TRS-80 DOS - NEWDOS/80 v2.0 for the Model III - SYS17/SYS Disassembled

SYS17/SYS

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SYS17/SYS is a NEWDOS/80 v2.0 overlay module for the TRS-80 Model III that implements two DOS commands: SYSTEM and WRDIRP. The module loads into memory at 4D00H and occupies 1256 bytes (4D00H–51E7H), of which approximately 555 bytes are executable code, 115 bytes are data tables and I/O parameter blocks, 115 bytes are residual code from a previous overlay, and 329 bytes are NOP padding.

SYSTEM Command (4D00H–4E81H)

WRDIRP Command (4F6DH–500EH)

Address range allocation for SYS17/SYS (4D00H–51E7H, total 1256 bytes). Entry point: 4D00H.

SYSTEM Command Code:

Display Routines:

Utility Routines:

SYSTEM Command Data:

WRDIRP Command Code:

WRDIRP Data:

Residual and Padding:

System Option Storage (in DOS buffer, loaded from diskette sector 2):

External DOS Locations Referenced:

SYS17 Internal Data Areas:

Self-Modifying Code Locations:

4D00H - SYS17/SYS ENTRY POINT — SYSTEM AND WRDIRP COMMAND DISPATCHER

SYS17/SYS is a DOS overlay module that implements two commands: SYSTEM (which displays and modifies the system configuration options stored in the 3rd sector of the system diskette) and WRDIRP (which rewrites directory sector address marks for Model I/III diskette interchangeability). On entry, Register A contains a command identifier byte passed by the DOS command dispatcher. If Register A = 53H (ASCII S), execution branches to the WRDIRP command handler at 4F6DH. If Register A = 33H (the internal code for the SYSTEM command), execution falls through to the SYSTEM command handler. Any other value returns error code 2AH (general error) to the caller.

4D0CH - SYSTEM COMMAND: INITIALIZATION AND SECTOR READ

This section initializes the SYSTEM command by saving the drive number, setting up the sector I/O parameter block to reference the 3rd sector of the system diskette, reading that sector into the DOS buffer, and then beginning to parse the command line for option parameters. If no parameters are provided (the command line is exhausted), execution branches to the display routine at 4DC0H to show the current settings.

4D1EH - PARSE SYSTEM OPTION LETTER PAIR (e.g., "AA", "BK")

This routine parses a two-letter SYSTEM option identifier from the command line. SYSTEM options use a two-letter code where the first letter is 'A' or 'B' (the row) and the second letter is 'A' through 'Z' (the column). The two letters are converted into a single index (0–71) into the option descriptor table at 4F24H. The first letter ('A'=0, 'B'=1, etc.) is multiplied by 26 (19H × 2 iterations via DJNZ, yielding index × 26), and the second letter's offset (0–25) is added. This combined index is used to look up the option's descriptor byte, which encodes the option type and storage location.

4D4EH - CHECK FOR '=' SIGN AND DISPATCH BY OPTION TYPE

After successfully looking up the option descriptor byte (now in Register B), this section checks that the next character on the command line is '=' (3DH). It then examines Bit 7 of the descriptor byte to determine whether this is a Y/N flag option (Bit 7 = 0) or a numeric value option (Bit 7 = 1), and dispatches accordingly.

4D58H - Y/N FLAG OPTION HANDLER

This section handles SYSTEM options that take Y/N values (e.g., "AA=Y", "BK=N"). The descriptor byte (in Register B) with Bit 7 = 0 encodes the Y/N flag's storage information. Bits 4–1 of the descriptor (after rotation) determine an offset within the Y/N flag byte table at 43F0H, and specific bits within the descriptor select which bit position to set or reset. The code uses self-modifying instructions to construct the correct RES/SET opcodes for the target bit.

4D82H - NUMERIC VALUE OPTION HANDLER

This section handles SYSTEM options that take numeric values (descriptor Bit 7 = 1). Options with Bit 6 = 0 take a single-byte value (0–255); options with Bit 6 = 1 take a two-byte (16-bit) value (e.g., the HIMEM address for option AP). The routine parses the ASCII decimal or hex number from the command line, then stores the result at the appropriate offset within the system options area at 43A0H (for 1-byte values) or at 43A0H+30H (for 2-byte values stored as a word). Bits 4–0 of the descriptor byte encode the byte offset within 43A0H.

4DA6H - OPTION STORED — PARSE NEXT OPTION OR WRITE AND EXIT

After successfully storing a Y/N flag or numeric value, this section restores the command line pointer and loops back to parse the next option. If no more options remain, it validates the drive count (option AL) and writes the modified system options sector back to the diskette.

