Percom DoubleDOS 4.24 for the Model I - SYS0/SYS Disassembled

SYS0/SYS

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Percom DoubleDOS 4.24 SYS0/SYS Disassembly (Model I)

SYS0/SYS is the resident part of DoubleDOS (DBLDOS in Percom's manual), the double density disk operating system Percom Data Company supplied with the Percom Doubler for the Model I. The boot sector loads it from cylinder 1, sector 0 on and jumps to its start-up at 4E00H. It stays in memory from 4300H to 4CFFH (with a few bytes at 400CH-405CH) and holds the vector table, the interrupt routine and the clock, the double density disk driver, the file routines and the space allocation, the overlay dispatcher and the program loader. Everything else (DOS READY and the commands, opening and closing files, the error messages, DEBUG) is in the overlays SYS1/SYS to SYS7/SYS, which the dispatcher loads at 4E00H (SYS6/SYS at 5200H, SYS7/SYS at 4D00H) when a request code asks for them.

The file is 3186 bytes: a name record, the copyright record of 169 bytes (MODIFIED FOR DOUBLE DENSITY OPERATION BY JAMES W STUTSMAN - COPYRIGHT (c) 1980 - ALL RIGHTS RESERVED ...), 29 load blocks of 2887 bytes and the transfer address 4E00H. Every loaded byte is on exactly one row below. The name record is 05H 06H 11H S Y S 0 and a space: its first name byte, 11H, is the third byte of the diskette's first sector, which 4B35H reads as the directory block (see Notes). The name record and the copyright record are not loaded and have no rows.

Load Map

Addresses	Contents
400CH-405CH	The RST 28H and RST 30H vectors, the DOS exits 402DH and 4030H, the device vector 4033H, the interrupt counter, mask and vectors (400CH)
4300H-4314H	The drive tables and system bytes (4300H)
4358H-43FBH	Code DoubleDOS added: the error hook, the keyboard driver, the block and sector conversion, the start of a transfer, the overlay entry check (4358H)
4400H-4478H	The vector table, with the Restore routine at 444BH (4400H)
4480H-44FFH	The file specification text, the system FCB, the error and DEBUG entries, the device dispatcher, the text display and print routines; 44EFH-44FFH is the last load block of the file (4480H)
4500H-45FDH	The task table, the interrupt routine, the BREAK vector, the task scheduler and the clock (4500H)
4600H-46FBH	The disk driver: drive select, seek, sector read and write with retries (4600H)
4700H-48DBH	The file routines @POSN, @BKSP, @REW, @PEOF, @READ, @WRITE, @VERF and byte I/O (4700H)
48DCH-4B96H	Finding a file's sectors, giving it granules, the directory and GAT routines, multiply and divide (48DCH)
4BA0H-4CFFH	The overlay dispatcher, @RUN, @LOAD and the load module loader, the clock display and trace tasks (4BA2H)
4E00H-4F89H	The start-up and the banner, overwritten by the overlays after it has run (4E00H)

How the Disk Is Addressed

DoubleDOS counts the sectors of a diskette from cylinder 1, sector 0 (cylinder 0 holds only the single density boot sector) in blocks of ten. A block is the unit the directory and the extents name: the routine at 43A5H turns block D and sector E into the linear sector 10 x D + E, then into cylinder 1 + (10 x D + E) / 18 and sector (10 x D + E) mod 18, for the 18 sectors of a double density cylinder. A block holds two granules of five sectors, the unit of space given to files. The directory is block 17 (11H) of a system diskette: sector 0 the GAT, sector 1 the HIT, sectors 2-9 the entries. Its sectors carry the deleted data mark F8H, which a read reports as error 06H; 4B35H treats that as success and, when a directory sector has the ordinary mark, takes the directory block from byte 2 of the diskette's logical sector 0.

How a Sector Is Read or Written

Step	What the code does
1	The entry (46DDH read, 46E6H write, 46EFH write with the deleted data mark, 46F3H read back) passes the command and four parameter bytes to 4671H: the tries, the error code base and the two instruction bytes of the transfer loop at 4698H.
2	The drive is selected with its own head position (4600H): the controller's track register is saved and loaded per drive in 4300H-4303H, and a drive whose motor was off gets a delay of about a second.
3	The sector and cylinder go to the controller (43A5H) and a Seek 1BH is issued when the head is elsewhere (4647H).
4	With the interrupts off, the command is issued (43C9H) and the bytes are moved by polling DATA REQUEST three times for each test of BUSY (4692H).
5	An error is tried again: LOST DATA at once, other errors after a Restore on a ready drive (444BH), at most three more times on a drive that is not ready (4C9BH). The error code is the base plus the number of the lowest error bit (46C8H).

