TRS-80 DOS - NEWDOS/80 v2.0 for the Model III - SYS3/SYS Disassembled

SYS3/SYS

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4D00H - SYS3 Main Function Dispatcher

SYS3 is the third overlay module of NEWDOS/80 v2.0 for the TRS-80 Model III. It is loaded into RAM at 4D00H when any of its functions are invoked via RST 28H. Register A contains the function code on entry. SYS3 handles: FREE (25H), PURGE (45H), SETHOOK (65H), RELHOOK (85H), DEBUG/DUMP (A5H), KILL (C5H), and system configuration sub-functions (E5H with sub-function number in Register C).

4D43H - Invalid Function Error Return

Returns error code 2AH (general error) to the caller when an invalid or unrecognized function code is provided to SYS3. Sets the NZ FLAG to signal error.

4D47H - SETHOOK: Insert Hook Into System Hook Chain (Function 65H)

Inserts a new hook entry into the system hook chain. The hook chain is a singly-linked list anchored at 4584H (a 2-byte pointer to the head of the chain). On entry, Register Pair DE contains the address of the new hook entry to insert. The hook entry structure is: bytes 0-1 hold the "next" link pointer, which will be filled in by this routine. The new hook is inserted at the head of the chain. Interrupts are disabled during the modification to prevent corruption of the linked list.

4D55H - RELHOOK: Remove Hook From System Hook Chain (Function 85H)

Removes a specific hook entry from the system hook chain. On entry, Register Pair DE contains the address of the hook entry to remove. The routine walks the linked list starting from the anchor at 4584H, comparing each entry's address against DE. When found, it unlinks the entry by updating the previous entry's "next" pointer to skip over the removed entry. Interrupts are disabled during list traversal and modification.

4D70H - Display Configuration Handler (E5H Sub-function 2)

Handles the display configuration sub-function. Parses an N/Y parameter from the command line, and if Y was specified, calls ROM routine 02A1H to configure the display. If N was specified, calls ROM routine 0298H to reset the display. Returns with A=00H and Z FLAG set (success).

4D7BH - TRACE Mode Toggle (E5H Sub-function 10)

Toggles the TRACE mode flag at memory location 4019H bit 0. When TRACE is enabled (bit 0 = 1), execution tracing is active. Parses N/Y from the command line: N enables TRACE (sets bit 0), Y disables TRACE (clears bit 0).

4D88H - SCREEN Mode Toggle (E5H Sub-function 1)

Toggles the SCREEN mode flag at memory location 401CH bit 0. When SCREEN mode is enabled (bit 0 = 1), video display output is active. Parses N/Y from the command line: N enables SCREEN output (sets bit 0), Y disables SCREEN output (clears bit 0).

4D95H - FORMS Mode Toggle (E5H Sub-function 3)

Toggles the FORMS mode flag at memory location 4289H bit 7. When FORMS mode is enabled (bit 7 = 0), printer form-feed formatting is active. Parses N/Y: N enables FORMS (clears bit 7), Y disables FORMS (sets bit 7).

4DA2H - SKIP Mode Configuration (E5H Sub-function 4)

Configures the SKIP mode by writing self-modifying code at 4A2DH-4A2FH. When enabled (N), writes DDH at 4A2DH and 017EH at 4A2EH-4A2FH, which forms the instruction LD A,(IX+7EH). When disabled (Y), writes 00H at 4A2DH and FF3EH at 4A2EH-4A2FH, which forms the instruction NOP followed by LD A,FFH. [SELF-MODIFYING CODE]

4DB8H - DEBUG Hook Enable/Disable (E5H Sub-function 5)

Enables or disables the DEBUG hook by writing self-modifying code at 4478H. When enabled (N), writes JP 4546H (C3H followed by address 4546H) at 4478H, causing the debug hook to be active. When disabled (Y), writes RET (C9H) at 4478H, causing the debug hook to immediately return without doing anything. [SELF-MODIFYING CODE]

4DCCH - Parse N/Y Parameter From Command Line

Parses a single character (N or Y) from the command line pointed to by HL. First calls the inner parser at 4DD1H, then if neither N nor Y was found, skips whitespace and checks for end-of-line. Returns Z FLAG for N, NZ FLAG for Y. If the character is not N, Y, or end-of-line, jumps to the error handler at 4409H.

4DD1H - Inner N/Y Character Parser

Reads a single character from (HL), subtracts 4EH (ASCII N), and checks if the result is 00H (N) or 0BH (Y, since 59H - 4EH = 0BH). On match, advances HL past the character and returns with B holding the result (0 for N, 0BH for Y). On mismatch, restores HL and falls through to whitespace/end-of-line checking.