4DC0H - DISPLAY CURRENT SYSTEM OPTIONS

This routine displays all currently defined SYSTEM options and their values. It walks through the option descriptor table at 4F24H, and for each defined option (descriptor byte < FEH), it computes the two-letter option code, looks up the stored value, and displays it. Y/N flag options display as "Y" or "N"; numeric options display as both a decimal value and a hex value with an "H" suffix. The entry point at 4DC0H sets up HL to point to the descriptor table, DE = 0000H (initial offset into the system options data), and B = 0DH (ASCII carriage return, used as the first separator character — producing a new line before the first option). Execution then jumps into the middle of the display loop at 4E13H.

4DCAH - DISPLAY LOOP BODY: OUTPUT ONE OPTION'S NAME AND VALUE

For each defined option, this section displays: the separator character (CR for the first option, comma for subsequent ones), the two-letter option code (e.g., "AA", "BK"), an '=' sign, and the option's current value. For Y/N flag options (descriptor Bit 7 = 0), the current flag bit is read from the table at 43F0H and displayed as 'Y' or 'N'.
For numeric options (descriptor Bit 7 = 1), execution branches to 4E2AH for decimal+hex display.
The two-letter code is computed by dividing the current descriptor offset (DE) by 26 (1AH) using the SYS0 routine at 4C59H: the quotient becomes the first letter and the remainder becomes the second letter, each with 41H added to convert to ASCII 'A'–'Z'.

4DF1H - DISPLAY Y/N FLAG VALUE

For Y/N flag options (descriptor Bit 7 = 0), this section uses self-modifying code to construct a BIT n,(HL) instruction that tests the correct flag bit at the correct byte offset in the Y/N flag table at 43F0H. If the bit is set, 'Y' is displayed; if clear, 'N' is displayed. The self-modifying technique mirrors the SET/RES approach used in the option-setting code at 4D58H.

4E0CH - DISPLAY LOOP: ADVANCE TO NEXT OPTION AND CHECK DESCRIPTOR

After displaying one option's value, this section restores the saved registers, sets the separator character to comma (2CH) for all subsequent options, advances both the descriptor table pointer (HL) and the system options data offset (DE), and then checks the next descriptor byte. Descriptor values below FEH indicate a defined option (loop back to display it). A value of FEH means the option is undefined (skip it by advancing DE without displaying). A value of FFH signals the end of a display group (output a carriage return and terminate).

4E21H - SET UP SECTOR I/O PARAMETER BLOCK

This short utility routine initializes the sector I/O parameter block at 4F16H for accessing sector 2 (the 3rd sector, zero-indexed) of the system diskette. It stores sector number 02H into the sector field at 4F20H and points DE to 4F16H (the base of the I/O parameter block). Called before both sector read and sector write operations.

4E2AH - DISPLAY NUMERIC OPTION VALUE (DECIMAL / HEX)

This section handles the display of numeric SYSTEM option values (descriptor Bit 7 = 1). It extracts the storage offset from the descriptor byte, fetches the stored value (1-byte or 2-byte depending on Bit 6), then displays the value first as a decimal number using the repeated-subtraction division routine at 4EDEH, followed by a '/' separator and a hexadecimal representation using the SYS0 hex conversion routine at 44D2H. The hex display includes leading-zero suppression and an 'H' suffix. This section is reached via the JR NZ at 4DEFH.

4E50H - HEX DISPLAY: LEADING-ZERO SUPPRESSION

This section scans the hex digit buffer from left to right, suppressing leading zeros. For each position, if the character is ≥ 'A' (41H), it is a hex digit A–F and leading zeros end. If the character is '0' (30H) and there are still leading zeros to suppress, the count is decremented and the character is skipped. Once a non-zero digit or non-leading-zero is found, all remaining characters (including the 'H' suffix) are displayed.

4E6BH - CHARACTER OUTPUT WRAPPER

A utility routine that displays the character in Register A on the screen by calling the Model III ROM character output routine at 0033H. It preserves Register Pair DE and the AF register across the call, which is important because the display loop uses DE to track the system options data offset and A to hold various characters and values.

4E73H - PARSE DECIMAL NUMBER WITH VALIDATION / PARSE HEX NUMBER

These are two entry points for numeric parsing used by the SYSTEM command (for setting numeric option values) and by the WRDIRP command. Entry at 4E73H parses a decimal number; entry at 4E78H parses a hex number. Both call the core number parsing routine at 4EA7H/4EA9H. After parsing, 4E7BH validates the result: if D = 0 (value fits in 8 bits), the value in E is returned in A with Z FLAG set (success). If D ≠ 0 (value exceeds 255), execution falls through to the error handler at 4E7FH.

