CP/M Basic Input/Output System

The BIOS is the machine-dependent part of CP/M. In theory, all you need to do is change the BIOS and CP/M will work on a different machine. The same used to be true of MSDOS...

The BIOS begins with the following jumps to service routines:
        JMP     BOOT    ;Cold start routine
        JMP     WBOOT   ;Warm boot - reload command processor
        JMP     CONST   ;Console status
        JMP     CONIN   ;Console input
        JMP     CONOUT  ;Console output
        JMP     LIST    ;Printer output
        JMP     PUNCH   ;Paper tape punch output
        JMP     READER  ;Paper tape reader input
        JMP     HOME    ;Move disc head to track 0
        JMP     SELDSK  ;Select disc drive
        JMP     SETTRK  ;Set track number
        JMP     SETSEC  ;Set sector number
        JMP     SETDMA  ;Set DMA address
        JMP     READ    ;Read a sector
        JMP     WRITE   ;Write a sector
In CP/M 2 and later, the following extra jumps appear:
        JMP     LISTST  ;Status of list device
        JMP     SECTRAN ;Sector translation for skewing
In CP/M 3, a further set of jumps is present:
        JMP     CONOST  ;Status of console output
        JMP     AUXIST  ;Status of auxiliary input
        JMP     AUXOST  ;Status of auxiliary output
        JMP     DEVTBL  ;Address of devices table
        JMP     DEVINI  ;Initialise a device
        JMP     DRVTBL  ;Address of discs table
        JMP     MULTIO  ;Read/write multiple sectors
        JMP     FLUSH   ;Flush host buffers
        JMP     MOVE    ;Move a block of memory
        JMP     TIME    ;Real time clock
        JMP     SELMEM  ;Select memory bank
        JMP     SETBNK  ;Select bank for DMA operation
        JMP     XMOVE   ;Preload banks for MOVE
        JMP     USERF   ;System-depedent functions
        JMP     RESERV1 ;Reserved
        JMP     RESERV2 ;Reserved

BOOT

This function is completely implementation-dependent and should never be called from user code.

WBOOT

Reloads the command processor and (on some systems) the BDOS as well. How it does this is implementation-dependent; it may use the reserved tracks of a floppy disc or extra memory.

CONST

Returns its status in A; 0 if no character is ready, 0FFh if one is.

CONIN

Wait until the keyboard is ready to provide a character, and return it in A.

CONOUT

Write the character in C to the screen.

LIST

Write the character in C to the printer. If the printer isn't ready, wait until it is.

PUNCH

Write the character in C to the "paper tape punch" - or whatever the current auxiliary device is. If the device isn't ready, wait until it is.

READER

Read a character from the "paper tape reader" - or whatever the current auxiliary device is. If the device isn't ready, wait until it is. The character will be returned in A. If this device isn't implemented, return character 26 (^Z).

HOME

Move the current drive to track 0.

SELDSK

Select the disc drive in register C (0=A:, 1=B: ...). Entered with DE=0 or FFFFh. If DE=0, the disc will be logged in; if it is FFFFh, no attempt will be made to read it.
SELDSK returns the address of a Disc Parameter Header in HL.

SETTRK

Set the track in BC - 0 based.

SETSEC

Set the sector in BC. Under CP/M 1 and 2 a sector is 128 bytes. Under CP/M 3 the sector size is given in the Disk Parameter Block.
There has been discussion in comp.os.cpm about whether the parameter to this function is a byte or a word. The conclusion (based on examining the BDOS source) was that it is a word.

SETDMA

The next disc operation will read its data from (or write its data to) the address given in BC.

READ

Read the currently set track and sectorat the current DMA address. Returns A=0 for OK, 1 for unrecoverable error, 0FFh if media changed.

WRITE

Write the currently set track and sector. C contains a deblocking code:
C=0 - Write can be deferred
C=1 - Write must be immediate
C=2 - Write can be deferred, no pre-read is necessary.
Returns A=0 for OK, 1 for unrecoverable error, 2 if disc is readonly, 0FFh if media changed.

