DOSPLUS 3.5 for the Model III - DISKZAP/CMD Disassembled

DISKZAP/CMD

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DOSPLUS 3.5 DISKZAP/CMD Disassembly - The Disk Zapper and Editor (Model III)

DISKZAP/CMD is DOSPLUS's sector editor, run from the DOS command level as DISKZAP with no parameters; its heading reads DISKZAP - DOSPLUS Disk zapper/editor - 3.50. It is a load module of 12 blocks, 2846 bytes, at 5200H-5D1DH with the transfer address 5200H, after the two comment records (the name DISKZAP and the copyright notice). DISKZAP/CMD is on the Model III master only.

DISKZAP needs HMEM$ at 89F8H or above; below that it returns to the DOS without a message. It uses its own stack at 41E5H, the sector buffer 5E00H and, for Copy and Format, the pages from 5F00H up to HMEM$. It keeps its own geometry for each of the eight drives in a table at 5B04H (as loaded 40 cylinders, one surface, 18 sectors of double density) and puts it into the drive's DCT whenever a drive or a cylinder is chosen, setting the DCT's log flag; the DCTs are not set back when DISKZAP ends.

The Parts of DISKZAP/CMD

How DISKZAP Works

The menu lists Set, Fill, Copy, Print, Verify, Format and Display; the up and down arrows move the asterisk, ENTER runs the option and O returns to the DOS (5219H). Each option asks its questions on row 11 of the screen; ENTER takes the default, BREAK at any question returns to the menu. Drivespec is the name of a drive in DCTTBL (0-7), Cylinder and Sector are hexadecimal, Sector count, Cylinder count and the values of Set decimal (587CH).

Display reads the sector and shows it in 16 rows of 16 bytes, with the drive's number, the cylinder and the sector at the left in hexadecimal (56DBH). Semicolon and minus step a sector, plus and equal sign a cylinder, wrapping round the geometry (5328H); CLEAR shows characters instead of hexadecimal; F flashes every byte of a value; M enters the modify mode, where the arrows (and SHIFT with them) move a block cursor, two hexadecimal digits or one character change a byte, @ fills, ENTER writes the sector back with the address mark it was read with (DWRITA for the directory's) and BREAK reads it again (53A2H).

Fill writes one byte to every byte of a run of sectors; Copy reads as many sectors as the pages up to HMEM$ hold, keeping each one's address mark, then writes them to the destination, pass after pass (5562H); Print prints the screen of each sector; Verify reads each sector and, with N to Ignore data AM, stops at the directory's address mark (562CH); Format builds each track at 5F00H with the drive's geometry and an interleave and writes it by DFORMT, both sides of a two sided drive (566EH, 59EFH). A run counts down at 5D1FH and stops when BREAK is pressed between sectors (57C5H).

Differences from the Manual

The density questions take D for double density and any other answer for single (the manual says 5 and 0). The drive shown at the top left is the drive's number. Copy, Print, Fill and Verify step through the sectors of the geometry set, wrapping from the last cylinder to cylinder 0. BREAK in the modify mode reads the sector again and returns to the display, not to the menu. @ takes up to four hexadecimal digits: with four the first two are the value and the last two the count, with two the value is 00H and the digits are the count, and a count of 00H fills to the end of the sector; with four digits the number goes on with whatever follows in the buffer 5D25H. F, the find of the display, is not in the manual. Format refuses a rigid drive (DRIVE NOT SUPPORTED!) and asks for the drive again. Verify waits for any key after an error; BREAK returns to the menu.