Differences Between the Versions

The load blocks of SYS0/SYS are the same in DoubleDOS 4.20, 4.23 and 4.24 except for the version digit of the banner at 4F15H (30H, 33H, 34H). The name record differs: 4.20 has 05H 06H S Y S 0 and two spaces; 4.23 and 4.24 have 05H 06H 11H S Y S 0 and a space. That is the change of Percom's memo DPM-0003 (record 1, offset 2 of SYS0/SYS: 11H, then SYS0), for programs that read the directory: they take its block from byte 2 of the diskette's first sector, and 4.20 gave them 53H.

Notes

SYS0/SYS's Own Bytes

Address Size	Contents
4040H 1 byte	Interrupt Counter Incremented on every 25 ms interrupt (4577H); its low three bits choose task slot 0-7.
404BH 1 byte	Interrupt Latch Copy 37E0H as read at the last interrupt (4520H).
404CH 1 byte	Interrupt Mask Bits 7 (clock) and 6 (disk controller) set by the start-up.
404DH-405CH 16 bytes	Interrupt Vectors One word for each latch bit 0-7; 4059H = 45F7H, 405BH = 4560H, the rest 4537H (RET).
4300H-4303H 4 bytes	Head Position of Drives 0-3 The track register of each drive, swapped by 4600H.
4304H-4307H 4 bytes	Directory Block of Drives 0-3 11H; changed by 4B51H from byte 2 of logical sector 0.
4308H 1 byte	Drive in Use
4309H 1 byte	Select Bit of the Drive in Use Stored by 4621H, used by 445EH.
430AH-430BH 2 bytes	FCB of the Last File Call Stored by 489BH.
430CH-430DH 2 bytes	Return Address of the Last File Call Stored by 4898H; SYS4/SYS reads it.
430EH 1 byte	Request Code of the Overlay Last Loaded 02H at load (no overlay); compared by its low four bits (4BAAH).
430FH 1 byte	DEBUG Flag 01H after the start-up; bit 7 makes @RUN enter DEBUG (4C0EH).
4358H-435EH 7 bytes	Error Hook OR A, JP M,(435AH), JP (435DH); only never run code of BASIC/CMD (5444H) would set the addresses and point @ERROR here.
44A0H-44AFH 16 bytes	System FCB The FCB through which the dispatcher loads the overlays from drive 0, buffer 4200H.
4500H-4517H 24 bytes	Task Table Twelve slots, each the address of a task header; 45A3H is the empty header.
45AFH-45B1H 3 bytes	Clock Task Header Routine 45B2H and a count of runs; slot 7.
4CA9H-4CABH 3 bytes	Clock Display Task Header Routine 4CACH and a count of runs; installed by SYS6/SYS.
4CD9H-4CDAH 2 bytes	Trace Task Header Routine 4CDBH; installed by SYS1/SYS.

Operands Changed by the Code

Address Size	Contents
439AH 1 byte	Last Key The character the keyboard driver returned last (439BH).
43CEH 1 byte	Disk Command 88H, A8H or A9H, stored by 4671H.
444CH, 4657H 1 byte each	Step Rate Bytes The Restore and Seek commands, changed by the STEPnn/PAT patches.
45A6H-45A7H 2 bytes	Slot of the Running Task Stored by 457FH.
4645H 1 byte	SET n,A Second byte of the bit mask instruction (4640H).
4698H-4699H 2 bytes	Transfer Instructions From the parameters of 4671H (4681H).
46C9H 1 byte	Error Code Base 01H read, 09H write (467AH).
485EH 1 byte	Read Back Switch 00H @WRITE, 01H @VERF and full buffers in byte mode.
499AH, 49AAH, 49AEH 1 byte each	Place in the Extent, Sector in the Granule, Blocks After the Extent The adds of 4999H, 49A9H and 49ADH.
4ABEH 1 byte	Directory Record to Write Back Stored by 49BDH.
4B68H 1 byte	BIT n,B Second byte of the bit test (4B64H).
4BFCH-4BFDH 2 bytes	Overlay Transfer Address Stored by 4BE8H.
4C00H 1 byte	Saved BREAK Flag Stored by 4BF1H around an overlay call.
4C8DH-4C8EH 2 bytes	FCB of the Loader Stored by 4C39H.

