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

SYS9/SYS

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SYS9/SYS is one of the NEWDOS/80 v2.0 DOS overlay modules, loaded into the overlay area at 4D00H–51FFH when any of its commands are invoked. SYS9 handles a diverse group of DOS library commands: COPY, DUAL, FORMS, KILL, MASTER, PATCH, RELO, ROUTE, SETCOM, and WP (Write Protect). It also contains handlers for the DOS FATAL ERROR and RUN ONLY STOPPED system messages.

When SYS1/SYS (the command interpreter) identifies one of these commands, it loads SYS9 into the overlay area and transfers control to the entry point at 4D00H. Register A contains a function code identifying which command group to execute, and Register C contains a sub-function number for the EBH dispatch group. Register HL points into the DOS command buffer at the current parse position.

The overlay is organized into several functional areas: the main command dispatch table (4D00H–4D41H), error exit stubs (4D42H–4D4FH), individual command handler routines, shared utility subroutines for option parsing and chaining control, text message strings, and the COPY command's file transfer engine. The ROUTE command at 4E22H contains self-modifying code that adjusts its initial setup value on first execution.

The following RAM locations, system variables, and self-modifying code addresses are referenced throughout SYS9/SYS.

Variables and Storage:

Address	Description
4058H	Timer interrupt chain head pointer. Points to the first entry in the linked list of user 25ms timer interrupt routines enqueued via 4410H (Model III: 447BH).
4280H	Start of PDRIVE storage area. IY is pointed here (4280H) at overlay entry to provide indexed access to PDRIVE parameters and system configuration.
4288H	System flags byte (within PDRIVE/system storage). Used to track the current file number or operation index during file I/O sequences.
4289H	System option flags word (2 bytes at 4289H–428AH). Byte at 4289H (Register C in the code) holds option bits including: Bit 5 = chaining Y/N flag. Byte at 428AH (Register B in the code) holds option bits including: Bit 6 = D-option valid flag, Bit 4 = D-option selected flag.
428DH	SYSTEM option flags. Bit 7 = flag checked by the ROUTE command to determine stack scanning behavior. Bit 6 = BREAK key enable flag checked during chaining pause operations.
42C5H	Chaining saved-character byte. When the ROUTE/CHAIN system is reading characters from a chain file, this byte holds the last character read or a state indicator (00H = no saved character, non-zero = saved character or state code).
44A0H	Start of a 32-byte (20H bytes) work area used for filespec storage and parameter passing. Byte at 44A0H bit 7 is tested by the MASTER command to check file status. The area from 44A0H–44BFH is loaded from the command buffer during RELO processing.
44A1H	Second byte of the work area. Bit 5 is set by the ROUTE command during initialization to indicate the route is active.
44A3H	2-byte pointer within the work area. The RELO command stores a pointer to the default filename extension string (4FF7H) here.
44A5H	Current byte position counter within the chain file read routine. Tracks how many bytes have been consumed from the current record.
44A9H	Byte difference value used in chain file positioning. Stores the difference between the starting and current position for record repositioning.
4480H	FCB (File Control Block) base area used for file I/O operations. Passed as DE parameter to DOS file position routines.
4483H	2-byte pointer storing the address of a routine to call after file operations (set by the COPY file transfer engine at 4FDDH).
4488H	File record length or mode byte. Tested by the COPY routine to determine whether to decrement the record count.
448CH	2-byte value holding the current file record count or size. Read by the COPY routine to determine the number of records to transfer.
3840H	Keyboard memory-mapped row 6: ENTER, CLEAR, BREAK, UP, DOWN, LEFT, RIGHT, SPACE. Bit 0 = @ key (address row); Bit 3 = UP ARROW key; Bit 6 = RIGHT ARROW key. Various bits tested for BREAK key checking and keyboard pause detection.
51E0H	Start of a temporary buffer area within the overlay's unused padding space. Used by the COPY command and the FORMS/PATCH line-input routine to store filespec data and input text. Extends through 51EFH (16 bytes for filespec) or further as needed.
51EAH	2-byte value within the 51E0H buffer area. The COPY routine stores the source file's record count here.
51ECH	2-byte value within the 51E0H buffer area. The COPY routine stores the destination file's ending record count here.
51E8H	Byte within the 51E0H buffer area. The COPY routine stores the file record length/mode byte here.

