Percom DoubleDOS 4.24 for the Model I - DOUBLE/CMD Disassembled

DOUBLE/CMD

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Percom DoubleDOS 4.24 DOUBLE/CMD Disassembly (Model I)

DOUBLE/CMD is the DBLDOS FORMAT/BACKUP UTILITY of DoubleDOS (DBLDOS in Percom's manual), version 1.21. It takes the place of the FORMAT and BACKUP commands, which DoubleDOS does not have (manual 3.3): DOUBLE FORMAT :n formats a diskette, DOUBLE BACKUP :s TO :d copies one, with Tnn to give the copy more cylinders than the source. It is the only program of the system diskette that formats a track: SYS0/SYS has no such routine, so DOUBLE/CMD drives the controller and the Doubler itself, through the registers 37ECH-37EFH and the drive latch 37E1H, with the interrupts off from start to end. From SYS0/SYS it uses only the display routine 4467H, the date text 4470H, the multiply 4B6AH and the divide 4B84H, and the table of head positions 4300H-4303H.

The file is 2816 bytes: a name record, 22 load blocks (twenty of 128 bytes, then 101 and 47), 5200H-5C93H (2708 bytes), then the transfer address 5200H. Every loaded byte is on exactly one row below: 1867 bytes of code and 841 bytes of variables, tables and texts. It is the same in 4.20 and 4.23 (version 1.20) apart from the differences listed further down, and those of 4.24 are named on their rows.

Load Map

Addresses	Contents
5200H-5239H	The start, the choice of FORMAT or BACKUP, the ends (5200H, 5214H)
523AH-5392H	DOUBLE FORMAT (523AH, 52D1H)
5393H-54A3H	DOUBLE BACKUP and its variables (5393H, 53F4H, 5461H)
54A4H-5649H	Enlarge the GAT for Tnn, the diskette prompts, the read, write and verify passes, the command line routines (54A4H to 5647H)
5650H-5770H	Format a diskette, the format tables, build and write a track (5650H, 56BCH, 56FBH)
5771H-5880H	The controller: commands, waits, motor, seek, read and write of a sector or a track (577BH to 583FH)
5881H-5964H	Verify a cylinder with the flaw table, density switch, compare, digits, the two conversions of cylinder and logical track (5881H to 5949H)
5965H-5982H	The NO SYSTEM boot sector (5965H)
5983H-5C64H	The texts and the directory entry of DIR/SYS (5983H, 5C45H)
5C65H-5C93H	New in 4.24: the end of the passes at the source's size, the verify of extra cylinders, the 6DH B6H data (5C65H, 5C75H, 5C89H)
5D00H up	Not in the file: the buffer of the track images and of the backup passes, up to the top of memory (4049H)

DOUBLE FORMAT

DOUBLE FORMAT :n asks DISKETTE NAME? (1-8 characters), CREATION DATE? (only when no DATE has been given to DoubleDOS; eight characters with slashes in the third and sixth places) and NUMBER OF TRACKS (DEFAULT 40)? (two digits, 18-80, or ENTER for 40). It builds the GAT at 4D00H (52D1H), formats every cylinder (5650H), verifies every sector (5881H), then writes the NO SYSTEM boot sector to cylinder 0, the GAT, the HIT (byte 0 2CH) and the eight directory sectors with DIR/SYS in record 00H. FORMAT makes a data diskette only: it writes no SYS file, so a system diskette can only be made by DOUBLE BACKUP of one, which copies every sector to its place, the SYS files' fixed directory records included.

GAT byte	What FORMAT writes
00H to n-1	FCH, both granules free, for each of the n whole logical tracks of ten sectors on cylinders 1 to the last (n = (cylinders - 1) x 18 / 10, 61 for 35 cylinders, 70 for 40); then the bit of the granule of each flawed sector is set
n to BFH	FFH: logical tracks the diskette does not have are in use, so FREE counts only real granules
C0H-C9H	The flaw table: five entries of a cylinder and a sector, FFH when free
CAH	FFH
CBH	21H (but see bug B38)
CCH	n, the number of logical tracks
CDH	The number of cylinders
CEH-CFH	E0H 42H, the word 42E0H
D0H-D7H	The diskette name, spaces after it
D8H-DFH	The creation date MM/DD/YY
E0H-FFH	Spaces (the system diskette has 0DH at E0H: no AUTO command; a diskette made by FORMAT has no system, so it is never booted)

