Buffer #0 command and status registers. Bits:

• Bits #0-#6: Status or command code.

• Bit #7: 0 = Job finished, register contains status code; 1 = Job to be executed, register contains command code.

Values:

• $00: "24,READ ERROR" (only during disk format).

• $01: "00,OK" (no error).

• $02: "20,READ ERROR".

• $03: "21,READ ERROR".

• $04: "22,READ ERROR".

• $05: "23,READ ERROR".

• $06: "24,READ ERROR" (may not occur).

• $07: "25,WRITE ERROR".

• $08: "26,WRITE PROTECT ON".

• $09: "27,READ ERROR".

• $0A: "28,READ ERROR" (may not occur).

• $0B: "29,DISK ID MISMATCH".

• $0F: "74,DRIVE NOT READY".

• $80: Read sector.

• $90: Write sector.

• $A0: Verify sector.

• $B0: Read in sector header and fetch header ID.

• $C0: Bump head.

• $D0: Execute code in buffer.

• $E0: Read in sector header and then execute code in buffer.

• $F0: Read in sector header.

Buffer #1 command and status register.

Buffer #2 command and status register.

Buffer #3 command and status register.

Buffer #4 command and status register.

Unused. (Command and status register of not existing buffer #5.)

Buffer #0 track and sector register.

Buffer #1 track and sector register.

Buffer #2 track and sector register.

Buffer #3 track and sector register.

Buffer #4 track and sector register.

Unused. (Track and sector register of not existing buffer #5.)

Unit #0 expected sector header ID.

Unused. (Expected sector header ID of not existing unit #1.)

Header ID from header of sector last read from disk.

Track and sector number from header of sector last read from disk.

Header checksum from header of sector last read from disk.

Unused.

Unit #0 disk change indicator. Bits:

• Bit #1: 1 = Status of write protect photocell has changed.

Unused. (Disk change indicator of not existing unit #1.)

Previous status of unit #0 write protect photocell (in bit #4).

Unused. (Previous status of write protect photocell of not existing unit #1.)

Unit #0 disk controller internal command register. Bits:

• Bit #4: 1 = Switch motor off.

• Bit #5: 1 = Idle.

• Bit #6: 1 = Seeking in progress.

• Bit #7: 1 = Waiting for motor to spin up.

Unused. (Disk controller internal command register of not existing unit #1.)

Unit #0 current track number.

Serial bus communication speed switch. Values:

• $00: C64, lower speed, extra waits are needed. (Compensation for the delays caused by sprite DMA in the host.)

• $01-$FF: VIC20, higher speed.

Header of sector last read from or next to be written onto disk, in GCR-encoded form.

Unused.

Pointer to current byte in buffer during GCR-encoding/decoding.

Pointer to beginning of current buffer.

Pointer to track and sector registers of current buffer.

GCR-byte counter during GCR-encoding/decoding.

Unused.

Byte counter during GCR-encoding/decoding.

Unused.

Data block signature byte of sector last read from disk.

Expected value of sector header signature byte.
Default: $08.

Computed checksum of data in buffer.

Unused.

Disk controller current unit number. Values:

• $00: Unit #0.

• $01: Unit #1. (Makes no sense, will not work.)

Default: $00, 0.

Disk controller previous unit number. Values:

• $00: Unit #0.

• $01: Unit #1. (Makes no sense, will not work.)

• $FF: Motor is off, must spin it up before seeking.

Default: $00, 0.

Disk controller current buffer number.

Current track number.

Buffer number that needs seeking.

Number of tracks to move during seeking.

Number of sectors on current track.

Data density (in bits #5-#6).
Temporary register for buffer command.

Disk controller buffer command register.

Unused.

Expected value of data block header signature byte.
Default: $07.

Motor spin up/down delay counter.

Original value of stack pointer before execution of disk controller interrupt.

Number of halftracks to move during seeking.

Retry counter for reading sector header.
Temporary area during seeking.

On current track, the distance of the sector, that matches the sector of a buffer and is nearest to the one last read from disk. (The execution of the command whose sector is optimal in time?)

On current track, the sector that is two sectors away from the one last read from disk. (Apparently, the optimization takes about two sectors worth of time.)

Pointer to beginning of auxiliary buffer during GCR-encoding (byte swapped).

