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

SYS9/SYS

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NEWDOS/80 v2.0 SYS9/SYS Disassembly - Chaining, *name Routines, MDCOPY, BASIC2, BOOT, PAUSE and STMT (Model I)

SYS9/SYS runs chain files (CHAIN and DO), keeps the list of *name routines, and holds MDCOPY, BASIC2, BOOT, PAUSE, STMT, CHNON and the two stop messages used by SYS1's exit code. It loads into the overlay area at 4D00H-51DFH (1,248 bytes in five load records, transfer address 4D00H); 5147H-51DFH are unused zero bytes. The SYS0 dispatcher (4BC2H) loads it when 4317H does not already hold its number (0BH).

The page shows every byte of the file once, in address order.

RST 28H Codes Handled by SYS9

How Chaining Works

*name Routines

Variables and Work Areas

4D00H - Function Dispatcher

SYS9 is entered by the SYS0 RST 28H dispatcher (4BC2H) with Register A = the RST 28H code and the return address 4C20H on the stack. 2BH and 4BH add a *name routine to the list at 4051H or remove it (the SYS0 vectors 4461H and 4464H); CBH gives the SYS0 keyboard driver its next key from the chain file; EBH (function 7) takes a sub-function in Register C: 1 return from 4DD0H, 2 MDCOPY, 3 CHAIN and DO, 4 abort chaining after an error, 5 CHNON, 6 BASIC2, 7 run a *name routine, 8 PAUSE, 9 STMT, 0AH BOOT, 0BH DOS FATAL ERROR, 0CH 'RUN ONLY' STOPPED. Anything else returns error 2AH.

4D42H - Error Exits

4D42H: error 2AH. 4D4CH: error 2FH. 4D4EH: the error in Register A through SYS0 4409H. 4D46H is used when the *name list is found damaged (50E8H): it pushes error 33H and calls SYS4 function 2 (code 46H), which sets bit 2 of 436AH and shows the error; the SYS1 exit then stops the system through sub-function 0BH.

4D52H - PAUSE Command (Sub-Function 8)

PAUSE text: Register Pair HL = the text. It is shown (only in the case 4D63H describes), then PRESS 'ENTER' WHEN READY TO CONTINUE, and SYS9 waits for ENTER.

4D63H - STMT Command (Sub-Function 9)

STMT text only shows the text at Register Pair HL, and only when bit 6 of 436AH (a DOS-CALL is running) is set and chaining is off (4369H bit 5 clear). Also used by PAUSE.

4D72H - CHNON Command (Sub-Function 5)

CHNON Y, N or D, with Register Pair HL = the parameter. It works only while a chain file is open (the chain FCB at 43E0H), otherwise error 26H. Anything else gives error 34H.

4D82H - Read Y, N or D

Used by CHNON and by the 85H line of a chain file (4E87H), with Register Pair HL = the letter. N turns chaining off (4369H bit 5 clear), Y turns it on, D turns it on and also sets 436AH bit 4 when bit 6 (a DOS-CALL) is set. Only the end of the line may follow. Returns the Z FLAG with Register A = 00H, or the NZ FLAG (nothing stored) for anything else; Register B and Register C keep the new flags in either case.

4DACH - DOS FATAL ERROR and 'RUN ONLY' STOPPED (Sub-Functions 0BH and 0CH)

Called by the SYS1 exit. 0BH when bit 2 of 436AH is set (by SYS4 function 2 after an error at reset or a damaged *name list); 0CH when a command has ended while option AB (RUN-ONLY, 436CH bit 6) is on and no chain file is running. Both show their message and !! KEY 'R' FOR RESET., wait for R and restart the system (4DC6H).

4DC6H - BOOT Command (Sub-Function 0AH)

Deselects the drives and executes HALT, which resets the Model I: the system boots again.

4DD0H - Run an Overlay Function and Come Back

Used by MDCOPY, whose file buffer lies inside SYS9 and whose calls load other overlays. Pushes the word EB01H and 49D3H. With Register A not 00H it then jumps to 49DDH (RST 28H with code A); with Register A = 00H it jumps to the address in Register Pair HL' (4439H, set at 5006H). The function returns to SYS0 49D3H, which shows an error (NZ) or does RST 28H with EB01H: SYS9 function 7 sub-function 1 (reloaded if needed), which returns to the caller of 4DD0H.