Self-Modifying Code:

Address	Description
4E22H	The ROUTE command entry point. Initially contains LD A,0AH (3E 0AH). On first execution, the instruction at 4E26H overwrites the immediate operand at 4E22H+1 with 18H, changing the instruction to LD A,18H for all subsequent calls. This implements a one-time initialization: the first ROUTE invocation uses function code 0AH, and all subsequent invocations use 18H.

The following is a summary of the major routines within SYS9/SYS and their entry points.

Command Dispatch and Error Handling

Address	Description
4D00H	Main entry point and command dispatcher. Dispatches to individual command handlers based on function code in Register A and sub-function in Register C.
4D42H	Error exit: loads error code 2AH (general error) and falls through to the common error exit at 4D4EH.
4D46H	Error exit with file close: loads error code 33H, pushes it, calls RST 28H with function 46H to close the current file, then loads error code 2FH and exits.
4D4EH	Common error exit: OR A to clear the Z flag and set NZ, then jumps to SYS0 error handler at 4409H.

Shared Utility Routines

Address	Description
4D63H	Display pending chaining message. Checks system option flags at 4289H: if bit 6 of B (428AH) is set and bit 5 of C (4289H) is clear, calls 4467H to display a message. Returns with A=0 and Z flag set.
4D82H	Parse Y/N/D option from command line. Reads a character from (HL), converts to uppercase via 4548H, and sets bits in Register B and C based on whether the character is Y, N, or D. Stores updated flags back to 4289H on success.
4DC7H	DOS-CALL / SVC invocation setup. Saves alternate register set, sets BC'=EB01H and DE'=4978H (SVC parameters), then tests Register A and dispatches accordingly.
4EC5H	Reset chaining state. Clears byte at 44A0H to 00H, resets bit 5 of (4289H) and bit 4 of (428AH), clearing all chaining-active flags.
4ED3H	Check for SHIFT+DOWN ARROW key combination. Reads keyboard row at 3840H, tests bit 3 (UP ARROW), then checks system option flag bit 6 at 428DH. Returns Z if the key combination is not pressed or not enabled.
4F45H	Display two messages: first the "CHAINING " prefix from 4F98H, then the message pointed to by HL on entry. Used by the ABORTED and PAUSE message display routines.
4F50H	Read next character from chain file. Calls the low-level chain read at 4F8CH, then checks for slash-delimited embedded commands (section markers, drive specs). Returns the character in A with Z flag set on success, or NZ on error.
4F8CH	Low-level chain file character read. Calls DOS routine 0013H (read next byte from file) with DE pointing to the work area at 44A0H. Skips null bytes (00H). Returns character in A with Z set, or NZ on error.