DOUBLE BACKUP

DOUBLE BACKUP :s TO :d [Tnn] asks for the SOURCE and the DESTINATION diskettes, reads the source GAT into 4D00H, formats the destination with the source's number of cylinders (GAT byte CDH), or with Tnn when that is larger (at most 80; 54A4H makes the added logical tracks free), then copies cylinder 0 in single density and cylinders 1 to the source's last in passes. With two drives a pass is one cylinder; with one drive it is as many sectors as there are whole pages from 5D00H to the top of memory (at least 18, else INSUFFICIENT MEMORY FOR 1 DRIVE BACKUP), and the diskettes are asked for before each read and each write. Each pass is read (5532H), written (558DH; the sectors of logical track 17 with the deleted data mark of the directory) and read back (55BBH). In 4.24 the destination's cylinders beyond the source are then verified for flaws (5C75H). Last, the GAT image, the source's with the new size, flaws and today's date, is written to the destination. The source's name, files and flaw table are kept: the copy is the mirror image the manual describes.

A sector of the source that cannot be read in four tries (two, a step in and out, one, a Restore, one: 57CBH) shows - SOURCE READ ERROR. When its cylinder and sector are in the source's flaw table the copy goes on; else the user is asked (C)ONTINUE, (R)ETRY, OR (A)BORT?. Any error on the destination ends the backup.

The Tracks DOUBLE/CMD Writes

Cylinder 0 is written in single density with ten sectors 0-9 in the order 0 5 1 6 2 7 3 8 4 9; cylinders 1 to the last in double density with eighteen sectors 0-17, on cylinder 1 in the order 0 6 12 1 7 13 2 8 14 3 9 15 4 10 16 5 11 17, each cylinder's order being turned four places from the last (5683H). Every sector is 256 bytes. The image of a track is built at 5D00H from the tables at 56BDH and 56CEH and written with the Write Track command F4H. The density is set by the Doubler's commands FEH (single) and FFH (double), each followed by D0H (58F7H).

Part	Single density	Double density (4.24)
After the index hole	31 x FFH	100 x 4EH (62 in 4.23)
Before the ID field	6 x 00H	12 x 00H, 3 x F5H (written as A1H)
ID field	FEH, cylinder, 00H, sector, 01H, CRC	FEH, cylinder, 00H, sector, 01H, CRC
After the ID field	11 x FFH, 6 x 00H	22 x 4EH, 12 x 00H, 3 x F5H
Data field	FBH, 256 bytes, CRC	FBH, 256 bytes, CRC
After the data field	11 x FFH	20 x 4EH (22 in 4.23)
End of the track	256 x FFH	256 x 4EH

The 256 data bytes are 6DH B6H repeated in 4.24 (5C89H) and E5H in 4.20 and 4.23.

The Controller Routines

Every command goes through 5797H, which first selects the drive again and, when the status shows NOT READY, waits about half a second for the motor. 577DH waits until BUSY ends, selecting the drive on each turn, and reports NOT READY after 65536 turns. Sectors are read by 57E4H and written by 583FH, both polling DATA REQUEST several times for each test of BUSY so that a double density byte is not missed. A sector written is verified by reading it with Read Sector 88H and no data taken: the controller then always reports LOST DATA, which the mask 98H leaves out, and NOT READY, RECORD NOT FOUND or a CRC error remain. The deleted data mark (F8H) of the directory, logical track 17, is written with Write Sector A9H; the read does not test the mark. Cylinders and sectors are turned into the logical tracks of ten sectors of DoubleDOS and back by 592DH and 5949H; the GAT is cylinder 10, sector 8.