Three findings stand from the bytes. ENTER at Fill data does not give 00H: Register E keeps the low byte of the sector count from the question before, so the sectors are filled with that byte (5541H). ENTER at Interleave factor leaves Register H at 5DH, the high byte of the answer's buffer, an interleave of 93; and an interleave greater than the sectors per track can put a sector number outside the table of the sector order, as each place is brought back round the track only once (568EH, 5A30H). What such a track holds depends on the bytes the sector buffer 5E00H has left above the table, for a place outside the table is taken when its byte has bit 7 set (5A3DH). With 18 sectors and the interleave of 93, bytes with bit 7 clear give all 18 sectors, in the order 00H 05H 0AH 0FH 03H 08H 0DH 01H 06H 0BH 10H 04H 09H 0EH 02H 07H 0CH 11H round the track; bytes with bit 7 set (E5H or FFH) leave only sectors 00H, 0AH, 07H and 11H on the track, and the other 14 IDs carry the sector number FFH. ENTER at Cylinder count formats one cylinder (590DH), so Format with ENTER at every question after the drive formats cylinder 0 alone and returns to the menu. The decimal numbers take a character from 21H to 2FH as a digit of value 241-255 (5948H).

5200H - DISKZAP

DISKZAP/CMD's transfer address, entered from the DOS command level as DISKZAP (no parameters). DISKZAP is a menu driven sector editor: Set (the geometry of a drive), Fill, Copy, Print, Verify, Format and Display (with the modify mode). It keeps its own geometry for each of the eight drives (5B04H) and puts it into the drive's DCT when the drive or cylinder 0 is chosen. It needs HMEM$ at 89F8H or above, else it goes back to the DOS at once; the room from 5F00H up to HMEM$ is the buffer of Copy.

5219H - The Main Menu

The stack is set again and 5219H pushed as the return address of every option, so each one ends back here; the screen is cleared and the menu shown, with the asterisk at the line kept at 5D1BH (1 Set to 7 Display). The arrows move it, ENTER runs the option, O goes back to the DOS. BREAK at any prompt of an option also comes back here (57A3H).

527CH - Display

Line 7: the drive, the cylinder and the sector are asked for, the sector is read into 5E00H (an error asks continue ?) and shown. Keys: semicolon the next sector, minus the one before, plus the next cylinder, equal sign the cylinder before (each wrapping round the drive's geometry), F flashes every byte of a value, M the modify mode, CLEAR hexadecimal or characters, BREAK the menu.

52D4H - F: Flash a Value

The value is one key in character mode, else a hexadecimal number typed (not shown) and ended by a key that is not a digit; its low byte is looked for in the sector, and every place that holds it is shown as two graphic blocks or as the byte, in turn, every 512 scans of the keyboard, until a key is pressed. Then the sector is shown again.

5328H - The Sector and the Cylinder Before and After

5328H the sector before (below the first sector, DCT+08H, the last of the cylinder before), 5348H the cylinder before (below 0 the last, DCT+13H less 1), 5365H the next sector (past the last, DCT+08H plus DCT+11H less 1, the first of the next cylinder), 5382H the next cylinder (past the last, 0). On reaching or leaving cylinder 0 the geometry is put into the DCT again (595CH). The sector is at 5D2FH, the cylinder at 5D30H.

53A2H - The Modify Mode

Two graphic blocks show the byte at the cursor, with its offset at 3D00H and its value at 3D40H. The arrows move the cursor (with SHIFT to the end or the start of the row, SHIFT up arrow to the first byte; the table at 541BH), wrapping round the sector; a hexadecimal digit pair (or in character mode a key) changes the byte; @ fills; CLEAR switches the display; ENTER writes the sector back with its own address mark and returns to the display; BREAK reads the sector again and returns to the display.

5431H - Fill, CLEAR, ENTER and BREAK

@ takes a hexadecimal number (not shown, up to four digits, 545EH): the high byte is the value and the low byte the count, 00H for the rest of the sector; the bytes from the cursor are filled, wrapping round the sector. 5444H and 5447H change a flag between 00H and FFH. ENTER writes the sector (59CFH) and, on an error, asks continue ? (Y or C writes again). BREAK drops the loop's return address and reads the sector again (5285H).

545EH - A Hexadecimal Number Typed

Up to four keys into 5D25H, not shown, ended early by a key below 21H or one that is not a hexadecimal digit; 5920H takes the number, which is returned in Register Pair BC. With four digits the number goes on with what follows in 5D25H.

547DH - The Screen Place of a Byte

Register Pair HL = the video address of byte Register E of the sector: row E/16 of the screen (3C00H plus 64 for each row) and column 8 plus 2.5 times E's low digit, as 56DBH shows the bytes.