Command Handlers

Address	Description
4D72H	MASTER command handler. Checks file status bit, then calls the Y/N/D parser. Exits with error if the parse fails.
4DACH	DOS FATAL ERROR handler. Points HL to the "DOS FATAL ERROR" message at 50F4H and falls into the reset prompt routine.
4DB1H	RUN ONLY STOPPED handler. Points HL to the "'RUN ONLY' STOPPED" message at 50E1H and falls into the reset prompt routine.
4DB4H	Reset prompt routine. Displays the error message, then displays "!! KEY 'R' FOR RESET." and waits for the R key, then executes RST 00H (system reset).
4DC6H	System reset (RST 00H). Immediate hardware reset.
4DD9H	COPY command entry (sub-function 1). Pops return address manipulation for stack-based dispatch.
4DDCH	RELO command entry (sub-function 3). Copies 32 bytes from command buffer to the 44A0H work area, sets up the default extension pointer, then processes the file operation.
4E06H	Filename search/match routine. Scans through a data stream reading bytes via 4F50H looking for an 80H marker byte followed by character matches against the string at (HL).
4E22H	ROUTE command entry. [SELF-MODIFYING CODE] First call uses function 0AH; subsequent calls use 18H.
4E57H	ROUTE command main processing. Checks BREAK key, calls chain character reader, and processes the chain file command stream.
4EBFH	Display "CHAINING ABORTED" message and reset chaining state.
4EE2H	Chain error handler. Calls the ABORTED display, then exits to either the no-error exit (4030H) or the error exit (4409H) depending on the error state.
4F03H	Display "CHAINING PAUSE" message and wait for a key release.
4F1BH	Line input from chain file. Reads up to 32 characters from the chain file into a buffer at 4FD7H, terminated by 0DH (ENTER).
4FD7H	COPY command file transfer engine. Sets up FCB parameters, opens files, and transfers records between source and destination.
5048H	KILL command handler. Validates address range, then manipulates the timer interrupt chain linked list to remove an entry.
505FH	WP (Write Protect) command handler (actually the enqueue side - adds to timer chain).
5070H	FORMS command handler. Parses up to 9 characters of filename from the command buffer into the 51E0H buffer area.
50A3H	Address range validation. Checks that Register H >= 52H (address must be above the overlay area at 5200H). Returns NZ with HL advanced by 4 if valid.
50ADH	Timer interrupt chain search. Walks the linked list at 4058H looking for a matching entry. Returns Z if found, NZ if not found.
511BH	ROUTE final check. Tests Register A for zero, then examines bits 4 and 6 of Register B to determine exit path.

Text Message Strings

Address	Message
4F98H	"CHAINING " (terminated by 03H at 4FA1H, but note the space at 4FA0H is followed by 03H)
4FA2H	"ABORTED" + 0DH
4FAAH	"PAUSE. PRESS 'ENTER' WHEN READY TO CONTINUE" + 0DH
50E1H	"'RUN ONLY' STOPPED" + 03H
50F4H	"DOS FATAL ERROR" + 03H
5104H	"!! KEY 'R' FOR RESET." + 0DH

4D00H – SYS9 Main Entry Point and Command Dispatcher

This is the entry point for SYS9/SYS. SYS0 loads this overlay and jumps here with Register A containing a function code that identifies which command group was requested. Register IY is set to 4280H (PDRIVE storage base) for indexed access to system parameters. Register HL points into the DOS command buffer at the current parse position, and Register C contains a sub-function number used by the EBH dispatch group.

4D42H – Error Exit Stubs

These are short error exit routines used by the dispatcher and individual command handlers. They load an appropriate NEWDOS/80 error code into Register A and jump to the SYS0 error handler at 4409H.

4D52H – SETCOM Command Handler

The SETCOM command (Model III only) sets RS-232 interface parameters. This handler calls the DUAL display routine first, then displays the "PAUSE" prompt message and waits for the user to press ENTER before returning.

4D63H – Display Pending Chaining Message / DUAL Command Handler

This routine checks the system option flags at 4289H–428AH. If bit 6 of the byte at 428AH (the "D-option valid" flag in Register B) is set AND bit 5 of the byte at 4289H (the "chaining Y flag" in Register C) is clear, it calls the message display routine at 4467H to show a pending notification. This routine also serves as the DUAL command handler (sub-function 9). It always returns with Register A = 0 and the Z flag set.

4D72H – MASTER Command Handler

The MASTER command (sub-function 5) manages the master diskette designation. It first checks bit 7 of the byte at 44A0H (the file status/work area byte) to determine the current state. If bit 7 is clear (Carry not set after RLCA), an error is returned. If bit 7 is set, the Y/N/D option parser at 4D82H is called to read the user's option character from the command buffer. On success (Z flag), the routine returns to the caller. On failure (NZ flag), error code 34H (parse error) is returned.

4D82H – Parse Y/N/D Option from Command Line

This utility routine parses a single-character option (Y, N, or D) from the command line buffer pointed to by Register HL. It loads the current system option flags from 4289H–428AH, clears the relevant flag bits, reads and converts the character to uppercase, sets the appropriate bits based on the character found, verifies the command line is terminated by 0DH, and stores the updated flags back to 4289H. Returns Z and A=0 on success, or NZ on failure (invalid character or unterminated command).