Differences Between the Versions

Address	4.24 (version 1.21) against 4.20 and 4.23 (version 1.20)
59B4H	The version digit: 1 (1.21) for 0 (1.20).
53E1H, 5534H, 558FH, 55BDH, 5715H, 5762H	The buffer starts at 5D00H, where 4.23 had 5C65H, the first byte after its last load block; the room is taken by the new code 5C65H-5C93H.
546FH, 5C65H	4.23 had LD (54A1H),A; 4.24 calls 5C65H, which also makes the passes end at the source's number of cylinders. In 4.23 56BCH kept the destination's number while copying: with Tnn larger than the source and one drive, a pass that did not end exactly at the source's last cylinder went on reading cylinders the source does not have, and the end test at 5488H was then never met.
548BH, 5C75H	4.23 called 577BH (Restore); 4.24 calls 5C75H, which first verifies the destination's cylinders beyond the source's, locking out their flaws, and then makes the Restore. 4.23 never verified them.
574BH, 5C89H	4.23 called 5776H, writing 256 bytes E5H (Register A, loaded at 5749H) as the data of each sector; 4.24 calls 5C89H, 6DH B6H.
56D1H, 56DDH	The double density gaps: 100 bytes 4EH after the index hole for 62, and 20 bytes 4EH after each data field for 22.

Percom's patch memo DPM-0001 (September 29, 1980) is for DOUBLE version 1.00, whose BACKUP of an enlarged diskette copied only 35 cylinders; the memo warns that version 1.20, reassembled on September 13, 1980, must not be patched. No set in hand has version 1.00; versions 1.20 and 1.21 take the size of the copy from the source's GAT byte CDH (5410H), which an enlarged diskette carries.

Bugs

No. Address	What happens
B37 5585H	A source sector that cannot be read leaves Register Pair BC on its place in the buffer (57E4H moves it on only after a good read). When the copy goes on, for (C)ONTINUE or because the sector is in the flaw table, the next sector is read into that place: every following sector of the pass is written one sector early on the destination, and the pass's last sector gets whatever the buffer held after the data read. The manual says that with C only the bad sector's data may differ.
B38 58BAH	The verify after FORMAT turns a flawed sector of cylinder 0 into a logical track with 592DH, which counts from cylinder 1: cylinder 0 gives logical track CBH. A flaw in sectors 5-9 of cylinder 0 sets bit 1 of GAT byte CBH (21H becomes 23H); one in sectors 0-4 sets bit 0, already set. The flaw also takes one of the five places of the flaw table, though cylinder 0 holds no granule.
B39 5663H	The jumps on the NZ FLAG after 56FBH (5663H, 567CH) and after 583FH (576AH) are never taken: 583FH jumps to 56A8H itself on an error and returns only with the Z FLAG.

Every bug found in DoubleDOS is listed, with what a user would do to set it off, on the DoubleDOS reference page.

Differences From the Manual

• The manual (3.3.1) writes the command as DOUBLE FORMAT N and gives the prompts DISK NAME?, CREATION DATE? and HOW MANY TRACKS?. DOUBLE/CMD needs :n, and its prompts are DISKETTE NAME?, CREATION DATE? (asked only when DoubleDOS has no date) and NUMBER OF TRACKS (DEFAULT 40)?; a one digit answer is asked again.
• The manual says FORMAT and BACKUP work totally in double density; cylinder 0 is formatted, verified and copied in single density, ten sectors.
• The manual says a destination smaller than the source cannot be given: a Tnn not larger than the source is not used, and the copy gets the source's size. A Tnn above 80 is taken as 80.
• The manual writes TOO MANY FLAWS, USE ANOTHER DISK!; the text is TOO MANY FLAWS - USE ANOTHER DISK!.
• With (C)ONTINUE the manual expects only the bad sector to differ; see B37.
• The test for enough memory is made for a backup with two drives too, though its message names one drive.

Notes

• The image of a double density track takes 6404 bytes from 5D00H, up to 7603H; nothing tests that the top of memory lies above it. On a Model I with 16K or more the top is 7FFFH or higher.
• The wait for the second byte of a write (5854H) does not test BUSY; a command that ended after the first byte would hang there.
• The single key typed at a prompt is stored at 5983H, over the banner, which is not shown again.