Indicator of buffer data currently being in GCR-encoded form. Values:

• $00: Data in normal form.

• $01-$FF: Data in GCR-encoded form. Upon exiting the disk controller interrupt, data must be decoded.

Current track number during formatting. Bits:

• Bit #7: 0 = Disk is currently being formatted; 1 = Formatting has not started yet.

Temporary area for data bytes during GCR-encoding/decoding.

Temporary area for data nybbles and GCR bytes during GCR-encoding/decoding.

Number of halftracks to accelerate/decelerate through during accelerated seeking.
(Twice this value must be less than value of address $0064.)

Acceleration/deceleration factor during accelerated seeking.
(Value of address $1C07 plus/minus value of address $005E times this value must not be too low – below about 12-20, depends on drive mechanics – or too high – above 255.)

Delay counter after seeking. (So that head stops vibrating.)
Halftrack counter for acceleration/deceleration during accelerated seeking.

Halftrack counter for full speed during accelerated seeking.

Pointer to routine next to be executed during seeking.

Lower distance limit of accelerated seeking, in halftracks.
(When moving to a track further than this many halftracks, seeking will be accelerated.)
Default: $C8, 200.

Pointer to warm reset ("UI" command) routine.
Default: $EB22.

Unknown.

Automatic disk initialization switch. Values:

• $00: When initializing units upon processing commands, the BAM must be loaded.

• $01-$FF: Unit initialization does not include loading the BAM.

Default: $00.

Soft interleave. (Distance, in sectors, for allocating the next sector for files.)
Default: $0A, 10.

Number of retries on disk commands. Bits:

• Bits #0-#5: Number of retries.

• Bit #6: 1 = Do not retry on adjacent halftracks.

• Bit #7: 1 = Do not bump head.

Default: $05, 5.

Pointer to "Ux" user command pointer table.
Default: $FFEA.

Temporary pointer for BAM operations.

Temporary pointer for various operations.

Unknown.

Pointer to current byte during memory test upon startup.
Execution address of current "Ux" user command.

Serial bus LISTEN command to accept. (Device number OR $20.)

Serial bus TALK command to accept. (Device number OR $40.)

Serial bus LISTEN command indicator. Values:

• $00: No LISTEN command active.

• $01-$FF: LISTEN command currently active.

Serial bus TALK command indicator. Values:

• $00: No TALK command active.

• $01-$FF: TALK command currently active.

Unknown.

Serial bus ATN arrival indicator. Values:

• $00: ATN inactive.

• $01-$FF: ATN signal arrived.

End of command indicator. Values:

• $00: Command fully arrived, ATN became inactive.

• $01-$FF: Command still transferring.

Track number of previously opened file. (Used when opening "*".) Values:

• $00: No file has been opened yet.

• $01-$FF: Track number of previously opened file.

Current unit. Values:

• $00: Unit #0.

• $01: Unit #1. (Makes no sense, will not work.)

Default: $00, 0.

Track and sector number for various operations.

Current channel number. Values:

• $00-$04: Buffers #0-#4.

• $05: Error message buffer.

Current secondary address (only bits #0-#3).

Current secondary address.

Data byte read from serial bus.
Data byte read from buffer or to be written into buffer.

Pointer to current byte in directory buffer when writing entries into directory.
Pointer to current byte of error message.
$0086: Counter of files deleted during "SCRATCH" command.
Start switch during "&" command ($01-$FF: Execute code.)
Temporary unit number during job processing.
$0087: Computed block checksum during "&" command.

Pointer to current byte during "&" command.
Execution address of user code during "&" command.

Unknown.

Temporary area for integer division. (Used to compute the side sector of relative files.)

Unknown.

Pointer to current directory entry.

Unused.

Bit counter during serial bus input/output.

Pointer to buffer #0.
Default: $0300.

Pointer to buffer #1.
Default: $0400.

Pointer to buffer #2.
Default: $0500.

Pointer to buffer #3.
Default: $0600.

Pointer to buffer #4.
Default: $0700.

Pointer to input buffer.
Default: $0200.

Pointer to error message buffer.
Default: $02D5.

Primary buffer number assigned to channels. Bits:

• Bits #0-#6: Buffer number.