4DACH – DOS FATAL ERROR / 'RUN ONLY' STOPPED / Reset Prompt Handlers

These routines handle unrecoverable system conditions. The DOS FATAL ERROR handler (sub-function 11) and the 'RUN ONLY' STOPPED handler (sub-function 12) each load a pointer to their respective error message string and then fall into the shared reset prompt routine at 4DB4H. The reset prompt displays the error message, displays "!! KEY 'R' FOR RESET.", then waits in a loop for the user to press R. Once R is detected, RST 00H performs a hardware system reset.

4DC6H – System Reset (RST 00H)

This entry point executes RST 00H, which performs a hardware system reset on the TRS-80 Model III. The CPU jumps to address 0000H in ROM, which reinitializes the system and boots from disk. This is reached after the user presses R in response to a DOS FATAL ERROR or 'RUN ONLY' STOPPED condition. It is also the target of sub-function 10 from the dispatch table.

4DC7H – DOS-CALL / SVC Invocation Setup

This routine sets up the alternate register set with SVC (Supervisor Call) parameters and then dispatches based on the value of Register A. It is called by the COPY file transfer engine and other routines that need to invoke DOS services. The alternate registers BC' and DE' are loaded with EB01H and 4978H respectively — these are the SVC function code (EB01H = overlay load with sub-function 01) and the SVC parameter pointer (4978H). If Register A is non-zero, execution jumps to 4982H in SYS0. If Register A is zero, the alternate HL' is pushed and execution returns through the stack.

4DD9H – COPY Command Entry (Stack Manipulation)

The COPY command entry point (sub-function 1) performs a stack manipulation to redirect the return path. It exchanges the top of the stack with HL, then pops the result into HL. The effect is to replace the current return address with the value in HL (the command buffer pointer), and load the old return address into HL. This sets up HL with the return address from the dispatcher while putting the command buffer pointer where a later RET will find it.

4DDCH – RELO Command Handler

The RELO command (sub-function 3) relocates or transfers file data. It copies 32 bytes (20H) from the current command buffer position into the 44A0H work area, stores a pointer to the default filename extension string at 44A3H, restores saved registers from the stack, calls 4C7AH in SYS0 for the main relocation processing, and then conditionally calls the filename search routine at 4E06H. On success, it sets the chaining Y flag (bit 5 of 4289H), conditionally sets the D-option flag, clears the saved character byte at 42C5H, and returns.

4E06H – Filename Search/Match in Chain Data Stream

This routine searches through the chain file data stream for a filename that matches the string pointed to by Register HL. It reads bytes from the chain file via 4F50H, looking for an 80H marker byte that signals the start of a filename entry. When an 80H byte is found, the routine reads subsequent characters, converts them to uppercase via 4548H, and compares them against the corresponding characters in the match string at (HL). If all characters match and the final character is 0DH (indicating the end of the filename), the routine returns Z. If a mismatch is found, it resets HL to the start of the match string and continues scanning. Returns NZ on error or if the data stream ends without finding a match.

4E22H – ROUTE Command Entry [SELF-MODIFYING CODE]

The ROUTE command manages I/O device routing in NEWDOS/80. This entry point contains self-modifying code: the instruction at 4E22H is initially LD A,0AH, but on first execution, the code at 4E26H overwrites the immediate operand byte at address 4E23H (the byte after the 3EH opcode) with the value 18H. This changes the instruction permanently to LD A,18H for all subsequent calls. The first invocation uses function code 0AH (initial route setup), and all later invocations use 18H (route continuation). After the self-modification, the routine sets the chaining active flag in 44A1H, then checks the system option flag at 428DH bit 7 to decide whether to perform a stack scan or jump directly to the main ROUTE processing loop.

4E35H – ROUTE Stack Scan for Return Address Validation

When bit 7 of 428DH is 0, the ROUTE command scans the stack looking for a specific return address pattern that indicates the call came from a known DOS-CALL context. The scan examines up to 10 word-sized entries on the stack, looking for the byte pair 94H followed by 06H (which represents a return address within SYS0). If found, it then checks bytes at offset +0CH from that position for the pair E3H/05H. If the full pattern is found, execution continues to the main ROUTE processing at 4E57H. If the pattern is not found after scanning 10 entries, the routine exits with OR 01H (setting NZ and bit 0 of A) at 4E7BH.