Places in DOUBLE/CMD

Address Size	Contents
5239H 1 byte	Drive 0 Used Bit 0 set when drive 0 is formatted or is the source or destination; the end then asks for the DBLDOS diskette.
549DH-549EH 2 bytes	The Select Bits of the Source and the Destination
549FH 1 byte	Two Drives 00H while each diskette must be asked for, else the two select bits exclusive ORed.
54A0H-54A2H 3 bytes	The Sectors of a Pass The pages of memory from 5D00H, the size of the pass, and the count left.
54A3H 1 byte	The Cylinders of the Source
56BCH 1 byte	The Cylinders to Format, Verify or Copy
56E9H-56FAH 18 bytes	The Double Density Sector Order Turned four places after each cylinder formatted.
576FH-5770H 2 bytes	The First Pair of a Sector in the format table being used.
5B29H, 5B2EH, 5B3EH, 5B48H, 5BAFH	Places in the Texts The digits of the cylinder being formatted, the word of the progress text, its cylinder and sector digits, the drive digit of the prompt.
5983H-5984H 2 bytes	The Key Buffer The start of the banner, used by 0040H for a single key.
5C76H 1 byte	The Cylinders of the Destination The operand of LD C at 5C75H, stored by 5C6BH.

Places Outside DOUBLE/CMD

Address Size	Contents
37E1H 1 byte	Drive Select Latch Written from 4309H by 57B7H.
37ECH-37EFH 4 bytes	The Controller Command and status, track, sector and data registers; the Doubler takes the commands FEH and FFH.
4045H, 4044H-4046H 3 bytes	The Date of DoubleDOS A day of 00H means no date set.
4049H-404AH 2 bytes	The Top of Memory
4300H-4303H 4 bytes	The Head Positions of Drives 0-3 Kept by 54DEH when the drive changes, as SYS0/SYS keeps them.
4308H, 4309H 2 bytes	The Drive in Use and Its Select Bit
4D00H-4DFFH 256 bytes	The GAT Image Built by FORMAT or read from the source; also the buffer of the HIT and the directory sectors of FORMAT and of the typed answers. 4DC0H-4DC9H is the flaw table.
5D00H up	The Buffer The image of a track, or the sectors of a backup pass.

Outside Routines

Address	Use
0033H	ROM: Display a Character
0040H	ROM: Line Input Up to Register B characters at Register Pair HL; Register B returns the count.
4467H	SYS0/SYS: Display a Text up to 03H or 0DH.
4470H	SYS0/SYS: The Date as Text MM/DD/YY at Register Pair HL.
4B6AH	SYS0/SYS: Multiply Register Pair HL by Register A.
4B84H	SYS0/SYS: Divide Register Pair HL by Register A.
402DH	DOS READY

5200H - Start: Show the Banner and Choose FORMAT or BACKUP

The transfer address of DOUBLE/CMD, which SYS1/SYS runs through @RUN (4433H) for the command DOUBLE, with Register Pair HL past the space after the name (SYS1/SYS 4E80H). The interrupts are turned off for the whole run and turned on again only at 5235H, so the clock task of SYS0/SYS stops while DOUBLE works. The first letter of the rest of the line picks the job: F for DOUBLE FORMAT, B for DOUBLE BACKUP; anything else is COMMAND SYNTAX ERROR.

5214H - The Ends of DOUBLE: Error, Abort and Completion

5214H shows COMMAND SYNTAX ERROR; 5217H shows the error text at Register Pair HL; both go on to DOUBLE DENSITY UTILITY ABORTED (521AH). 521DH is the end of a finished job, with FORMAT COMPLETED or BACKUP COMPLETED in Register Pair HL. Every way out puts the Doubler back in double density and, when drive 0 took part, asks for the DBLDOS diskette in drive 0 before DOS READY, as the manual says (3.3.1).

523AH - DOUBLE FORMAT: Ask for the Name, the Date and the Tracks

The form is DOUBLE FORMAT :n with n the drive 0-3; the manual writes DOUBLE FORMAT N (3.3.1) and gives the prompts as DISK NAME?, CREATION DATE? and HOW MANY TRACKS?, but the texts here are DISKETTE NAME?, CREATION DATE? and NUMBER OF TRACKS (DEFAULT 40)?. The GAT of the new diskette is built at 4D00H: the name at 4DD0H, the date at 4DD8H (the date of DoubleDOS when one is set, else typed), the track count 18-80 at 56BCH. No test is made that the diskette is blank, as the manual warns.

