TRS-80 DOS - VTOS 4.0.2 for the Model I - SYS9/SYS Disassembled

SYS9/SYS

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VTOS 4.0.2 SYS9/SYS Disassembly - The Extended DEBUG Commands (Model I)

SYS9/SYS adds commands to the DEBUG monitor of SYS5/SYS. DEBUG (EXT) (SYS6/SYS member 6, 523EH) issues request 8BH; the request handler loads SYS9/SYS at 4E00H (directory entry code 23H) and calls it. It copies its command reader (4E27H-51F3H, 973 bytes) to the top of memory, lowers the top of memory 4049H below it and puts the copy's address at 4310H, where every request below 80H goes. SYS5/SYS hands each key that is not one of its own commands to RST 28H (its 4E97H), so the copy gets the key and runs the extended command, using SYS5/SYS's routines for keys, hex numbers and hex output and its cells for the addresses. The manual: "An extended DEBUG facility may be loaded into high memory by setting the 'EXT' flag."

The file starts with a record of type 05H (the name SYS9), a record of type 1FH holding the copyright notice of Randolph Cook (1980) and four load blocks that fill 4E00H-51F3H, 1012 bytes; the last record gives the transfer address 4E00H, and one byte 03H follows it, which the loader never reads. The file carries no patch. The rows show the command reader at its file addresses; it runs from the copy, and its relative jumps land on the rows of this page. Its calls to 518AH, 51A3H, 51D0H, 51D4H, 51D9H and 51F2H go to SYS5/SYS, which is in memory whenever the copy is entered from DEBUG, not to this file's rows at those addresses.

The Installation

The Commands

Points to Note

4E00H - Entry: Install the Extended Commands

SYS9/SYS is loaded at 4E00H by the request handler of SYS0/SYS (4C31H) for a request code whose bits 3-0 are BH (directory entry code 23H); DEBUG with EXT (SYS6/SYS member 6, its 523EH) issues 8BH. The code does not look at the request code. It first asks whether the extended commands are already in memory: RST 28H with Register A = 40H (a request below 80H) goes to the routine whose address is at 4310H, and the copied command reader answers 40H with Register A = 00H, while SYS0/SYS's default 4C2FH returns Register A unchanged. When they are not there, the top of memory 4049H is lowered by 03CDH bytes, the command reader 4E27H-51F3H is copied just above the new top, and 4310H is pointed at the copy; the old contents of 4310H are kept as the operand at 5135H, where the copy sends every key it does not know.

4E27H - The Command Reader

Copied to the top of memory by 4E1FH and run there; the rows show it at its file addresses, and its relative jumps land on the rows of this page. SYS5/SYS's command loop sends every key that is not one of its own commands to RST 28H (its 4E97H) with 4E4BH, its command loop, pushed as the return address; for a key below 80H the request handler jumps through 4310H to here with Register A = the key. Each command ends with RET to SYS5/SYS's 4E4BH, which shows the display again. The calls to 518AH (a key), 51A3H (a hex number), 51D0H, 51D4H and 51D9H (hex digits) and 51F2H (a space) are SYS5/SYS's routines, which are in memory at 4E00H whenever the copy is entered from DEBUG. Numbers are hex, ended by a space, a comma or ENTER. Some fields have a default when no digits are typed, which most commands show in its place (V not after ENTER; F, P, Q and the disk command have none).

4E2DH - O: Return to VTOS

O followed by ENTER leaves DEBUG for VTOS READY; any other key after O goes back to the monitor.

4E38H - B: Block Move

B source,destination,count: copies count bytes (default 0100H; 0000H moves 65536) from the source (default the display address 4063H) to the destination (default the M address 4060H) with LDIR, low addresses first; a destination above the source and inside the block repeats its first bytes. The M address is left just past the destination block. ENTER right after the source drops the command.

4E84H - E: Enter Bytes

E address: shows each byte from the address (default the M address 4060H) followed by a minus sign; a hex number typed replaces it (its low byte), a space or comma with no digits keeps it; ENTER ends the command after its byte. The M address follows the byte being changed.

4EB7H - F: Fill

F start,end,byte: stores the byte at every address from the start to the end, the end included; a start equal to or above the end stores nothing, and an end of FFFFH never stops (4ED5H). Each field must be typed: a field with no digits drops the command.