The FCB

Offset Size	Contents
+00H 1 byte	Open Bit 7 set when the file is open (4892H).
+01H 1 byte	Flags Bits 0-2 the access level (reading needs below 6, writing below 5, loading below 7); bit 4 the buffer has been changed; bit 5 the buffer must be read; bit 6 positioned; bit 7 byte mode.
+03H-+04H 2 bytes	Buffer Address
+05H 1 byte	Byte Offset in the Buffer
+06H 1 byte	Drive
+07H 1 byte	Directory Record Bits 7-5 the entry in the sector, bits 4-0 the entry sector (4B1EH).
+08H 1 byte	End of File Offset
+09H 1 byte	Record Length 00H for 256.
+0AH-+0BH 2 bytes	Sector The next sector (sector mode) or the sector of the position (byte mode).
+0CH-+0DH 2 bytes	End of File Sector
+0EH-+1FH 18 bytes	Extents Five extents: the block and granule byte of the first at +0EH-+0FH, then for each later one the count of granules before it, its block and its granule byte (48DCH).

Error Codes Returned by SYS0/SYS

The messages in capitals are those SYS4/SYS shows for each code (it uses the low six bits; bit 6 has no effect in 4.24).

Code	Meaning and message (where set)
03H-08H	Read, by status bit 2-7: LOST DATA DURING READ, PARITY ERROR DURING READ, DATA RECORD NOT FOUND DURING READ, ATTEMPTED TO READ LOCKED/DELETED DATA RECORD, ATTEMPTED TO READ SYSTEM DATA RECORD, DEVICE NOT AVAILABLE (46C8H)
0BH-10H	Write, by status bit 2-7: LOST DATA DURING WRITE, PARITY ERROR DURING WRITE, DATA RECORD NOT FOUND DURING WRITE, WRITE FAULT ON DISK DRIVE, WRITE PROTECTED DISKETTE, and for NOT READY 10H, ILLEGAL LOGICAL FILE NUMBER (a bug, 46CDH)
10H	Bad directory record: ILLEGAL LOGICAL FILE NUMBER (4B31H)
11H, 12H	DIRECTORY READ ERROR, DIRECTORY WRITE ERROR (4AD1H, 4AEBH)
14H, 15H	GAT READ ERROR, GAT WRITE ERROR (4B00H, 4B19H)
1BH	DISK SPACE FULL (4A85H)
1CH, 1DH	END OF FILE ENCOUNTERED; past the end of file, RECORD NUMBER OUT OF RANGE (48D4H, 48D7H)
1DH	Also the end of the extent search at 44A0H (43F9H)
25H	ILLEGAL ACCESS ATTEMPTED TO PROTECTED FILE (4817H, 4847H, 4C33H)
26H	FILE NOT OPEN (48AFH)
58H	Overlay missing: 18H with bit 6, FILE NOT IN DIRECTORY (43E6H)
5FH, 62H-64H	@LOAD and the loader, with bit 6 set: PROGRAM NOT FOUND (4C25H), LOAD FILE FORMAT ERROR, MEMORY FAULT, ATTEMPTED TO LOAD READ ONLY MEMORY (4C5EH-4C87H)

Memory and Hardware Outside SYS0/SYS

Address Size	Contents
4012H-4014H 3 bytes	RST 38H Vector JP 4518H, stored by the start-up.
4016H-4017H 2 bytes	Keyboard Driver Address 4378H, stored by the start-up.
4036H-403CH 7 bytes	Keyboard Row Images The ROM's, used by the driver at 4378H.
4041H-4043H 3 bytes	Seconds, Minutes, Hours Counted by the clock task.
4044H-4046H 3 bytes	Date Written by 4CD2H from 4046H down.
4047H-4048H 2 bytes	Lowest Free Address 5200H (4E09H).
4049H-404AH 2 bytes	Top of Memory Found by the start-up (4E1DH).
4315H 1 byte	BREAK Flag Not 00H: BREAK goes to the vector at 4316H-4317H (454FH).
4318H-4357H 64 bytes	Command Line The AUTO command is copied here (4EE2H); SYS1/SYS takes up to 63 characters and a carriage return here.
4200H-42FFH 256 bytes	Sector Buffer Directory sectors and the overlays' sectors.
4D00H-4DFFH 256 bytes	GAT Buffer 4AF0H, 4B03H.
37E0H 1 byte	Interrupt Latch Bit 7 clock, bit 6 disk controller.
37E1H 1 byte	Drive Select Latch
37ECH-37EFH 4 bytes	Controller Command and status, track, sector, data.
3840H, 38C0H 1 byte each	Keyboard Rows BREAK (3840H bit 2); the keys tested by the start-up (38C0H).
3C2EH, 3C35H 4 and 8 bytes	Screen Where the trace and clock display tasks write.