52D1H - DOUBLE FORMAT: Build the GAT, Format and Verify

With the number of tracks in Register A the GAT image at 4D00H is finished: two free granules (FCH) for every logical track that fits on cylinders 1 to the last, FFH from there to CFH, so that FREE counts only real granules and the flaw table 4DC0H-4DC9H starts empty (FFH); CBH 21H, CCH the logical tracks, CDH the cylinders, CEH-CFH 42E0H. Then every track is formatted (5650H) and every sector verified (5881H): a flawed sector locks its granule out and goes into the flaw table.

5393H - DOUBLE BACKUP: Read the Drives and the Track Count

The form is DOUBLE BACKUP :s TO :d, then optionally Tnn, two digits, the cylinders of the destination (manual 3.3.2 and 3.3.2.2). The select bits of the source and the destination go to 549DH and 549EH; Tnn goes to 56BCH (00H without it). Every pass reads into a buffer from 5D00H up to the top of memory (4049H): its size in pages, 54A0H, must be at least 12H (18 sectors, one double density cylinder), else INSUFFICIENT MEMORY FOR 1 DRIVE BACKUP, a test made for two drives as well, which also read one cylinder a pass.

53F4H - DOUBLE BACKUP: Read the Source GAT, Format the Destination, Copy Cylinder 0

The source GAT is read into 4D00H, where it stays as the GAT of the destination. The destination gets the source's number of cylinders (GAT byte CDH), or Tnn when that is larger (54A4H enlarges the GAT image for it), and today's date of DoubleDOS when one is set; the name stays. Every cylinder of the destination is formatted, then cylinder 0 is copied in single density, ten sectors in one pass. The prompts for the source and the destination before the format are shown even with two drives; 549FH is set only after them.

5461H - DOUBLE BACKUP: Copy Cylinders 1 to the Last

Each pass reads a number of sectors from the source into 5D00H, writes them to the destination and reads them back: eighteen (one cylinder) with two drives, else as many as memory holds (54A0H), with the diskettes changed in between. 5C65H (new in 4.24) makes the passes stop at the source's last cylinder; 5C75H then verifies the destination's extra cylinders of Tnn. Last, the GAT image is written as the GAT of the destination.

54A4H - Enlarge the GAT Image for Tnn

Called from 5417H when Tnn is larger than the source. Tnn is held to 80 cylinders at most; the logical tracks the extra cylinders add are made free (FCH) after the source's last one, and GAT bytes CCH and CDH get the new counts. The manual: the destination is formatted for the larger size and the GAT enlarged by the extra granules (3.3.2.2).

54D0H - Select the Source or the Destination and Ask for Its Diskette

54D0H takes the source (549DH), 54D8H the destination (549EH), 54DEH any drive whose select bit is in Register A, with Register Pair HL the word to show. The track register of the controller belongs to the drive in use, so the head position of the old drive is kept in its byte of 4300H-4303H (the table of SYS0/SYS) and that of the new drive put in its place; 4308H and 4309H get the drive and its select bit. When 549FH is 00H the text MOUNT word DISK IN DRIVE n AND PUSH "ENTER" is shown and a key awaited.

5527H - Find the Byte of a Drive in 4300H-4303H

Called with a select bit in Register A (01H, 02H, 04H or 08H). Returns Register Pair HL 4300H plus the drive number and Register D the drive number.

5532H - Read a Pass from the Source

Reads 54A1H sectors from cylinder Register D, sector Register E on into the buffer at 5D00H, each with up to four tries (57CBH), showing READING TRACK nn SECTOR nn in place. A sector that cannot be read shows - SOURCE READ ERROR; when it is in the flaw table of the GAT image (4DC0H-4DC9H) the pass goes on, else the user is asked (C)ONTINUE, (R)ETRY, OR (A)BORT?. On (C)ONTINUE, and for a sector of the flaw table, the buffer place of the bad sector is not passed over: the next sector is read into it, so the rest of the pass is written one sector early (a bug: the Bugs part of the introduction). Returns Register Pair DE past the last sector.