LISTST

Return status of current printer device.
Returns A=0 (not ready) or A=0FFh (ready).

SECTRAN

Translate sector numbers to take account of skewing.
On entry, BC=logical sector number (zero based) and DE=address of translation table. On exit, HL contains physical sector number.

CONOST

Return status of current screen output device.
Returns A=0 (not ready) or A=0FFh (ready).

AUXIST

Return status of current auxiliary input device.
Returns A=0 (not ready) or A=0FFh (ready).

AUXOST

Return status of current auxiliary output device.
Returns A=0 (not ready) or A=0FFh (ready).

DEVTBL

Return in HL the address of the devices table, or 0 if the devices table isn't implemented.
The devices table will be visible to programs without the need for bank switching, ie. it will be in common memory.
The device table contains one entry for each character device. Each entry is formed:
        DEFB    'NAME  '        ;Name, 6 bytes. If the first byte is zero,
                                ;this is the end of the table.
        DEFB    mode            ;Bitmapped value:
                                ;Bit 0 set => can input from this device
                                ;Bit 1 set => can output to this device
                                ;Bit 2 set => can change the baud rate
                                ;Bit 3 set => supports XON/XOFF
                                ;Bit 4 set => is using XON/XOFF
                                ;Bits 5,6,7 set to 0.
                                ; Amstrad extension: If bit 7 is set, output
                                ;to the device does not time out. 
        DEFB    baudrate        ;Coded speed, 1-15 or 0 if speed can't be
                                ;changed.
                                ;Rates are 50,75,110,134.5,150,300,600,1200,
                                ;         1800,2400,3600,4800,7200,9600,19200.
The maximum number of devices allowed in CP/M Plus is unclear. The documentation variously says there can be 12 or 13, while the DEVICE.COM source code suggests 15.

DEVINI

Reinitialise character device number C - called when the device's settings (baud rate, mode etc.) are changed.

DRVTBL

Return in HL the address of the drive table, or 0 (or 0FFFFh, or 0FFFEh) if the drive table isn't implemented. The drive table contains 16 pointers to the Disc Parameter Headers of the 16 disc drives A-P; if a pointer is 0 it means that the corresponding drive does not exist.
The drive table is usually (but not always) in common memory (ie accessible by user programs).

MULTIO

Notify the BIOS that the BDOS is intending to transfer a number of consecutive disc sectors with READ or WRITE. Entered with C = number of calls that will be made; up to 16k of data will be transferred.
The idea is that after the MULTIO call, the BIOS can choose to transfer all the data in the first READ/WRITE operation, and then not to do anything on the subsequent (n-1) operations.

FLUSH

Write any pending data to disc.
Returns A=0 if successful, 1 for physical disc error, 2 for drive R/O.
This function is only useful when the BIOS is doing the deblocking - ie, the physical sector size is not the size that the BIOS reports to the BDOS.

MOVE

Move BC bytes of memory, from the address in HL to the address in DE. Should return HL and DE pointing to the first addresses not copied. If XMOVE is used before this function, data are moved between two memory banks.

TIME

Get the current date and time into the SCB (at CBOOT-0Ch). HL and DE must be preserved. If C=0FFh, then set the time from the SCB instead.
The format of the 5-byte buffer is:
        DW      day     ;Day 1 is 1 Jan 1978
        DB      hour    ;packed BCD
        DB      minute  ;packed BCD
        DB      second  ;packed BCD

SELMEM

Set the current bank to the number in A. Bank 1 is the bank in which user programs run (the TPA); Bank 0 and any other banks are used by CP/M for disc buffers or as a RAMdisc.

SETBNK

Set the bank to be used for the next read/write sector operation. The bank number is passed in A.

XMOVE

After XMOVE, the next call to MOVE will move data between different memory banks. Call XMOVE with C=source bank and B=destination bank.

USERF

This function is reserved for the author of the BIOS to add any extra features. On Amstrad computers, for example, this call accesses the extended BIOS functions.

RESERV1 and RESERV2

These calls are reserved and contain JMP 0 instructions.
Letzte Änderung: 4.Juni 2004