4E57H – ROUTE Command Main Processing Loop

This is the main processing loop for the ROUTE command's chain file execution. It first checks for a BREAK key condition, then reads characters from the chain file and dispatches them based on their value. Characters with code 80H trigger a chain reset. Characters 81H–83H are special chain commands (81H–83H after subtracting 80H give 01H–03H). Characters 84H and 85H trigger additional processing. Regular printable characters in the range 01H–7FH are echoed to the display if bit 0 of Register B is set. The loop processes the entire chain file until an error, end-of-file, or explicit stop condition is encountered.

4E7EH – Chain Command Y/N/D Option Processing

This routine is called when the chain file contains a command that requires a Y/N/D option (such as CHNON,Y or CHNON,N). It calls the Y/N/D parser at 4D82H to read the option from the chain data, then calls 511BH for final ROUTE status checking. Based on the result, it either returns with failure or loops back to the main ROUTE processing at 4E57H.

4E90H – Chain Character Dispatcher

This routine receives a character from the chain file (either freshly read or retrieved from the saved-character byte at 42C5H) and dispatches it based on its value. The character is saved in both Registers B and C. After clearing the saved-character byte, it subtracts 80H to classify: 80H (result 0) triggers a chain reset; 81H–83H (result 01H–03H) triggers character echo/pass-through; 84H–85H (result 04H–05H) triggers special command processing; 86H+ triggers line input from the chain file.

4EBFH – Display "CHAINING ABORTED" Message

This routine displays the "CHAINING ABORTED" message by first calling 4F45H (which displays "CHAINING " as a prefix, then displays the message pointed to by HL). It then falls through to 4EC5H to reset the chaining state flags.

4EC5H – Reset Chaining State Flags

This routine clears all chaining-related state flags: it zeroes the byte at 44A0H (the work area status byte), clears bit 5 of (4289H) (the chaining Y flag), and clears bit 4 of (428AH) (the D-option selected flag). This fully deactivates the chaining system.

4ED3H – Check for SHIFT+DOWN ARROW Key Combination

This utility routine checks whether the SHIFT+DOWN ARROW key combination is currently pressed and whether the system option at 428DH bit 6 enables this check. It reads keyboard row 3840H and tests bit 3 (the UP ARROW key position in the keyboard matrix). If the key is pressed (bit is 0, active low) AND the system option bit 6 at 428DH is set, the routine returns NZ with A=0. Otherwise it returns Z. This key combination is used as a user-initiated pause or intervention during chaining.

4EE0H – Chain Error Handler

This is the common error handler for chaining operations. It saves the error code in Register A, calls 4EBFH to display "CHAINING ABORTED" and reset the chaining state, restores the error code, and then dispatches to the appropriate DOS exit: if A is zero, it jumps to the no-error exit at 4030H (error already displayed); if A is non-zero, it jumps to the error exit at 4409H to display the error message. Note that the entry at 4EDFH (XOR A; fall through) and 4EE0H (PUSH AF) form two different entry paths.

4EEEH – Chain Character Echo / Pass-Through Loop

This routine handles characters 81H–83H from the chain dispatch. These characters represent printable text that should be read from the chain file and optionally echoed to the display. The loop reads characters one at a time from the chain file via 4F50H, conditionally displays each character via the ROM display routine at 0033H (if bit 0 of Register B is set), and continues until a 0DH (carriage return) terminator is found. After the line is complete, it checks if the original chain command was 81H and if so, enters the PAUSE handler. Otherwise, it returns to the main ROUTE processing loop at 4E57H.