558DH - Write a Pass to the Destination

Writes 54A1H sectors from 5D00H to cylinder Register D, sector Register E on, showing WRITING TRACK nn SECTOR nn. Sectors of logical track 17, the directory, get the deleted data mark (Write Sector A9H), every other sector the ordinary mark (A8H). The sectors are read back afterwards by 55BBH; a write error ends at 56A8H.

55BBH - Verify a Pass on the Destination

Reads back the 54A1H sectors written from cylinder Register D, sector Register E on with the Read Sector command 88H and no data taken, showing VERIFYING TRACK nn SECTOR nn. The controller ends each command with LOST DATA, which the mask 98H leaves out: NOT READY, RECORD NOT FOUND and a CRC error end the backup at VERIFY ERROR, as the manual says of any error on the destination (3.3.1). Returns Register Pair DE past the last sector.

55E3H - Next Sector of a Pass

Moves cylinder Register D, sector Register E to the next of the eighteen sectors of a cylinder and counts 54A2H down. Returns the Z FLAG when the count reaches 0, or when the next cylinder is 56BCH, the end (from 4.24 the source's number of cylinders while copying, 5C71H).

55F9H - Show the Progress of a Pass

Copies the six characters at Register Pair HL (two spaces and READ, two spaces and WRIT, or VERIFY) over the start of VERIFYING at 5B2EH and puts the two digits of cylinder Register D at 5B3EH and of sector Register E at 5B48H, then shows the line from 5B2DH, which starts with 1DH (the cursor to the start of the line), so that each sector's line is written over the last.

561EH - Read :n, the Drive Number

Called with Register Pair HL on the command line. Spaces are passed over; a colon and a digit 0-3 must follow. The select bit of the drive (01H, 02H, 04H or 08H) is stored at 4309H and returned in Register A, with Register Pair HL on the digit. Anything else ends at INVALID DRIVE NUMBER.

563FH - Pass Over Spaces

Called with Register Pair HL on the text. Returns Register Pair HL on the first character that is not a space, with that character in Register A.

5647H - Read a Digit

Called with Register Pair HL one before the character. Moves Register Pair HL on to it and returns its value 0-9 in Register A with the NC FLAG, or the C FLAG when it is not a digit.

5650H - Format the Diskette

Formats 56BCH cylinders of the selected drive: cylinder 0 in single density with ten sectors (the table at 56BDH), then each of cylinders 1 to the last in double density with eighteen sectors (56CEH), stepping in by one each time. After each double density cylinder the sector order at 56E9H-56FAH is turned by four places, so that sector 0 of the next cylinder comes four places later. A drive not ready or write protected at the first Restore ends the job at 56A8H.

56A8H - Name a Disk Error and Abort

Reached with the status of the controller in Register A. Bit 7 (NOT READY) gives DRIVE INOPERATIVE OR DOOR OPEN, bit 6 (WRITE PROTECT) DISK IS WRITE PROTECTED, anything else UNEXPECTED DISK I/O ERROR (56B6H, also reached from 5409H when the source GAT cannot be read). Each ends at 5217H, DOUBLE DENSITY UTILITY ABORTED.

56BCH - The Format Tables

56BCH holds the number of cylinders. Then the table of a single density cylinder (56BDH) and of a double density cylinder (56CEH), each a sector count, the address of its sector order, and pairs of a count and a byte that 56FBH writes into the track image: one pair before the first sector, then six pairs for each sector around its ID field and its data. The single density sector order (56DFH) is 0 5 1 6 2 7 3 8 4 9; the double density one (56E9H) starts 0 6 12 1 7 13, as found on the 4.24 diskette, and is turned by 5683H after each cylinder. Two counts of the double density table differ in 4.23: 3EH at 56D1H and 16H at 56DDH.