Outside Routines

Address	Use
0033H	ROM: Display a Character 44D5H, 4EB2H.
003BH	ROM: Print a Character 44E5H.
002BH	ROM: Scan the Keyboard 4EBAH.
03FAH	ROM: Decode a Key 4392H.
SYS1/SYS-SYS7/SYS	The Overlays Called through RST 28H with the request codes of the vector table.

400CH - The Restart Vectors and the DOS Exits in the 4000H Area

SYS0/SYS loads a few bytes into the communication area that the ROM set up at 4000H-40FFH. The ROM's restarts RST 28H and RST 30H jump to 400CH and 400FH; programs leave through 402DH and 4030H; the ROM's device routine reaches DoubleDOS through 4033H.

4300H - Drive Tables and System Bytes

The load block 4300H-4314H. The bytes 4315H-4357H (the BREAK flag and vector, the command line buffer at 4318H) are not in the file.

4358H - The Error Hook

Three instructions meant as an error handler for BASIC/CMD: code at BASIC/CMD 5444H-5447H would point the @ERROR vector at 4409H to 4358H and store its own addresses at 435AH and 435DH, but that code (CMD with a DOS command) is never run, so nothing sets this hook and it is never reached.

435FH - Step to the Next Sector

Entered from 47CFH and 4868H when the byte offset at FCB+05H has wrapped to 00H: the buffer is used up, the sector number goes up by one and the next sector must be read before its first byte is used.

4366H - Read the Sector of the Next Byte

Called from 47C5H when a file read byte by byte needs a new sector in its buffer. 480EH reads the sector and adds one to the sector number at FCB+0AH-+0BH; 436AH-4373H take the one away again, so that in byte mode FCB+0AH-+0BH is the sector in the buffer.

4378H - The Keyboard Driver

The start-up stores 4378H at 4016H, the driver address of the keyboard device control block (4E29H-4E2CH), unless keys of bits 0 and 3 of 38C0H are held. The scan at 4378H-438DH is the ROM's own scan of 03E3H-03F8H; the key is decoded by the ROM at 03FAH, and 4397H-43A4H add a check that a key reported twice is still down.

43A5H - Convert a Block and Sector to Cylinder and Sector

DoubleDOS numbers the sectors of a diskette in blocks of ten from cylinder 1, sector 0 on. Called from 4651H with Register D the block, Register E the sector in it and Register A the cylinder in the track register: the sector goes to the sector register 37EEH and the cylinder to the data register 37EFH, and the Z FLAG returns set when the head is already on that cylinder.

43C9H - Issue the Read or Write Command

Reached by the jump at 45F0H with the command at 43CEH (stored by 4671H) and the buffer address on the stack. The interrupts are turned off for the transfer.

43DDH - Check the Overlay's Directory Entry

Called from 4BD4H with Register Pair HL at the directory entry the overlay dispatcher has read and Register B its directory record.

43ECH - Stop the Extent Search at 44A0H

Entered by the jump at 4912H with Register Pair HL at the next extent of the FCB being searched: when Register Pair HL has reached 44A0H the search ends with error 1DH, else it goes on at 48F9H. 44A0H is the end of the extents of an FCB at 4480H, the FCB that SYS1/SYS passes to @RUN for a program (SYS1/SYS 4E52H, 4E81H).

4400H - The Vector Table

The fixed entry points of DoubleDOS at 4400H-4478H. Each overlay entry is two instructions, LD A,nn and RST 28H, that pass a request code to the dispatcher at 4BA2H: the low four bits name the overlay (3 SYS1/SYS, 4 SYS2/SYS, 5 SYS3/SYS, 6 SYS4/SYS, 7 SYS5/SYS) and the code is handed to it. The bytes between the entries (4403H-4404H, 4408H, 440CH, 441FH, 4423H, 4427H, 442BH, 442FH, 4466H) are not in the file.

444BH - Restore the Head of the Drive in Use

Called through the jump at 4C9CH from the retry code of 46C2H when a sector read or write has failed on a ready drive: the head goes back to cylinder 0 and the next try seeks again. The routine sits in the space after the vector at 4448H.

4467H - Display, Print and the Time and Date Texts

Four more entries of the vector table.

4480H - The File Specification Template and the System FCB

A text whose 32 bytes SYS1/SYS uses as the buffer of a command's name and as the FCB of a program (4E52H there), and the FCB through which the overlay dispatcher loads the overlays.

44B0H - Hand an Error to SYS4/SYS

Reached by the jump at 4409H with the error code in Register A.

44B4H - Enter DEBUG

Reached by the jumps at 400FH (RST 30H) and 440DH.