4ED7H - J: Jump Over a Byte

Adds 1 to the saved program counter 407BH: the program will go on one byte further.

4EE3H - L: Locate a Byte

L address,byte: searches memory for the byte (default the last byte searched, at 4062H) from the address (default the M address plus 1, so that L alone finds the next one) through all 65536 addresses; since 4062H holds the byte, it is always found, at 4062H when nowhere else. The M address 4060H is then the byte and the display address 4063H the start of its 64 byte block, the four memory lines of SYS5/SYS's display. When an address is typed and the byte is not, the value shown as the byte is the low byte of the address, while the search uses 4062H.

4F2AH - N: Next Block

N address: takes the byte after the address (default the M address 4060H) as a length and moves the M address past it: the new M address is the address plus the length plus 2, with a length 00H taken as 256. On a record of a load module (type, length) this is the next record. The length is not taken as signed, so on a JR with a jump back it moves forward. The display address goes to the start of the 64 byte block of the new M address.

4F4AH - Q: Query or Set a Port

Q port ENTER: reads the port with IN and shows = and the value, then waits for a key. Q port,value: writes the low byte of the value to the port with OUT. A field with no digits drops the command.

4F6EH - T: Type Text Into Memory

T address: shows each byte from the address (default the M address 4060H) as a character (bit 7 cleared, then a period for codes below 20H) after a space, then a minus sign, and waits for a key: a space keeps the byte, ENTER ends the command, any other key (a comma too) is stored as it is. The M address follows the byte.

4FADH - V: Verify Two Blocks

V first,second,count: compares count bytes (default 0000H, 65536) at the first address (default the display address 4063H) with those at the second (default the M address 4060H), stopping at the first difference. 4063H and 4060H are left at the two bytes that differ, or just past the blocks when they are equal.

5008H - W: Search for a Word

W address,word: searches memory from the address (default the M address plus 2) for the word, low byte first (default the last word searched, kept at 4062H low and 405FH high); after a low byte whose high byte differs, CPIR goes on with the count it has left, so the search can run past 65536 bytes. Found: the M address 4060H is the byte before the word (the opcode when the word is the operand of a CALL, JP or LD) and the display address 4063H the start of its 64 byte block; the default start of the next W, the M address plus 2, is the byte after the word's low byte. When an address is typed and the word is not, the value shown as the word is the address, while the search uses 4062H and 405FH.

505EH - 0-7: Sectors to or From a Disk

d,cylinder,sector,o,address,count: d is the drive 0-7 (the key itself), o the operation: R reads, W writes with the ordinary data mark, an asterisk writes with the directory's data mark FAH; any other key (a space or a comma too) only counts the sectors. The cylinder defaults to the directory cylinder (drive code table +09H). With a sector, count sectors (default 1) are moved from that sector on; with no sector, a whole cylinder from sector 0 (the sectors of a cylinder, doubled for a drive with two sides). Each sector takes the next 256 bytes of memory from the address; after the last sector of a cylinder the next cylinder follows. A failed sector shows its error code between two asterisks and waits for a key, then the next sector is done. After R the display address and the M address are set to the start of the 256 byte page of the address and the full screen display is chosen. The count is kept in 4062H and the sectors of a cylinder in 405FH, SYS5/SYS's cells. ENTER after the drive, the cylinder or the sector drops the command; a key that is not a hex digit in a number field ends it in SYS5/SYS.

5132H - P: Print Memory

P start,end: prints on the printer (ROM 003BH) lines of 16 bytes from the start rounded down to a multiple of 16: four hex digits of the address, two spaces, the 16 bytes in hex with a space after each and one more after the 4th, 8th and 12th, then one more space (two before the characters) and the 16 bytes as characters (a period for codes below 20H and from 80H up). Lines go on while the next line's address is below the end, so an end that is the first byte of a line is not printed. Two carriage returns come first and three after. An end of FFF1H-FFFFH never stops the listing (51E7H). Both fields must be typed. A key that is no command at all goes on at 5134H to the routine 4310H held before SYS9/SYS was put in, normally SYS0/SYS's 4C2FH, which returns to the monitor.