• Bit #7: 1 = No buffer assigned to channel.

Secondary buffer number assigned to channels. Bits:

• Bits #0-#6: Buffer number.

• Bit #7: 1 = No buffer assigned to channel.

Length of file assigned to channels, low byte. For relative files, number of records, low byte.

Length of file assigned to channels, high byte. For relative files, number of records, high byte.

Offset of current byte in buffer assigned to channels.

Record length of relative file assigned to channels.

Buffer number holding side sector of relative file assigned to channels. Values:

• Bits #0-#6: Buffer number.

• Bit #7: 1 = No buffer assigned to channel.

Comma counter during fetching unit numbers from command.

Offset of current byte in relative file record.

Side sector number belonging to current relative file record.

Offset of track and sector number of current relative file record in side sector.

Offset of record in relative file data sector.

Sector number of directory entry of files.

Offset of directory entry of files.

Unit number of files. Bits:

• Bit #0: Unit number.

• Bit #7: 1 = No valid unit number has been specified in command, must try both units.

File type and flags of files. Bits:

• Bits #0-#2: File type; 0 = DEL; 1 = SEQ; 2 = PRG; 3 = USR; 4 = REL.

• Bit #5: 0 = File has been closed.

• Bit #6: 1 = File is write protected.

• Bit #7: 1 = Wildcards are present in the file name.

Unit number, file type and flags of files assigned to channels. Bits:

• Bit #0: Unit number.

• Bits #1-#3: File type; 0-4: Usual file types; 7 = "#", direct disk access.

• Bit #5: 1 = End of record.

• Bit #6: 1 = End of file.

• Bit #7: 1 = Directory entry of file must be updated.

Input/output flags of channels. Bits:

• Bit #0: 1 = Write allowed.

• Bit #3: 0 = End of file.

• Bit #7: 1 = Read allowed.

End of file indicator of current channel. Values:

• $00: End of file.

• $01-$FF: File has not ended yet.

Current buffer number.

Unknown.

Unit #0 BAM input/output error indicator. Values:

• $00: No error.

• $01-$FF: An error occurred while reading/writing the BAM.

Reserved, do not use. (BAM input/output error indicator of not existing unit #1.)

Unit #0 BAM version code. (Byte at offset #$02 in sector 18;00.)
Expected: $41, "A".

Reserved, do not use. (BAM version code of not existing unit #1.)

Actual processor stack.

Unused.

Auxiliary buffer for GCR-encoding/decoding.

Input buffer (42 bytes). (Used for accepting commands from host.)

DOS command number. Values:

• $00: OPEN.

• $01-$0B: DOS commands.

• $0C: OPEN "$".

• $80-$FE: "B-x" commands.

• $FF: Command too long.

Channel number assigned to secondary addresses. Values:

• $00-$FE: Channel number.

• $FF: No channel assigned.

Temporary area of next data byte to be written from buffers #0-#4 to serial bus.

Temporary area of next data byte to be written from error message buffer to serial bus.

Offset of last data byte in buffers #0-#4.

Offset of last data byte in error message buffer.

File type of current file.

Length of name of current file.

Temporary area for secondary address.

Temporary area for disk controller command.

Number of sectors on current track.

Buffer allocation register. Bits:

• Bits #0-#4: 1 = Buffer #x is being used.

• Bit #5: 1 = Error message buffer is being used.

Default: $FFE0, all existing buffers are free.

Unit #0 BAM change indicator. Values:

• $00: No changes.

• $01-$FF: BAM has been changed, must be written onto disk.

Reserved, do not use. (BAM change indicator of not existing unit #1.)

File found indicator during searching for a file name in directory. Bits:

• Bits #0-#2: Current entry number.

• Bit #7: 0 = A file matching the name has been found in the directory.

LOAD channel directory indicator. Values:

• $00: Data file is being LOAD'ed.

• $01-$FF: Disk directory is being LOAD'ed.

End of command indicator. Values:

• $00: Command is still arriving.

• $01-$FF: Command fully arrived.

Channel allocation register. Bits:

• Bits #0-#6: 1 = Channel #x is being used.

Temporary area for channel number. Values:

• $00-$06: Assigned buffer number in primary table ($00A7-$00AD).