5499H - Set

Line 1: the drivespec, then the six values of that drive's entry in the table at 5B04H: Cylinder count (decimal), Surface count, CYL 0 sec/trk, CYL 0 density (D double, any other answer single), Sectors/track and Track density. ENTER keeps a value, 0 asks again, BREAK goes back to the menu; only the low byte of a number is kept. The DCT gets the new values when the drive or cylinder is next chosen (595CH).

552BH - Fill

Line 2: drive, cylinder, sector, sector count (decimal, ENTER 1) and Fill data (hexadecimal). The buffer at 5E00H is filled with the low byte of the value and written to each sector with the normal address mark, the status line showing each one. ENTER at Fill data leaves Register E as the last prompt left it, the low byte of the sector count, so the sectors are filled with that byte, not 00H. An error is shown and a key returns to the menu.

5562H - Copy

Line 3: the source drive, cylinder and sector, the destination drive, cylinder and sector, and the sector count. The two positions are swapped by 55DDH (the other one kept at 5D2BH-5D2DH). Each pass reads as many sectors as the pages above 5F00H hold (5D24H), keeping each one's address mark at 5E00H on, then writes them to the destination with the same marks. A read error asks continue ?; a write error is shown and a key returns to the menu.

55EFH - Print

Line 4: drive, cylinder, sector and sector count. Each sector is read and shown, then the 16 rows of 64 characters of the screen are sent to the printer by the ROM's 003BH (a byte of 80H or above as a period), each row ended by a carriage return and the sector by a line feed. A read error asks continue ?.

562CH - Verify

Line 5: drive, cylinder, sector, sector count and Ignore data AM. Each sector is read into 5E00H (DREAD). A read error is shown and waits for a key (BREAK the menu, any other key goes on); with N to Ignore data AM a sector with the directory's address mark shows AM/WRITE FAULT! and waits too. Holding BREAK ends the run (57C5H).

566EH - Format

Line 6: drive, cylinder, cylinder count (decimal, ENTER 1) and Interleave factor. A rigid drive shows DRIVE NOT SUPPORTED! and asks for the drive again. The drive is sent to cylinder 0 (DHOME), then each cylinder is formatted (59EFH) with the drive's geometry from the table, both sides when it has two. With ENTER at Interleave factor Register H keeps 5DH from the address of the answer (5D25H), so the interleave is 93.

56B7H - Keep the Registers

Called at the start of a routine: the rest of the routine, from the return address, is called as a subroutine (the address stored into the CALL at 56C1H) with Register Pairs BC, DE, HL and Index Register IX saved; Register A and the flags come back as the routine left them.

56CAH - A Hexadecimal Digit

Register A, a character (kept in Register B by the entry at 56CAH), as a digit 0-9 or A-F: the NC FLAG and its value 00H-0FH in Register A, else the C FLAG.

56DBH - Show the Sector

The sector at 5E00H fills the 16 rows of the screen: on each row four columns at the left (blanked only on the first display, 5D18H 00H), the offset and a colon, eight groups of a space and two bytes (four hexadecimal digits, or two characters each with a space), a space and the 16 characters (spaces in character mode). Then the drive's number at 3C00H, the cylinder at 3C40H and the sector at 3C80H, in hexadecimal, in those columns.

576DH - Characters and Digits on the Screen

576DH the byte at Register Pair HL (576EH Register A) as a character at Register Pair DE: below 20H or C0H and above as a period. 577AH Register A as two hexadecimal digits at Register Pair DE. 578BH Register A at Register Pair DE. Register Pair DE ends after them.

578EH - The Keyboard and the Display

578EH waits for a key and makes a lower case letter upper case (5793H only the latter); 579CH scans the keyboard once (the NZ FLAG for a key); 57A3H the ROM's line input, BREAK going back to the menu; 57AEH clears the screen; 57B5H displays and 57BDH prints Register A, keeping Register Pair DE.