4F03H – Chaining PAUSE Handler

This routine displays the "CHAINING PAUSE" message (the prefix "CHAINING " followed by "PAUSE. PRESS 'ENTER' WHEN READY TO CONTINUE"), then enters a loop waiting for the user to release a specific key. The loop calls the SHIFT+DOWN ARROW check at 4ED3H and reads keyboard row 3840H testing bit 0 (the @/ENTER key area). When the key condition is met (bit 0 of 3840H becomes non-zero, meaning the key is released), the routine exits to the main ROUTE processing loop at 4E57H.

4F16H – CHNON Continuation Processing (Chain Command 85H)

This routine handles chain command code 85H, which signals a CHNON (chain on/off) continuation. It saves the current command code into Register C, saves Register B (the original character) to the chaining saved-character byte at 42C5H, and then falls through to the line input routine at 4F1BH. This preserves the chain state so that after the line input is processed, chaining can continue from where it left off.

4F1BH – Line Input from Chain File into Buffer

This routine reads a line of text from the chain file into a buffer at 4FD7H. It reads up to 32 characters (20H), storing each one and checking for the 0DH (carriage return) terminator. If the line is successfully read (terminated by 0DH within 32 characters), the routine dispatches based on bit 0 of Register C: if bit 0 is set (from CHNON processing at 4F16H), it jumps to the Y/N/D option handler at 4E7EH; if bit 0 is clear, it processes the line as a DOS command by positioning the file and searching for a filename match. If the chain file read fails or the line exceeds 32 characters, the chain error handler at 4EE0H is invoked.

4F45H – Display Two-Part Chaining Message

This utility routine displays a two-part message: first the "CHAINING " prefix string from 4F98H, then the message pointed to by HL on entry. It preserves HL across the first display call and then displays the second part. Used by the "CHAINING ABORTED" and "CHAINING PAUSE..." message routines.

4F50H – Read Next Character from Chain File (High-Level)

This is the high-level chain file character reader. It calls the low-level reader at 4F8CH to get the next byte, then checks if the character is a slash (2FH) which introduces an embedded command sequence. Embedded commands have the format /x.y/n where x is checked against the current position byte at 44A5H, y is a separator (2EH = period), and n is a drive/section number (ASCII digit). If the slash sequence is recognized as an embedded command, the routine processes the position change and returns the adjusted character. If the slash is not followed by a valid command sequence, the routine falls back to normal character return. Register BC is preserved across the call.

4F8CH – Low-Level Chain File Character Reader

This is the low-level routine that reads the next byte from the chain file. It calls the DOS routine at 0013H (read next byte from file) with DE pointing to the work area at 44A0H. If the byte is 00H (null), it skips it and reads again, looping until a non-null byte is found. Returns the character in Register A with Z set on success, or NZ with an error code on failure.

4F98H – Text Message Strings

These are the text message strings used by the SYS9 overlay module. Each string is terminated by either 03H (ETX, used by the SYS0 display routine at 4467H as an end-of-message marker) or 0DH (carriage return, which also terminates the display and moves the cursor to the next line).

4FD7H – WP / COPY File Transfer Engine Entry

This routine serves as the entry point for the COPY file transfer engine (also used by the WP command's sub-function 2). It sets up the source file by loading its filespec pointer (51E0H), stores the RELO handler address (4DDCH) as a callback pointer at 4483H, invokes the DOS-CALL mechanism via 4DC7H with function code 44H, initializes the file tracking byte at 4288H, reads the source file's record count from 448CH and record length/mode from 4488H, then validates the source file. If the source file is empty (record count = FFFFH after adjustment), it skips directly to copying the destination file information. Otherwise, it saves the source record count and sets up for the record-by-record copy loop.

500EH – COPY Main Record Transfer Loop

This is the main copy loop that transfers records from the source file to the destination file. For each iteration, it positions the destination FCB to the current record (via 4436H), reads a record from the source file buffer (via 4439H with the source FCB at 51E0H), and repeats until all records are transferred or an end-of-file condition is reached. When done, it checks for expected end-of-file errors (1CH or 1DH) and proceeds to finalize the destination file.

5026H – COPY Destination File Finalization

This routine saves the destination file's parameters (record count and record length/mode) from the DOS file control area into the temporary buffer at 51E0H, then positions the destination file's FCB to the start and begins writing records from the start. When all records have been written (detected by a 1CH or 1DH positioning error), it calls the SYS0 file close routine at 4428H.