• $07-$0D: Assigned buffer number in secondary table ($00AE-$00B4).

Record length of current relative file.

Track and sector number of first side sector of current relative file.

Original disk controller commands of buffers #0-#4.

Sector number of directory entry of files specified in command.

Offset of directory entry of files specified in command.

Switch for displaying warning messages of relative files. Values:

• $00: Always display "00,OK" instead.

• $01-$FF: Display warning messages.

Delay counter for LED blinking that indicates hardware problem upon startup.

Unit #0 LED bit. Values:

• $00: Switch LED off.

• $08: Switch unit #0 LED on.

Unit number of previously opened file. (Used when opening "*".)

Sector number of previously opened file. (Used when opening "*".)

Temporary area for channel number.

Unused.

BASIC line number for entries sent to host during LOAD'ing "$". (For header, unit number; for files, length of file, in blocks; for footer, number of free blocks.)

Length of command.

First character of command.
Character to search for in input buffer.

Offset of first character after file name in command.

Temporary area for number of commas in command.

Number of commas or unit numbers in command.

Number of commas before equation mark in command.
Number of current file in command during searching for files specified in command.

Offset of character before colon in command. (Probably, the character resembles a unit number.)

Offset of file names in command. (Last offset specifies end of command.)

Track number of files specified in command. Values:

• $00: File not yet found in directory.

• $01-$FF: Track number of first sector of file.

For "B-x" commands, upper byte of parameters.

Sector number of files specified in command. For "B-x" commands, lower byte of parameters.

Number of wildcards found in current file name.

Command syntax flags. Bits:

• Bit #0: 0 = Equation marks are present in the command.

• Bit #1: 1 = Equation marks are present in the command.

• Bit #2: 1 = Commas after equation marks are present in the command.

• Bit #3: 1 = Wildcards after equation marks are present in the command.

• Bit #6: 1 = Commas before equation marks are present in the command.

• Bit #7: 1 = Wildcards before equation marks are present in the command.

Number of units to process during reading the directory. Values:

• $00: Only one unit.

• $01: Both units.

Current unit number during reading the directory. Values:

• $00: Unit previously used.

• $01: Switch to inactive unit.

Previous unit number during reading the directory.

Indicator to keep searching in the directory. Values:

• $00: More files are to be searched for.

• $01-$FF: All files have been found, no need to continue search.

Current directory sector number.

Directory sector number to read. Values:

• $00: First directory sector is to be read.

• $01-$FF: Sector number of next directory to read.

Offset of current directory entry in directory sector.

End of directory indicator. Values:

• $00: Current directory sector is the last.

• $01-$FF: More directory sectors are present.

Offset of current directory entry in directory sector.

Number of remaining directory entries in directory sector minus 1. Values:

• $00-$07: Number of remaining directory entries.

• $FF: No more entries left.

File type of file being searched in directory. Values:

• $00: No file type has been given, any file type will match.

• $01-$04: File type specified in command.

File open mode. Values:

• $00: "R", Read or LOAD.

• $01: "W", Write or SAVE.

• $02: "A", Append.

• $03: "M", Modify, salvage improperly closed file.

Message display switch for disk errors. Bits:

• Bit #7: 0 = Silently ignore "26,WRITE PROTECT ON", "29,DISK ID MISMATCH" and "74,DRIVE NOT READY" errors when executing disk commands; 1 = Display error message.

Offset of current byte in halftrack seek table during retrying disk operations on adjacent halftracks.

Direction of seeking back to original halftrack during retrying disk operations on adjacent halftracks.

Unit #0 track number of current BAM entry.

Reserved, do not use. (Track number of current BAM entry of not existing unit #1.)

Unit #0 track numbers of two cached BAM entries.

Reserved, do not use. (Track numbers of two cached BAM entries of not existing unit #1).

Unit #0 two cached BAM entries.

Reserved, do not use. (Two cached BAM entries of not existing unit #1.)

Buffer for constructing current entry (BASIC line) while LOAD'ing "$".

Unused.

Error message buffer.

Disk update upon BAM change switch. Different BAM-related operations expect different values here.

Unit #0 number of free blocks, low byte.

Reserved, do not use. (Number of free blocks, low byte, of not existing unit #1.)

Unit #0 number of free blocks, high byte.