56FBH - Build and Write the Image of a Cylinder

Called with Register Pair HL a format table (56BDH or 56CEH) and the head on the cylinder. FORMATTING TRACK nn is shown; the image is built at 5D00H (5C65H in 4.23): the first pair, then for each sector two pairs, the ID field (FEH, the cylinder, side 00H, the sector from the order, size 01H for 256 bytes, F7H for the CRC), three pairs, the data mark FBH, 256 bytes of data, F7H and the last pair; then 256 bytes of the gap byte. The Write Track command F4H writes it. The caller's registers are kept in the alternate set.

5771H - Write a Pair of the Format Table

5771H takes a count and a byte from Register Pair DE and writes the byte count times at Register Pair HL; 5776H writes Register A Register B times. Register Pair HL returns past the bytes written and Register B 00H.

577BH - Give a Command to the Controller and Wait

577BH gives the Restore command 03H; 577DH gives the command in Register A. The controller is waited for until it is no longer busy, the drive being selected again on each turn so that its motor runs; when it stays busy for 65536 turns bit 7 (NOT READY) is set in the status returned in Register A.

5797H - Give a Command

Called with the command in Register A. The drive's motor is made to run (57A4H), the command goes to 37ECH, and two pushes and pops give the controller time to set BUSY before its status is read.

57A4H - Select the Drive and Wait for Its Motor

When the status at 37ECH has bit 7 (NOT READY) clear, the drive is only selected again. Else the drive is selected, a loop of 65536 turns gives the motor time to reach speed, and the drive is selected once more.

57BEH - Put the Head on a Cylinder

Called with Register D the cylinder and Register E the sector. Both go to the controller (Register E to the sector register 37EEH, Register D to the data register 37EFH); when the track register 37EDH is already Register D nothing more is done, else the Seek command 13H moves the head.

57CBH - Read a Sector with Retries

Reads cylinder Register D, sector Register E into the buffer at Register Pair BC: two tries, then the head is stepped in and out by one cylinder without changing the track register (43H, 63H) and a third try, then a Restore and a fourth try (57E1H falls into 57E4H). Returns the Z FLAG and Register Pair BC 256 higher, or the NZ FLAG and Register Pair BC unchanged.

57E4H - Read a Sector

Reads cylinder Register D, sector Register E into the buffer at Register Pair BC with the Read Sector command 88H. The data register is read whenever DATA REQUEST is set, testing it three times for each test of BUSY so that a double density byte is not lost. The status is masked with 9CH (NOT READY, RECORD NOT FOUND, CRC ERROR, LOST DATA); the data mark (bit 5) is not tested, so the deleted mark of the directory reads as any other.

581AH - Write a Sector and Read It Back

581AH writes the 256 bytes at 4D00H with the deleted data mark (Write Sector A9H), for the directory; 5825H writes the bytes at Register Pair BC with the ordinary mark (A8H). Register D is the cylinder and Register E the sector. The sector is then read with 88H and no data taken; NOT READY, RECORD NOT FOUND or a CRC error ends at VERIFY ERROR.

583FH - Write to the Controller

Gives the write command in Register A (Write Sector A8H or A9H, Write Track F4H) and sends bytes from Register Pair BC to the data register 37EFH whenever DATA REQUEST is set, testing it six times for each test of BUSY, until the controller ends the command; Register Pair BC returns past the bytes sent. The wait for the second byte (5854H) has no test of BUSY. An error bit in the final status (mask FCH) ends at 56A8H; the return is always with the Z FLAG.

5881H - Verify a Cylinder and Lock Out Its Flaws

Reads every sector of the cylinder the head is on (ten on cylinder 0, else eighteen) with Read Sector 88H and no data taken, showing VERIFYING TRACK nn SECTOR nn. For a sector that does not read back (NOT READY, RECORD NOT FOUND, CRC ERROR) the bit of its granule is set in the GAT image at 4D00H (the five sectors 0-4 of a logical track are granule 0, bit 0; sectors 5-9 granule 1, bit 1) and its cylinder and sector go into the first free entry of the flaw table 4DC0H-4DC9H, with - ERROR, GRANULE LOCKED OUT; a sixth flaw ends at TOO MANY FLAWS - USE ANOTHER DISK!. Cylinder 0 has no logical tracks: the conversion makes it logical track CBH, so a flaw in sectors 5-9 of cylinder 0 sets bit 1 of GAT byte CBH (a bug: the Bugs part of the introduction).

