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EXTENSIONS.md

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BIOS Extensions

Traditionally there are two reserved BIOS functions, RESERVE1(31) and RESERVE2(32), I've claimed the first as virtual syscalls that the emulator will handle.

They can be called like so:

; Set the function-number to call in HL
ld hl, 0x00

; Invoke the BIOS function
ld a, 31
out (0xff), a

We've implemented a small number of custom BIOS calls, documented below.

Function 0x00: CPMUlator?

Test to see if the code is running under cpmulator, the return value is split into two parts:

  • Registers are set to specific values:
    • H -> S
    • L -> K
    • A -> X
  • The DMA buffer is filled with a text-banner, null-terminated.

Function 0x01: Get/Set Ctrl-C Count

  • If C == 0xFF return the value of the Ctrl-C count in A.
  • IF C != 0xFF set the Ctrl-C count to be C.

Example:

;: get the value
LD H, 0x01
LD C, 0xFF
LD A, 31
OUT (0xFF), A
;; Now A has the result

;; Set the value to 4
LD H, 0x01
LD C, 0x04
LD A, 31
OUT (0xFF), A

Demonstrated in samples/ctrlc.z80.

Function 0x02: Change Console Driver

On entry DE points to a text-string, terminated by NULL, which represents the name of the console output driver to use.

Demonstrated in samples/console.z80

Function 0x03: Change CCP Driver

On entry DE points to a text-string, terminated by NULL, which represents the name of the CCP to use.

Demonstrated in samples/ccp.z80

Function 0x04: Set Quiet

If C is 0 quiet-mode is enabled, otherwise it is disabled.

Quiet mode prevents the display of a banner every time the CCP is restarted.

Demonstrated in samples/quiet.z80