLY" program has been stopped, then waits for the user to press R to continue.

4DB1H - SUBCOMMAND 12 HANDLER ("RUN ONLY" MESSAGE)

This routine handles subcommand 12 (0CH) by displaying a different "RUN ONLY" related message.

4DC7H - REGISTER SWAP AND SETUP ROUTINE

This routine performs register exchanges and setup operations. It appears to prepare for a special operation by swapping the alternate register set, setting up stack values, and then deciding whether to continue or jump to an error handler based on the value in Register A.

4DD9H - SUBCOMMAND 1 HANDLER (STACK MANIPULATION)

This routine handles subcommand 1 of the EBH command group. It performs a simple stack manipulation by exchanging and popping values.

4DDCH - SUBCOMMAND 3 HANDLER (BUFFER COPY OPERATION)

This routine handles subcommand 3 of the EBH command group. It exchanges DE and HL registers, then copies 32 bytes (20H) from the area pointed to by HL into the filename parsing buffer at 44A0H, sets up a pointer at 44A3H, and performs file parsing/loading operations.

4DF5H - SET SYSTEM FLAGS AND RETURN

This section completes the processing from 4DDCH by setting specific bits in the DOS system flags at 4289H-428AH to indicate file operations are active, then clears a state flag and returns.

4E06H - FILE VERIFICATION LOOP

This routine performs file verification by reading characters from the filename buffer and comparing them against characters from the current file position. It loops through the filename, checking for match conditions and special markers (80H indicates start of data, 0DH indicates end).

4E22H - CHAIN/DO COMMAND HANDLER ENTRY POINT

This is the main entry point for the CHAIN and DO commands (command code CBH). These commands redirect keyboard input to come from a disk file instead of the physical keyboard. The handler sets up chaining mode, opens the specified file, and manages the state flags that control how input is redirected.

4E57H - CHECK KEYBOARD STATE AND BEGIN CHAINING LOOP

This section checks keyboard and system state, then enters the main chaining loop which processes input from the chain file instead of the keyboard.

4E7EH - PROCESS FILENAME FLAGS AND HANDLE ERRORS

This routine checks filename buffer flags using the Y/N/D checker at 4D82H, then handles various error conditions and continues chaining operations. It processes saved characters and manages the flow based on system flag states.

4E90H - PROCESS CHARACTER FROM CHAIN FILE

This section processes a character read from the chain file. It saves the character in multiple registers, clears the saved character flag, and then performs various checks to determine how to handle the character (whether it's a special code requiring file processing, a regular character, or an error condition).

4EBFH - DISPLAY "ABORTED" MESSAGE

This short routine displays the "ABORTED" message and then falls through to the chaining cleanup routine.

4EC5H - CLEAR CHAINING FLAGS AND RESET STATE

This routine clears all chaining-related flags and resets the system to normal keyboard input mode. It clears the chaining active flag, clears buffer flags, and resets write protection flags.

4ED3H - CHECK MINI-DOS AND KEYBOARD CONDITIONS

This routine checks if the system is in a special state where MINI-DOS is active with specific keyboard conditions. If both conditions are met, it clears Register A; otherwise it returns immediately.

4EE0H - ERROR HANDLER WITH STACK MANIPULATION

This routine handles error conditions by displaying an "ABORTED" message, saving the error code on the stack twice, calling the cleanup routine, and then deciding whether to jump to DOS READY (4030H) or the error display handler (4409H) based on whether Register A is zero.

4EEEH - HANDLE CHARACTERS 80H-83H (OUTPUT TO DISPLAY)

This routine handles special characters in the 80H-83H range read from the chain file. It processes these as commands to output characters to the display, with bit 0 of the character code controlling whether the character should be echoed to the display via ROM routine 0033H.

4F03H - PAUSE HANDLER DURING CHAINING

This routine handles the pause condition when BREAK is pressed during chaining (or when command 81H is processed). It displays a "PAUSE." message and waits for the user to press a key before continuing.

4F16H - HANDLE 85H SPECIAL CODE

This routine handles the special 85H code from the chain file. It saves the code in Register C and sets up the original command character for special processing.