Reserved, do not use. (Number of free blocks, high byte, of not existing unit #1.)

Unit #0 direction of seeking of adjacent halftrack. Values:

• $00: No seeking.

• $01: Seek a halftrack upwards.

• $02: Halftrack seeking complete.

• $FF: Seek a halftrack downwards.

Reserved, do not use. (Direction of seeking of adjacent halftrack of not existing unit #1.)

Buffer #0.

Buffer #1.

Buffer #2.

Buffer #3.

Buffer #4.

Port B, serial bus. Bits:

• Bit #0: DATA IN; 0 = Low; 1 = High.

• Bit #1: DATA OUT; 0 = Low; 1 = High.

• Bit #2: CLOCK IN; 0 = Low; 1 = High.

• Bit #3: CLOCK OUT; 0 = Low; 1 = High.

• Bit #4: ATNA OUT; 1 = Enable device presence detection by automatically acknowledging ATN IN signals on DATA OUT.

• Bits #5-#6: Device number, set with jumper, minus 8; %00 = 8; %01 = 9; %10 = 10; %11 = 11. Default: %00, 8.

• Bit #7: ATN IN; 0 = Low; 1 = High.

Port A. Read to acknowledge interrupt generated by ATN IN going high.

Port B data direction register. Bits:

• Bit #x: 0 = Bit #x in port B can only be read; 1 = Bit #x in port B can be read and written.

Default: $1A, %00011010.

Port A data direction register.
Default: $FF, %11111111.

Timer. Read low byte or write high byte to start timer or restart timer upon underflow.

Timer latch. Read/write starting value of timer from/to here.

Unused.

Timer control register. Bits:

• Bit #6: 0 = Stop timer; 1 = Start timer.

Unused.

Interrupt status register. Bits:

• Bit #1: 1 = ATN IN went high.

• Bit #6: 1 = Timer underflow occurred.

Interrupt control register. Read bits:

• Bit #7: An interrupt has been generated.

Write bits:

• Bit #1: 1 = Enable interrupts generated by ATN IN going high.

• Bit #7: Fill bit; bits #0-#6, that are set to 1, get their values from this bit; bits #0-#6, that are set to 0, are left unchanged.

Unused.

Port B. Bits:

• Bits #0-#1: Head step direction. Decrease value (%00-%11-%10-%01-%00...) to move head downwards; increase value (%00-%01-%10-%11-%00...) to move head upwards.

• Bit #2: Motor control; 0 = Off; 1 = On.

• Bit #3: LED control; 0 = Off; 1 = On.

• Bit #4: Write protect photocell status; 0 = Write protect tab covered, disk protected; 1 = Tab uncovered, disk not protected.

• Bits #5-#6: Data density; %00 = Lowest; %11 = Highest.

• Bit #7: 0 = SYNC marks are being currently read from disk; 1 = Data bytes are being read.

Port A. Data byte last read from or to be next written onto disk.

Port B data direction register. Bits:

• Bit #x: 0 = Bit #x in port B can only be read; 1 = Bit #x in port B can be read and written.

Default: $6F, %01101111.

Port A data direction register. Bits:

• Bit #x: 0 = Bit #x in port A can only be read; 1 = Bit #x in port A can be read and written.

Values:

• $00: Read from disk.

• $FF: Write onto disk.

Timer. Read low byte or write high byte to start timer or restart timer upon underflow.

Timer latch. Read/write starting value of timer from/to here.

Unused.

Timer control register. Bits:

• Bit #6: 0 = Stop timer; 1 = Start timer.

Auxiliary control register. Bits:

• Bits #1-#3: %111 = Attach Byte Ready line to oVerflow processor flag. (Whenever a data byte has been successfully read from or written to disk, V flag is set to 1.)

• Bits #5-#7: Head control; %111 = Read; %110 = Write.

Interrupt status register. Bits:

• Bit #6: 1 = Timer underflow occurred.

Interrupt control register. Read bits:

• Bit #7: An interrupt has been generated.

Write bits:

• Bit #6: 1 = Enable interrupts generated by timer underflow.

• Bit #7: Fill bit; bits #0-#6, that are set to 1, get their values from this bit; bits #0-#6, that are set to 0, are left unchanged.

Unused.