44B8H - The Device Dispatcher

Entered at 44BBH by the jump at 4033H with Index Register IX at a device control block (the ROM's device routine at 03C2H sets it). A block of type 10H is a link to another block; a type above 10H is an open FCB, served as a device by 47AEH; any other type goes to the driver at +01H-+02H.

44CFH - Display a Text

Called through 4467H with Register Pair HL at the text.

44DFH - Print a Text

Called through 446AH with Register Pair HL at the text.

44EFH - Start the Transfer Loop

Reached by the jump at 43D7H with Register A the command, Register Pair HL at 37ECH, Register Pair DE at 37EFH and Register Pair BC the buffer. These 17 bytes are the last load block of SYS0/SYS in the file, after the blocks of the start-up at 4E00H; nothing runs before the whole file is loaded, so the order has no effect.

4500H - The Task Table

Twelve slots of two bytes, each the address of a task header: a word, the address of the task's routine, followed by the task's own bytes. The scheduler at 4560H runs the tasks of slots 8-11 on every 25 ms clock interrupt and one of the slots 0-7 in turn, each every eighth interrupt (200 ms). An empty slot holds 45A3H, a header whose routine is the RET at 45A2H.

4518H - The Interrupt Routine

The start-up stores JP 4518H at 4012H (4E81H-4E8AH), where the ROM's RST 38H, the Z80's interrupt mode 1 entry, jumps. Every bit of the interrupt latch 37E0H that is also set in the mask at 404CH is served through its vector at 404DH-405CH (bit 0 first); then the BREAK key is checked.

4538H - Call the Vector of an Interrupt

Entered from 4527H with Register Pair HL at the low byte of the vector and Register A the latch bits not yet tested.

454FH - Take the BREAK Vector

Entered from 4532H when BREAK is held. When the BREAK flag at 4315H is set (a program has armed it), the flag is cleared and the interrupt returns to the address at 4316H-4317H instead of the interrupted program.

4560H - The Task Scheduler

The vector of latch bit 7, the 25 ms clock (405BH, stored by 4E90H). Slots 8-11 run on every interrupt; then the counter at 4040H chooses one of slots 0-7.

458EH - Remove the Running Task

Reached by the jump at 4419H from a task routine.

45A5H - Continue the Running Task After the Call

Reached by the jump at 4416H from a task routine: the address after its CALL 4416H becomes the routine address of its header.

45AFH - The Clock Task

Its header is in slot 7 (4EA4H-4EA9H), so the routine runs every 200 ms; it counts the seconds, minutes and hours at 4041H-4043H every fifth run.

45CFH - Write a Directory Sector and Read It Back

Writes the buffer at Register Pair HL to sector Register E of block Register D on drive Register C with the deleted data mark F8H, the mark of the directory sectors, and reads it back. The Z FLAG returns set when all went well. SYS2/SYS, SYS3/SYS and SYS6/SYS call it.

45D9H - Check a File Sector After It Is Written

Called from 4860H with the flags of the verify switch (485EH): the NZ FLAG asks for the check.

45E3H - Test the Overlay Area Before a Transfer

Reached by the jump at 468FH after the seek, with Register B the tries left and the buffer address in Register Pair HL.

45F3H - Wait and Read the Status

Called from 4455H and 4669H; 45F7H is also the vector of the disk controller's interrupt (4059H, stored by 4E99H), where reading the status ends the interrupt request.

4600H - Select a Drive

Called with Register C the drive number. Each drive keeps its own head position in 4300H-4303H, because the controller has one track register for all drives. A drive whose motor was off is given time to come up to speed.

4639H - Make a Bit Mask

Returns in Register A a byte with only bit n set, n being the low three bits of Register A: the routine writes the instruction SET n,A at 4644H. 461EH uses it for a drive's select bit, 4A9DH for a granule's bit in the GAT.

4647H - Select the Drive and Seek the Cylinder

Called with Register C the drive, Register D the block and Register E the sector in the block. The sector and cylinder go to the controller (43A5H) and a Seek is issued when the head is on another cylinder. 4652H holds the second step rate byte.

4658H - Issue a Command

Waits until the controller is not busy and stores the command in Register A in its command register.

4661H - Wait Until the Controller Is Not Busy

Entered at 4669H. While the controller is busy the status is read again; a busy controller on a drive that is not ready is stopped with Force Interrupt.

4671H - Read or Write a Sector

Called with Register A the command, followed by four parameter bytes after the CALL: the number of tries, the error code base, and the two instruction bytes of the transfer loop at 4698H. Register C is the drive, Register D the block, Register E the sector and Register Pair HL the buffer. The routine returns to the caller of the entry (46DDH, 46E6H, 46EFH, 46F3H) with Register Pair BC as it was, Register A the error code and the Z FLAG set when there was none.