4F1BH - HANDLE 84H-85H COMMANDS (READ LINE FROM FILE)

This routine handles commands 84H and 85H which read a line of text from the chain file into a buffer. It reads up to 32 (20H) characters or until a carriage return is encountered, storing them in a buffer starting at 4FD7H.

4F45H - DISPLAY "CHAINING " PREFIX THEN MESSAGE

This routine displays the "CHAINING " prefix message followed by whatever message is pointed to by HL. It's used to provide context for various chaining-related status messages.

4F50H - READ CHARACTER FROM CHAIN FILE BUFFER

This is the main character input routine for chaining operations. It reads characters from the filename buffer at 44A0H, handling special delimiter sequences ("/.." pattern) and performing automatic buffer refilling when needed. Returns characters in Register A with Z FLAG set for special conditions.

4F8CH - FETCH NEXT CHARACTER FROM FILENAME BUFFER

This routine fetches the next character from the filename buffer at 44A0H. If it reaches end of buffer (returns 00H), it attempts to refill the buffer by calling ROM input routine 0013H. Returns character in Register A with Z FLAG set if end of buffer.

4F98H - MESSAGE DATA AREA

This section contains all the text messages used by SYS9, stored as null-terminated ASCII strings. Each message ends with either a carriage return (0DH) or a null byte (03H) as a terminator.

4FD7H - BOOT COMMAND HANDLER AND LINE BUFFER

This section serves dual purposes: it's the entry point for the BOOT/reset command (subcommand 2 of EBH group), and it also contains a 32-byte buffer used for reading lines from chain files. The BOOT command initializes registers and performs a system reset sequence.

5048H - '+' COMMAND HANDLER (STACK PUSH)

This routine handles the '+' command (2BH) which manipulates the stack by pushing values. It stores the current return address and program counter onto a special stack area pointed to by 4058H.

505FH - 'K' COMMAND HANDLER (STACK POP/KILL)

This routine handles the 'K' command (4BH) which removes (kills) entries from the '+' command stack. It validates the stack pointer, reads values from the stack, and updates the stack pointer.

5070H - SUBCOMMAND 7 HANDLER (FILENAME PROCESSING)

This routine handles subcommand 7 of the EBH command group. It processes filename specifications by reading up to 9 characters from the buffer into the temporary area at 51E0H, padding with spaces if necessary, and then calling directory lookup routines.

50A3H - VALIDATE STACK ADDRESS RANGE

This routine validates that the address in HL is within the valid range for stack operations (must be >= 5200H). It performs the check and adjusts HL by adding 4, returning with Z FLAG set if the address is valid, or jumping to an error handler if out of range.

50ADH - DIRECTORY LOOKUP AND COMPARISON

This routine performs directory lookups by scanning through a linked list structure stored at 4058H. It reads entries, compares filenames, and returns information about matching entries. The routine handles the special stack structure used by the '+' and 'K' commands.

50E1H - FINAL MESSAGE DATA AND NOP PADDING

This section contains the remaining message strings used by SYS9, followed by NOP padding to fill out the module to its end address. The messages are for the "RUN ONLY" stopped condition and the fatal error display.

511BH - CHECK FLAGS AND CONDITIONAL RETURN

This routine checks the value in Register A and system flags to determine the appropriate return path. If A is non-zero, it jumps to 4E90H. If certain flag conditions are met, it performs a conditional DOS Ready return.

5128H - NOP PADDING TO MODULE END

This section contains NOP (No OPeration) instructions that pad the module from 5128H to 51DFH, filling the unused space in the SYS9 overlay area. This ensures the module occupies its full allocated memory region from 4D00H to 51FFH.

51E0H - TEMPORARY BUFFER AREA (NOT SHOWN IN DISASSEMBLY)

The disassembly ends at 51DFH. The memory region from 51E0H to 51FFH (32 bytes) is used as a temporary buffer by various routines in SYS9, particularly for file operations and the BOOT command handler. This buffer is not shown in the disassembly because it contains data, not code.