57C5H - The Count, the Status Line and the Errors

57C5H steps to the next sector, 57C8H counts one fewer at 5D1FH: the Z FLAG when the count is done or BREAK is pressed. 57DAH the status line Drive $nn, Cylinder nn, Sector nn on row 11. 57FFH clears the screen, shows the status line and the error's text and asks continue ? (the Z FLAG for Y or C); 5825H shows the error and waits for a key; 582BH shows the text of error code Register A, reduced to 1-8 by ((code less 1) AND 07H) plus 1, from the table at 5CAAH.

5846H - Texts and Prompts

5846H shows the text at Register Pair HL at the start of row 11 (the cursor 4020H set to 3EC0H) and clears the rest of the row; 5857H displays a text up to its 00H. 5860H shows a prompt as 5846H does, then a space, a question mark and a space, and takes up to Register B characters into 5D25H: the Z FLAG for ENTER alone.

587CH - The Questions

587CH Drivespec: ENTER is drive 0, else the answer's two characters (the second a space when the answer has one) are looked for among the names in DCTTBL (44E8H); not found asks again; the geometry is put into the DCT (595CH). 58BBH Cylinder and 58D4H Sector in hexadecimal (ENTER 0); 58EAH Sector count and 5905H Cylinder count in decimal (ENTER 1, 0 asks again). A bad number asks again.

5920H - Numbers

5920H a hexadecimal number at Register Pair HL into Register Pair DE, 593EH a decimal one: each ends at a character below 21H with the C FLAG, or at a bad character with the NC FLAG. In 593EH a character from 21H to 2FH is not refused (5948H): it is taken as a digit of value 241-255.

595CH - The Geometry into the DCT

The drive's entry in the table at 5B04H (59A0H) goes into its DCT, unless the DCT's type byte has bit 3 (no drive): DCT+13H the cylinders, DCT+0CH the surfaces, and for cylinder 0 (5D30H) the first pair, else the second: bit 7 of DCT+0BH the density, DCT+0DH the sectors per track, DCT+11H the sectors per cylinder (sectors times surfaces by 4491H). Bit 0 of DCT+03H is set, so the DOS logs the diskette at its next use.

59B0H - Read, Write and Home

59B0H reads the sector at 5D2FH (sector) and 5D30H (cylinder) of the drive into 5E00H, 59B3H into Register Pair HL, by DREAD; the code 06H (the directory's address mark) counts as good and sets 5D1EH to 01H, else 00H. 59CFH writes 5E00H, 59D2H Register Pair HL, by DWRITE (05H) or, when 5D1EH is 01H, DWRITA (08H). 59E9H DHOME.

59EFH - Format a Cylinder

Each side of cylinder Register D (Register E 00H, then DCT+0DH for the second side when DCT+0CH is 2). 5A03H: the track's number (the cylinder, or twice it plus the side), the first sector's place (twice the interleave Register H times the track, modulo the sectors per track), the sector order at 5E00H (each sector the interleave further on, past the end once round, or the next free place), the track built at 5F00H from the description at 5AD8H (single density) or 5AEEH (double): count and byte pairs, the ID (FEH, cylinder, side, sector, 01H, F7H), the data mark FBH and 256 bytes of 6CH, F7H; then DFORMT. An interleave greater than the sectors per track can place a number outside the order's table.

5AD8H - The Track Descriptions

Pairs of a count (00H for 256) and a byte, taken by 5ACEH: three at the start of the track, then for each sector two before the ID, three after it and one after the data, then two at the end of the track. 5AD8H single density, 5AEEH double density (F5H makes the controller write the A1H sync bytes).

5B04H - The Geometry of the Drives

Six bytes for each of the eight drives, as loaded 40 cylinders, one surface and 18 sectors of double density on every cylinder: +00H the cylinders, +01H the surfaces, +02H the density of cylinder 0 (bit 7 double), +03H its sectors per track, +04H the density of the other cylinders, +05H their sectors per track. Set changes them; 595CH puts them into the DCT.

5B34H - The Texts

The menu (shown after the codes 1CH 1FH that clear the screen), the prompts, the status line, continue ? and the texts of the errors, each ended by 00H (the menu by 0DH for each line).