4692H - The Transfer Loop

Reached from 44F1H for Read Sector and from 44FCH, with the first byte in Register A, for a write. Register Pair HL is 37ECH, Register Pair DE 37EFH and Register Pair BC the buffer; the interrupts are off.

46DDH - Read a Sector

Reads sector Register E of block Register D on drive Register C into the buffer at Register Pair HL, ten tries. Error codes 03H-08H (46CDH).

46E6H - Write a Sector

Writes the buffer at Register Pair HL to sector Register E of block Register D on drive Register C with the ordinary data mark FBH (Write Sector A8H), five tries. Error codes 0BH-10H. 46EFH writes with the deleted data mark F8H instead.

46F3H - Read a Sector Back

Reads sector Register E of block Register D on drive Register C without storing its bytes, to check that it can be read, five tries. A directory sector returns error 06H, the deleted data mark.

4700H - @POSN: Position to a Record

Reached through 4442H with Register Pair DE the FCB and Register Pair BC the record number. In byte mode (FCB+01H bit 7 set) the record is multiplied by the record length at FCB+09H to give the sector and the byte offset; else the record is the sector. 4717H is the common end of @POSN, @BKSP, @REW and @PEOF. The FCB is laid out as follows: +00H bit 7 open; +01H flags (bits 0-2 the access level, bit 4 the buffer has been changed, bit 5 the buffer must be read, bit 6 positioned, bit 7 byte mode); +03H-+04H the buffer; +05H the byte offset; +06H the drive; +07H the directory record; +08H the end of file offset; +09H the record length; +0AH-+0BH the sector; +0CH-+0DH the end of file sector; +0EH-+1FH the extents.

4737H - @BKSP: Back One Record

Reached through 4445H with Register Pair DE the FCB.

4756H - @REW: Back to the First Record

Reached through 443FH with Register Pair DE the FCB.

475FH - @PEOF: Position to the End of File

Reached through 4448H with Register Pair DE the FCB. The routine returns error 1CH, the end of file, from 472AH.

476DH - @READ: Read a Record

Reached through 4436H with Register Pair DE the FCB. In sector mode the next sector is read into the file's buffer; in byte mode a record of FCB+09H bytes (00H: 256) is taken byte by byte and stored at Register Pair HL.

478BH - @WRITE: Write a Record

Reached through 4439H with Register Pair DE the FCB, and from 47ACH for @VERF with Register A 01H. Register A is stored at 485EH, the switch for reading each sector back after it is written.

47A8H - @VERF: Write a Record and Read It Back

Reached through 443CH with Register Pair DE the FCB.

47AEH - A File Used as a Device

Reached from 44C8H when a device control block is an open FCB (type above 10H). Register B is the ROM's request: 01H takes a byte from the file into Register A, 02H (and 04H) puts the byte in Register C into the file.

47BBH - Take the Next Byte of a File

Called from 4780H and entered from 47AEH. Register A returns the byte with the Z FLAG set, or an error code with the NZ FLAG set.

47DAH - Put a Byte into a File

Called from 479FH and entered from 47B9H with the byte in Register A. When the buffer is full it is written as the sector; the end of file is moved to the new position.

47FFH - Set the End of File to the Position

Entered from 486EH and 4874H with Register Pair HL the sector and Register C the offset (from 48B9H).

480EH - Read the Next Sector

Reached from 4778H for @READ in sector mode and called from 4366H. The sector of FCB+0AH-+0BH is read into the buffer and the sector number goes up by one.

482DH - Find the Sector and Read It into the Buffer

Called from 47E1H and 481FH.

483EH - Write the Buffer as the Sector

Reached from 4795H, 47FDH and 487FH.

4878H - Write the Buffer Back When Changed

Called from 471BH and 4770H: in byte mode a changed buffer is written before the position moves.

4883H - The Address of the Byte in the Buffer

Returns Register Pair DE as the buffer address at FCB+03H-+04H plus the byte offset, and Register C the offset.

4892H - The Common Start of the File Routines

Called as the first instruction of every file routine. It checks that the FCB at Register Pair DE is open, keeps the FCB address and the caller's return address at 430AH and 430CH, saves Index Register IX, Register Pairs HL, DE and BC, loads Index Register IX with the FCB and goes on with the rest of the routine, which returns through 48B3H. SYS2/SYS and SYS3/SYS call it, SYS2/SYS at 4896H.