4DE6H - PURGE Command Handler (Function 45H)

The PURGE command deletes files matching a filespec. It first parses the filespec, validates it for wildcards, sets up the PURGE operation mode, and then enters the directory scanning loop shared with the FREE command.

4DF4H - PURGE Loop Continuation Entry

Re-entry point for the PURGE loop when continuing to delete additional files. The filespec is re-parsed because the directory may have changed.

4E02H - FREE Command Handler (Function 25H)

The FREE command calculates and displays free space on a disk. It opens the directory, scans all entries, counts allocated granules by examining extent tables, builds a granule allocation bitmap at 4D00H-4DBFH (overwriting SYS3's own dispatcher code which has already executed), and reports remaining free space. The bitmap is also used to verify and optionally repair the Granule Allocation Table (GAT).

4E9EH - Process File Extents Into Allocation Bitmap

Walks through a directory entry's extent table and clears bits in the allocation bitmap at 4D00H-4DBFH for each allocated granule. Called after the bitmap has been initialized to all-FFH. The entry's low buffer address was saved at 4EA1H. In PURGE mode, also marks entries for deletion.

4F0DH - GAT Verification and Bitmap Comparison

After all directory entries have been processed and the allocation bitmap at 4D00H-4DBFH is complete, this routine reads the disk's Granule Allocation Table (GAT) and compares it against the computed bitmap. If discrepancies are found, the GAT is updated. The bitmap is then used to count remaining free granules. The GAT is stored in the first sector of the directory track.

4F5AH - Build and Display Filename String

Builds a display string from the current directory entry's filename and extension fields, appends the drive number, and displays it on screen. The string is built in the area pointed to by IX (the FCB). The format is "FILENAME/EXT:d" where d is the drive number. Checks system flags first to determine if display should be suppressed.

4FC2H - Mark Granule in Secondary Tracking Area

Marks a granule as "processed" in the secondary tracking area at 4DC0H-4DDFH. Reads the directory entry position from the self-modifying code at 4EA1H, extracts the extent data, and sets the corresponding bit in the tracking area. This is used during PURGE operations to track which granules have been freed.

4FEAH - Open Directory File for Reading

Sets up the stack with return addresses and jumps to a SYS0 routine to open the directory file. Pushes two return addresses onto the stack before jumping: E507H (a post-processing routine) and 497CH (the directory open continuation). Then jumps to 4A07H which performs the actual directory file open.

4FF5H - File Position and EOF Marker Update (E5H Sub-function 7)

Updates a file's position and EOF marker information. Validates the file is open, checks its access mode, and reads/modifies the position fields in the FCB and directory entry. This is used for file truncation and position management operations.

502DH - KILL Command Access Check (Function C5H)

Validates whether a file can be accessed for the KILL command.

503AH - KILL Interactive File Deletion (E5H Sub-function 6)

Interactive file deletion with user confirmation. Parses the command line for optional qualifiers: /ext to filter by extension and USR to include system files. For each matching file, prompts "KILL IT? (Y/N/Q)".

5081H - KILL: Begin File Matching and Deletion Loop

Stores option flags, reads the first directory sector, sets up the upper limit for scanning, and iterates through directory entries checking each against the match criteria.

5136H - Display Characters From Buffer (Skip Spaces)

Displays B characters from the buffer at (HL), advancing HL after each character. Characters equal to 20H (space) are skipped and not displayed. All other characters are output via ROM routine 0033H.

5140H - Option Flags and Prompt String Data Area

This area serves dual purpose: at 5140H is the option flags byte (written at runtime by the KILL command at 5082H), followed by the extension filter buffer at 5141H-5143H, and at 5144H begins the "KILL IT?" prompt string. The data decodes as ASCII text: " KILL IT? (Y/N/Q) " followed by an ETX (03H) terminator.

515FH - DEBUG/DUMP Screen Memory Display (Function A5H)

Displays the contents of memory on screen in a readable format. Non-printable characters (below 20H) are replaced with periods (2EH). Characters above the current display threshold (stored at 4290H) are also replaced with periods. The routine continuously dumps memory starting at 3C00H (video RAM) or 4000H (start of user RAM), checking for the BREAK key to stop. Before each line, if the current column is 0, a carriage return is output. The routine reads memory byte by byte, filters through the printability check, and outputs via ROM routine 003BH (printer output).

518FH - NOP Padding (End of SYS3 Code)

NOP padding bytes filling the remainder of the SYS3 overlay space from 518FH through 51DFH. This unused space (81 bytes of 00H) pads the file to its full allocated size on disk.