4DE2H - Sub-Function 1: Back to the Caller of 4DD0H

The stack holds the dispatcher's return address (4C20H) and then the return address of the CALL 4DD0H. 4C20H is discarded and Register Pair HL kept.

4DE5H - CHAIN and DO Commands (Sub-Function 3)

SYS1 has opened the chain file (default extension /JCL) in the FCB at Register Pair DE and left the command line pointer on the stack. The FCB is copied to 43E0H-43FFH with its buffer at 5000H (inside SYS9). If a label follows the filespec it is searched for (4E0FH). Then chaining is turned on (4369H bit 5; 436AH bit 4 too when a DOS-CALL is running), and the SYS0 keyboard driver takes its keys from the file through code CBH.

4E0FH - Find a Label in the Chain File

Register Pair HL points to the label (ended by a carriage return). The file is read from where it is: a label line starts with the code 80H (4F59H turns /./0 into 80H too), followed by the name (compared in upper case) and a carriage return. Returns the Z FLAG with the file positioned after the label line, or the NZ FLAG with the error in Register A (1CH at the end of the file).

4E2BH - Code CBH: The Next Key From the Chain File

Called by the SYS0 keyboard driver (4516H) when chaining is on (4369H bit 5 set, bits 3 and 6 clear). Returns the Z FLAG with the key in Register A, or the NZ FLAG to make the driver read the real keyboard. The first time after SYS9 is loaded, 4E2BH-4E35H mark the chain file's buffer as empty (FCB+1 bit 5), because the buffer at 5000H has just been overwritten by SYS9 itself, and turn 4E2BH into JR 4E37H. Unless option AT (436DH bit 7) is on, keys are given only to the ROM's line input: SYS9 looks on the stack for the ROM return address 03DDH (inside the driver entry CIO, 03C2H-03E2H) and, 12 bytes further up, 05E3H (inside the keyboard line input KEYIN, 05D9H-0618H).

4E60H - Give the Next Character

With option BC (436DH bit 6) the right arrow held pauses the chain file (4F0CH) and the up arrow aborts it (4EDCH). A character read ahead earlier (43A5H) is used first; otherwise the next one is read (4F59H). At the end of the file chaining ends and the keyboard is used.

4E87H - A CHNON Line in the Chain File (85H)

The line read by 4F24H (at 4FE0H, Register Pair HL) is taken as CHNON's parameter Y, N or D; anything else aborts with error 01H. Then 513AH gives the character read ahead, if any, through 4E99H; the RET there comes back to 4E93H (the CALL at 4E90H is still on the stack), so that character is not handed to the keyboard driver and Register C then holds it rather than the 4369H flags. Without a character read ahead, if a DOS-CALL is running (436AH bit 6) without bit 4 the command ends at DOS READY (513AH); otherwise the chain goes on if it is still on.

4E99H - Handle a Character or a Code

Register A = the character. Codes: 80H (a label) ends chaining, 81H-83H lines go to 4EF7H, 84H and 85H lines to 4F24H. An ordinary character is given to the keyboard driver after the next one has been read ahead (43A5H): if that is the end of the file or 80H, chaining ends now, so the command being read is the last; if it is 85H its line is handled before this character is given (4F1FH).

4EC8H - CHAINING ABORTED and Turn Chaining Off

4EC8H shows CHAINING ABORTED. 4ECEH closes the chain (byte 0 of the chain FCB = 00H) and clears 4369H bit 5 and 436AH bit 4.

4EDCH - Abort With the Up Arrow

With option BC (436DH bit 6), the up arrow (keyboard row 6 bit 3) held aborts the chain file with no error code: it goes on at 4EE9H with Register A = 00H and never returns.

4EE9H - Abort the Chain File

Register A = the error (00H = none). Shows CHAINING ABORTED, turns chaining off and ends the command: with Register A = 00H through SYS0 4030H (error already shown), otherwise with the error in Register A through 4409H. 4EEBH is sub-function 4, called by the SYS1 exit after an error while chaining: the stack then holds the dispatcher's return address and SYS1's Register Pair AF with the error.

4EF7H - Lines Marked 81H, 82H and 83H

Register B = the code. The rest of the line is read to its carriage return. With 81H and 83H it is displayed, with 82H (a comment) it is not. After an 81H line the chain file pauses (CHAINING PAUSE. PRESS 'ENTER' WHEN READY TO CONTINUE) until ENTER is pressed; the up arrow can abort (with option BC).