48B9H - Compare the Position with the End of File

Returns Register Pair HL the sector at FCB+0AH-+0BH and Register C the offset at FCB+05H, with Register A 00H and the Z FLAG set before the end of file, 1CH at the end of file and 1DH past it, both with the NZ FLAG set.

48DCH - Find the Sector of the Position on the Disk

Called with Index Register IX the FCB. The sector number at FCB+0AH-+0BH is divided into granules of five sectors; the granule is looked for in the five extents kept in the FCB at +0EH-+1FH, then in the directory entry, and the file is given new granules when it has none there. Each extent of the FCB is four bytes: the count of granules before it (two bytes, 0 for the first extent, which has none), the block and the granule byte (bits 7-5 the first granule in the block, bits 4-0 the count of granules minus 1). Returns Register D the block, Register E the sector in it and Register C the drive.

4928H - Look in the Directory Entry

Reached when none of the five extents in the FCB holds the granule. 49B5H finds it in the directory entry (giving the file new granules when needed); the extent found is then put in the FCB as its second extent, the others moving up and the fifth dropped, unless it is there already.

496AH - Bring the Extent Forward

Entered from 4907H and 4926H with Register A the granule's place in the extent and Register Pair HL at the extent's granule byte in the FCB. The third, fourth or fifth extent is moved to the second place, so that the extents used last are found first.

4993H - Work Out the Block and the Sector

Entered with Register Pair HL the extent (Register L the block, Register H the granule byte), the granule's place in the extent at 499AH and the sector in the granule at 49AAH. A block holds two granules of five sectors.

49B5H - Find a Granule in the Directory Entry

Called with Register Pair BC the granule of the file. The directory entry of the file (record FCB+07H) is read into 4200H and its extents at +16H-+1FH are counted; an extent byte FEH is followed by the record of another entry of the same file, FFH ends the list and makes 4A00H give the file new granules. Returns Register Pair HL the extent and Register Pair DE the granules before it.

4A00H - Give the File New Granules

Called from 49F9H with Register Pair BC the granule wanted, Register Pair DE the granules the file has and Register Pair HL at the FFH that ends the extents in the directory entry at 4200H. The GAT is read into 4D00H; a granule following the last extent is added to it when free, else a new extent is begun at the first free granule from the GAT byte 4D00H plus four times the low three bits of the directory record. The GAT and the entry are written back at the end.

4AC1H - Read a Directory Record

Reads the directory sector of record Register B of drive Register C into 4200H. Returns Register Pair HL the address of the entry, or error 11H. SYS2/SYS calls it.

4AD6H - Write a Directory Record

Writes the directory sector at 4200H back as the sector of record Register B of drive Register C, with the deleted data mark, and reads it back. Returns error 12H if it fails. SYS2/SYS, SYS3/SYS and SYS6/SYS call it.

4AF0H - Read the GAT

Reads sector 0 of the directory block of drive Register C, the GAT, into 4D00H. Returns error 14H if it fails. SYS3/SYS and SYS6/SYS call it.

4B03H - Write the GAT

Writes the GAT at 4D00H back as sector 0 of the directory block of drive Register C and reads it back. Returns error 15H if it fails. SYS3/SYS and SYS6/SYS call it.

4B1EH - Find the Sector of a Directory Record

Register B is the directory record: bits 7-5 the entry in the sector (eight entries of 32 bytes), bits 4-0 the entry sector (0-7, sectors 2-9 of the directory block, after the GAT and the HIT). Returns Register D the block, Register E the sector and Register Pair HL the entry's address in 4200H, or error 10H for a sector above 7.

4B35H - Read a Directory Sector

Reads sector Register E of block Register D on drive Register C into the buffer at Register Pair HL and expects the deleted data mark of the directory. A sector with the ordinary mark means the directory is elsewhere on this diskette: the directory block is taken from byte 2 of logical sector 0 (the first sector of cylinder 1), kept at 4304H plus the drive, and the read is made again. Returns Register A 00H and the Z FLAG set, or the error code. SYS2/SYS, SYS3/SYS and SYS6/SYS call it, and the start-up reads the GAT with it (4ECBH).

4B55H - The Directory Block of a Drive

Loads Register D with the directory block of drive Register C from 4304H-4307H. SYS2/SYS, SYS3/SYS and SYS6/SYS use it.

4B5DH - Test a Bit

Tests bit n of Register B, n being the low three bits of Register A: the routine writes the instruction BIT n,B at 4B67H. The Z FLAG returns set when the bit is clear (in the GAT: the granule is free).