4E84H - DECIMAL NUMBER PARSER (WITH OPTIONAL HEX SUFFIX)

This routine parses an ASCII number from the command line at HL. It first attempts to parse the number as decimal. After parsing, it checks if the character following the digits is 'A'–'H'. If it is an 'H', the number is reinterpreted as hexadecimal by re-parsing from the beginning with the hex parser. The routine uses two internal entry points: 4EA7H (initialize B=0 for decimal mode) and 4EA9H (initialize DE=0000H and begin digit accumulation).

4EA2H - HEX NUMBER PARSER ENTRY POINT

This is the direct hex number parsing entry point. It pushes a return address of 4E9CH onto the stack so that after the digit accumulation loop returns, execution continues at the validation check. It then falls through to 4EA7H with B = 00H and calls 4EA9H.

4EA9H - DIGIT ACCUMULATION LOOP (DECIMAL OR HEX)

This is the core numeric parsing routine. It initializes DE = 0000H and then processes ASCII digit characters from the command line at HL one at a time. For each digit, the current accumulated value in DE is multiplied by 10 (decimal mode, Bit 0 of B = 0) or by 16 (hex mode, Bit 0 of B = 1), and the new digit value is added. The multiplication is performed using shifts and adds on a 24-bit accumulator (A:HL). The routine returns when a non-digit character is encountered, with the result in DE and Bit 1 of B set if at least one digit was parsed.

4EDEH - DISPLAY 16-BIT VALUE AS DECIMAL

This routine converts the 16-bit value in DE to ASCII decimal and displays it on screen. It uses repeated subtraction (actually, repeated addition of the two's complement of each divisor) to extract each decimal digit position. The divisor table at 4F08H contains the negated (two's complement) values of 10000, 1000, 100, and 10. Register B = 04H controls four iterations (ten-thousands, thousands, hundreds, tens), and the ones digit is displayed directly at the end. Register C serves as a leading-zero suppression flag: C = 0 means leading zeros are being suppressed; C ≠ 0 means a non-zero digit has been output.

4F08H - DECIMAL DIVISOR TABLE (NEGATED VALUES)

This 8-byte table contains the two's complement (negated) values of the four decimal place-value divisors used by the decimal display routine at 4EDEH. Each entry is a 2-byte little-endian word. The division is performed by repeatedly adding these negated values (equivalent to subtraction). The disassembler shows these bytes as instructions, but they are DATA, not executable code.

4F10H - HEX DISPLAY SCRATCH BUFFER AND I/O PARAMETER BLOCK

This area contains a 6-byte scratch buffer used by the hex display routine (4E46H calls 44D2H to convert DE to ASCII hex and store the result here), followed by the sector I/O parameter block used by the SYSTEM command for reading and writing the 3rd sector of the system diskette. The disassembler renders these data bytes as Z80 instructions, but they are NOT executable code — they are DATA structures.

4F16H - SYSTEM COMMAND SECTOR I/O PARAMETER BLOCK

This 14-byte structure at 4F16H–4F23H is the sector I/O parameter block used by the SYSTEM command to read and write the 3rd sector (sector 2, zero-indexed) of the system diskette. It is passed to the DOS sector read routine at 4436H and write routine at 443CH via Register Pair DE. The setup routine at 4E21H stores sector number 02H at offset +0AH (4F20H), and the entry point at 4D0DH stores the drive number at offset +06H (4F1CH).

4F24H - SYSTEM OPTION DESCRIPTOR TABLE

This table at 4F24H contains one descriptor byte per SYSTEM option, indexed by the combined option code: (first_letter − 'A') × 26 + (second_letter − 'A'). Each byte encodes the option's type and storage location. The table spans options AA (index 0) through BN (index 39 = 27H), followed by an FFH end-of-table marker and FFH padding to fill the remaining space.

Descriptor byte encoding for Y/N flag options (Bit 7 = 0):
Bits 6–4: Bit number (0–7) within the flag byte to SET or RES
Bits 3–0: Byte offset within the Y/N flag table at 43F0H

Descriptor byte encoding for numeric value options (Bit 7 = 1):
Bit 6: 0 = single-byte value (0–255); 1 = two-byte value (0–65535)
Bits 4–0: Byte offset within the numeric storage area at 43A0H (1-byte values) or 43D0H (2-byte values)

Special descriptor values:
FEH = Undefined/reserved option (skipped during display, rejected during set)
FFH = End-of-table marker (terminates display loop)

4F6DH - WRDIRP COMMAND: ENTRY POINT AND INITIALIZATION

The WRDIRP (Write Directory Protected) command rewrites the directory sectors of a diskette with the currently defined protected address mark for the current computer model. This is used when exchanging single-density diskettes between Model I and Model III under NEWDOS/80 v2, because the two models use different address mark bytes for protected directory sectors. Before executing, WRDIRP checks that SYSTEM option BK=Y (stored as bit 4 of the byte at 428DH, which is the in-memory copy of the 43F0H+1 flag byte). If BK=N, the command is rejected with error code 37H. The command syntax is: WRDIRP,dn1 where dn1 is the drive number.