4F1FH - Lines Marked 84H and 85H

4F1FH: the character after an ordinary one is 85H; the ordinary character (Register B) is kept in 43A5H to be given afterwards. 4F24H reads the rest of the line, at most 32 characters, into 4FE0H (over MDCOPY's own code, not needed while chaining). 84H: go to the label named on the line: the file is rewound and the label searched for (4E0FH). 85H: the line is a CHNON parameter (4E87H). A line that is too long, a missing label or a read error aborts.

4F4EH - Show CHAINING and a Message

Register Pair HL = the message that follows CHAINING.

4F59H - Read a Character From the Chain File

Returns the Z FLAG with the next byte that is not 00H in Register A, or the NZ FLAG with the error (1CH at the end of the file). The text /./ followed by 0-5 is returned as the code 80H-85H, so that a chain file made with an editor can hold the codes. If /, /. or /./ is followed by anything else, the file is moved back (SYS0 4445H with the record length at FCB+9 set to the number of extra bytes read) so that those bytes are read again one at a time, and the / is returned. Bytes 86H-FFH lose bit 7.

4F95H - Read a Byte From the Chain File

Reads bytes through the ROM input routine at 0013H with Register Pair DE = the chain FCB (SYS0 4ADBH reads them from the file) and skips 00H bytes. Returns the Z FLAG with the byte in Register A or the NZ FLAG with the error.

4FA1H - Chaining Messages

4FE0H - MDCOPY Command (Sub-Function 2)

MDCOPY source TO destination copies a file sector by sector; it can be used under MINI-DOS. SYS1 has opened the source in the FCB at 4480H and put the second filespec at 51E0H. Both FCBs use the sector buffer 4DE5H-4EE4H, inside SYS9, so 4317H is set to 00H after the calls that may load other overlays (SYS9 will be loaded again the next time it is needed). The destination is opened (new or existing, SYS2), positioned at the source's last sector and written there so that its space is allocated at once, rewound, then every source sector is read and written to it. Finally it gets the source's EOF, is read through once from the start and closed.

5051H - BASIC2 Command (Sub-Function 6)

Starts the ROM's Level II BASIC without the DOS: copies the ROM's table at 06D2H-0707H to 4000H-4035H, clears 4036H-405CH and jumps into the ROM's second stage of initialisation at 0075H.

5067H - Code 2BH: Enqueue a *name Routine

Register Pair HL = the address of the routine's block, which must be 5200H or higher. The block: +0/+1 a check word (set to the block address + 2), +2/+3 the link to the next block, +4 to +11 the name (8 characters), +12 the code. The block is put at the head of the list at 4051H. Error 35H if the name is already in the list, 2FH if the address is too low.

507EH - Code 4BH: Dequeue a *name Routine

Register Pair HL = the block address. The block is found by its name and removed from the list; its check word is spoiled so that it is no longer taken as a block. Error 18H if the name is not in the list.

508FH - Sub-Function 7: Run a *name Routine

SYS1 calls this for a command that starts with *, with Register Pair HL at the *. The name (1-8 characters, up to a separator or the end of the line) is looked for in the list; the routine is entered at its block + 12 with Register Pair HL = the rest of the command line, through SYS0 4C20H. No name gives error 2FH, an unknown one 18H.

50C2H - Look for a *name Block

50C2H checks that the block at Register Pair HL is at 5200H or higher (else error 2FH) and points Register Pair HL to its name; 50CCH looks for the 8-character name at Register Pair HL. The list starts at 4051H and ends with a link of 0000H. A block whose check word is not its own address + 2 means the list is damaged: 4D46H. Returns the Z FLAG (not found) with Register Pair HL = the name, or the NZ FLAG with Register A = 35H, Register Pair HL = the block's link word (+2), Register Pair DE = the address of the link that points to the block, Register Pair BC = the name.

5100H - Stop Messages

513AH - After a CHNON Line

Called from 4E90H with Register A = the character read ahead and Register B = the new 436AH flags from 4D82H: if there is a character it is given now (JP 4E99H). Otherwise, if a DOS-CALL is running (436AH bit 6) and bit 4 is clear, the command ends at DOS READY (402DH).

5147H - Unused Bytes