4B6AH - Multiply

Multiplies Register Pair HL by Register A. Returns the 24 bit product with its low byte in Register A and its upper 16 bits in Register Pair HL; Register Pair DE returns the multiplicand. DOUBLE/CMD calls it.

4B84H - Divide

Divides Register Pair HL by Register A. Returns the quotient in Register Pair HL and the remainder in Register A. DOUBLE/CMD and SYS3/SYS call it.

4BA0H - Return with Register A 00H

Reached by the jump at 4312H.

4BA2H - The Overlay Dispatcher

Reached by RST 28H through 400CH with Register A a request code. Bit 7 clear: nothing is done. Else the low four bits n name the overlay: directory record n (n below 8) or n plus 18H, read from drive 0 (records 03H-07H, SYS1/SYS to SYS5/SYS, are the first entry of entry sectors 3-7; records 20H and 21H, SYS6/SYS and SYS7/SYS, the second entry of entry sectors 0 and 1). The overlay is loaded through the system FCB at 44A0H unless the same low four bits were asked for last, then called at its transfer address with the request code in Register A. The return address of the RST is dropped, so the overlay returns to the caller of the vector entry.

4C06H - @RUN: Load and Run a Program

Reached through 4433H with Register Pair DE the FCB of the program file.

4C16H - @LOAD: Load a Program

Reached through 4430H with Register Pair DE the FCB of the program file. The file is opened with records of 256 bytes and the buffer 4200H; a file opened with an access level of 7 is refused, and bit 2 of the access level is cleared so that a file of level 4-6 can be read for loading. Returns the transfer address in Register Pair HL.

4C39H - Load a Load Module

Called from 4BE2H for an overlay and entered from 4C37H for a program, with Register Pair DE the FCB, whose buffer must be 4200H. Each record is a type byte, a length byte and the record: type 01H a load block (two address bytes and length minus 2 data bytes, a length of 02H giving 256), type 02H the transfer address, any other type below 20H passed over. Every byte stored is read back. Returns the transfer address in Register Pair HL, or with bit 6 set in Register A: 22H (a type of 20H or more), 23H (a byte reads back differently twice), 24H (a byte does not stay), or the error of @READ.

4C89H - Fetch the Next Byte of the Load Module

Returns the next byte of the file in Register A; Register E is the offset in the buffer 4200H-42FFH. When the buffer is used up the next sector is read through @READ. On an error the routine returns straight from the loader, taking its return address and the saved FCB off the stack.

4C9BH - Restore or Cut the Tries

Called from 46C2H after a failed transfer with Register A the error bits and Register B the tries left.

4CA9H - The Clock Display Task

A task header for the table at 4500H; SYS6/SYS puts its address in a slot (52C8H). Every fifth run the time is shown as HH:MM:SS at 3C35H, the right end of the top line of the screen.

4CB7H - Write the Time or the Date as Text

Entered at 4CB7H (through 446DH) for the time and at 4CD2H (through 4470H) for the date, with Register Pair HL where the eight characters go. Three bytes are taken from the highest address down and written as two decimal digits each, with a colon (time) or a slash (date) between them.

4CD9H - The Trace Task

A task header; SYS1/SYS puts its address in a slot (5194H). The routine shows the address the interrupted program was at as four hexadecimal digits at 3C2EH on the top line of the screen. Run from slots 8-11, through the CALL of 457BH, it finds that address 12H bytes above the Stack Pointer: the interrupt's own push, Register Pairs HL and AF (4518H), the five pushes of 4538H-453CH, the return address 4547H and the return address of the CALL.

4E00H - The Start-up

The transfer address of SYS0/SYS, reached from the boot sector at 424BH with the interrupts off. It is run once: SYS1/SYS and the other overlays later load over 4E00H. The start-up finds the top of memory, installs the keyboard driver, the interrupt routine, its vectors and the clock task, shows the banner and runs the AUTO command stored in the GAT unless ENTER is held.

4E32H - Code That Is Never Run

The jump at 4E2FH passes over these 68 bytes and nothing else reaches them. As instructions they store Register Pair HL and words from 4023H, 4026H and 402BH into 43BAH-43CBH, clear 43CCH-43FFH, and copy the three ROM device control blocks at 4015H-402CH to 4358H with eight bytes of 00H and sixteen of FFH after each. Run, they would overwrite the DoubleDOS code at 4358H-43FBH.

4E76H - Install the Interrupts and the Clock

Reached by the jump at 4E2FH.

4EEDH - The Banner

Shown by 4EACH-4EB8H when the system starts: four lines ended by 0DH and the end mark 03H. The version digit at 4F15H is the only byte of the load blocks that tells 4.20, 4.23 and 4.24 apart.