4F80H - WRDIRP: READ BOOT SECTOR AND VALIDATE DIRECTORY

WRDIRP reads the boot sector (sector 0) of the target diskette into the DOS buffer at 4300H. It then validates the boot sector by checking that the first byte at 4300H is 00H (indicating a valid boot sector format) and the second byte at 4301H is FEH (the directory granule marker). The third byte at 4302H contains the directory starting granule number, which will be used to locate the directory sectors. If validation fails, error code 2CH is returned.

4F96H - WRDIRP: SET UP DIRECTORY SECTOR ACCESS

Using the directory starting granule number from the boot sector, this section sets up the I/O parameter block to access directory sectors. It calls SYS0 routines to convert the granule number to a physical track and sector, then reads the first directory sector to verify it contains the expected system files (BOOT/SYS and DIR/SYS). Once verified, it loops through all directory sectors, rewriting each one with the protected address mark.

4FC5H - WRDIRP: DIRECTORY SECTOR REWRITE LOOP

This is the main loop that rewrites each directory sector with the protected address mark. For each iteration, it stores the current sector address, reads the sector, writes it back (which applies the current protected address mark), and advances to the next directory sector. Register B is the loop counter (total directory sectors). The loop uses the DOS write routine at 443CH which writes the sector with the protected address mark determined by the current system configuration.

4FD6H - WRDIRP ERROR EXITS

Error exit points used by WRDIRP when validation fails or when BK=N. These are shared with the SYSTEM command's error handler at 4E81H.

4FDBH - READ SECTOR WITH ERROR HANDLING

A utility subroutine used by WRDIRP to read a sector from the target diskette. It calls the DOS sector read routine using the I/O parameter block at 501EH. If the read succeeds (Z FLAG), it returns immediately. If the read fails with error code 06H (record not found / end of track), it returns with Z FLAG set (treating it as a non-fatal condition, since the directory may not fill an entire track). Any other error is fatal and branches to the error handler.

4FE7H - VERIFY DIRECTORY FPDE (FILE PRIMARY DIRECTORY ENTRY)

This subroutine reads a directory sector, verifies that it is a valid directory sector (checking specific attribute bits at 4300H and the drive number at 4316H), and then compares the filename in the sector against an expected filename. It is called twice by WRDIRP: once to verify BOOT/SYS and once to verify DIR/SYS. On entry, HL points to the expected filename string, and C contains the expected drive number. If any check fails, execution branches to error code 2CH.

5006H - BYTE-BY-BYTE COMPARISON LOOP

A simple comparison loop that checks B bytes at (DE) against B bytes at (HL). If any mismatch is found, execution jumps to the error handler at 4FD6H. On success, DE and HL are left pointing past the compared regions, and execution falls through to the RET.

500FH - WRDIRP STRING CONSTANTS AND I/O PARAMETER BLOCK

This data area contains the reference filename strings used by WRDIRP to verify that the directory contains BOOT/SYS and DIR/SYS entries, followed by the WRDIRP sector I/O parameter block at 501EH. The disassembler renders these bytes as Z80 instructions, but they are DATA.

501EH - WRDIRP SECTOR I/O PARAMETER BLOCK

This 14-byte structure at 501EH–502BH is the sector I/O parameter block used by the WRDIRP command. It is passed to the DOS sector read/write routines via Register Pair DE. The drive number at offset +06H (5024H) is set at 4F79H. The sector address pointer at offset +0AH (5028H) is updated during the directory rewrite loop.

502CH - RESIDUAL CODE (NOT PART OF SYS17)

The bytes from 502CH through 509EH are residual code left over from a different SYS overlay module that previously occupied this memory region. These instructions reference addresses far outside SYS17's range (5535H, 5784H, 56F9H, 5762H, 48C4H, 54CBH, 552DH) and memory locations in the 5900H range (59C1H, 5994H, 59C6H, 59BBH, 5981H, 5983H, 598BH, 59D1H, 59C3H, 59BCH), confirming this code belongs to a different overlay (such as SYS6 or a related FORMAT/COPY module). This residual code is NOT executed by SYS17 and is overwritten when other overlays load. It is listed here only for completeness.

509FH - NOP PADDING (UNUSED SPACE)

The region from 509FH through 51E7H contains 329 bytes of NOP (00H) padding. This is unused reserved space within the SYS17 overlay file, filling out the remainder of the overlay allocation to the file boundary. The PROGRAM ENTRY point is 4D00H as noted at the end of the source file.