5048H – KILL Command Handler (Timer Chain Insertion)

The KILL command (function code 2BH) enqueues a timer entry into the system timer interrupt chain at 4058H. It first calls the timer chain search routine at 50A3H to validate Register HL and search for any existing entry matching the caller's filespec. If the search succeeds (Z flag), the entry already exists and the routine returns error 18H. If the search returns NZ (no existing entry found), the routine inserts a new entry by linking it into the chain: the current chain head pointer at 4058H is stored into the new entry's link field, and the new entry's address is written to 4058H, making it the new head of the timer chain.

505FH – WP Command Handler (Timer Chain Removal)

The WP command (function code 4BH) removes a timer entry from the system timer interrupt chain at 4058H. It calls the timer chain search routine at 50A3H to find a matching entry. If a match is found (Z flag set), the entry is already absent or the search indicates an error, so error 18H is returned. If NZ is returned (the entry was found and its position identified), the routine unlinks the entry by copying its forward link pointer back to the predecessor's link field, effectively removing the entry from the chain.

5070H – FORMS Command Handler

The FORMS command (sub-function 7) manages printer form specifications. It reads up to 9 filespec/parameter entries from the command line. For each entry, it advances HL past one character, calls the SYS0 filespec parser at 4C7AH, and stores the parsed parameter bytes into a buffer at 51E0H. If the parser returns Carry (successful parse with data), the current character at (HL) is copied to the buffer and the loop continues. When the parser returns No Carry (end of parameters or error), the loop exits. If no parameters were parsed (B still equals 9), an error is returned via 4D4CH. Otherwise, remaining buffer positions are filled with 20H (spaces), and the routine calls the timer chain search at 50ADH to locate and update the FORMS entry.

50A3H – Timer Chain Search with HL Validation

This routine validates Register H (must be >= 52H, ensuring the pointer is in a valid memory range above the overlay) and then advances HL by 4 bytes before falling through to the timer chain search at 50ADH. If Register H < 52H, the routine exits with an error via 4D4CH (error code 33H).

50ADH – Timer Chain Search from Chain Head

This routine searches the timer interrupt chain starting from the head pointer at 4058H, looking for an entry whose 8-byte identifier matches the 8 bytes pointed to by HL on entry. The chain is a singly-linked list: each entry starts with a 2-byte forward link pointer (pointing to the next entry), followed by the entry data. The routine walks the chain, comparing 8 bytes at offset +2 of each entry against the 8 bytes at (HL). Returns NZ with A = OR 35H (non-zero) if a matching entry is found, or Z (from the OR A after LD A,D / OR E returning 0) if the end of the chain is reached (link = 0000H) without finding a match. On error (chain corruption detected by SBC HL,DE ≠ 0), jumps to the file-close error exit at 4D46H.

50E1H – Text Message Strings (Error Messages)

These are additional text message strings used by the SYS9 fatal error and reset handlers.

511BH – ROUTE Final Check Routine

This short utility routine is called from the ROUTE command's Y/N/D option processing at 4E87H. It tests whether Register A is zero and whether specific bits of Register B indicate a termination condition. If A is non-zero, execution jumps to the chain character dispatcher at 4E90H for further processing. If A is zero, the routine checks bits 4 and 6 of B: if bit 6 is set and bit 4 is clear (AND 50H = 40H), it signals a clean ROUTE termination by jumping to the no-error exit at 402DH.

5128H–51FFH – NOP Padding and Temporary Buffer Area

The region from 5128H to 51DFH consists of 184 bytes of 00H (NOP) padding. This unused space fills the remainder of the overlay's code area up to the end of the 512-byte overlay boundary. The region from 51E0H to 51FFH (32 bytes) is used as a temporary buffer area by the COPY, WP, and FORMS commands for storing filespecs, parameter data, and intermediate values during file transfer operations. Key locations within this buffer include: 51E0H (filespec/FCB storage), 51E8H (file record length/mode), 51EAH (source file record count), and 51ECH (destination file ending record count).