58F7H - Switch the Doubler to Single or Double Density

58F7H gives the Doubler's command FEH (single density), 58FBH its command FFH (double density), each followed by the Force Interrupt D0H. The track register 37EDH is read before and written back after.

5913H - Compare a Word

Called with Register Pair DE a word ended by 00H (FORMAT, BACKUP or TO) and Register Pair HL the command line. Every character must be the same; then Register Pair HL is past the word. A difference ends at COMMAND SYNTAX ERROR. What follows the word is not tested here, so FORMATX passes as FORMAT.

591EH - Turn a Number into Two Digits

Called with Register A 0-99. Returns Register C the tens digit and Register B the units digit as ASCII characters, so that LD (nnnnH),BC stores them in reading order.

592DH - Cylinder and Sector to Logical Track and Sector

Called with Register D a cylinder and Register E a sector of it. The sectors from cylinder 1, sector 0 are counted, (cylinder - 1) x 18 + sector, and divided by ten: Register D returns the logical track and Register E the sector of it, 0-9. Register Pairs BC and HL are kept. Multiplies and divides with 4B6AH and 4B84H of SYS0/SYS.

5949H - Logical Track and Sector to Cylinder and Sector

Called with Register D a logical track and Register E a sector of it (0-9). The sector counted from cylinder 1, sector 0 is Register D x 10 + Register E; divided by 18 it gives Register D the cylinder (quotient plus one) and Register E the sector. Register Pairs BC and HL are kept.

5965H - The Boot Sector of a Diskette Without a System

256 bytes from 5965H to 5A64H are written by FORMAT to cylinder 0, sector 0 (532DH). The ROM loads them at 4200H and runs them: the text at 4210H (clear the screen, 32 characters a line, 40 spaces, NO SYSTEM) is shown, a line is awaited, and the processor halts until RESET. Only the first 30 bytes are used; the rest of the sector is the banner and the texts that follow in DOUBLE/CMD (5983H-5A64H), never run.

5983H - The Texts

The banner, the messages and prompts, the words compared with the command line and the progress texts. 03H ends a text that leaves the cursor on its line; 1DH puts the cursor back at the start of the line, so that the progress texts write over themselves; 0AH is a line feed. 5983H-59DCH also serve as the buffer of a single key (5521H, 5572H) once the banner has been shown.

5C45H - The Directory Entry of DIR/SYS

The 32 bytes that FORMAT copies to the first entry of the first directory sector (5359H), directory record 00H: the same as the entry of DIR/SYS on the 4.24 system diskette.

5C65H - New in 4.24: Copy Only the Source's Cylinders

Called from 546FH with Register A the sectors of a pass. Besides the store at 54A1H that 4.23 made at 546FH, the number of cylinders of the destination (56BCH) is put into the operand of LD C at 5C75H and 56BCH is set to the source's number (54A3H), so that 55E3H ends the passes at the source's last cylinder. In 4.23, with Tnn larger than the source, 56BCH stayed at the destination's number: a pass with one drive that did not end exactly at the source's last cylinder went on reading cylinders the source does not have, and the test at 5488H then never found the end.

5C75H - New in 4.24: Verify the Destination's Extra Cylinders

Called from 548BH after the last pass, with the head of the destination on the source's last cylinder. While the head is not on the destination's last cylinder it steps in by one and 5881H verifies that cylinder, locking out the granules of its flawed sectors in the GAT image and the flaw table, which 5494H then writes to the destination. Last, the head is restored to cylinder 0 (as 4.23 did at 548BH). Without Tnn, or with a Tnn not larger than the source, the head is already on the last cylinder and only the Restore is made.

5C89H - New in 4.24: The Data of a Formatted Sector

Called from 574BH with Register Pair HL the data field in the image of the cylinder. Writes 128 times the two bytes 6DH B6H, 256 bytes in all; 4.23 wrote 256 bytes E5H through 5776H. The pattern stays in every sector that is not written later, so a diskette formatted by 4.24 has 6DH B6H in its unused sectors (the DOS identification page uses it to tell 4.24).