68000 Second Processor

CiscOS API Calls

Programs communicate with the kernal and MOS with the TRAP 12 opcode with parameters arranged as for an ARM/RISC OS SWI call, with A0=SWI number.

D0: SWI R0         A0: SWI number
D1: SWI R1         A1: SWI R8         ; rarely used
D2: SWI R2         A2: SWI R9         ; rarely used
D3: SWI R3         A3: SWI R10        ; only for OS_CallASWIR10
D4: SWI R4         A4: 'RISC OS R12'  ; private workspace pointer
D5: SWI R5         A5: SWI R12        ; only for OS_CallASWIR12
D6: SWI R6         A6=>stack frame
D7: SWI R7         A7=>stack

On exit:
If VC, D0=any returned data
If VS, D0=>error block: EQUW errnum:EQUS errstr:EQUB 0
All registers that are not used by the SWI call are preserved, including A0.

The BASIC SYS command only passes and returns R0-R7, which are passed through D0-D7.

Examples

MOV #ASC"B",D0
MOV #0,A0          ; OS_WriteC
TRAP 12            ; Writes "B" to the current output stream


MOV #ASC"B"+256,A0 ; OS_WriteI+ASC"B"
TRAP 12            ; Writes "B" to the current output stream


MOV #5,D0          ; Read file info
MOV filename,D1    ; D1=>filename
MOV #8,A0          ; A0=OS_File
TRAP 12            ; Read information on the specified file/object
BVS erroroccured

A0 is preserved, so multiple non-X calls can be concatonated:

MOV #0,A0          ; OS_WriteC
MOV ASC"H"
TRAP 12            ; H
MOV ASC"E"
TRAP 12            ; E
MOV ASC"L"
TRAP 12            ; L
TRAP 12            ; L
MOV ASC"O"
TRAP 12            ; O

See also

• 68000 BBC BASIC
• mdfs.net

Jgharston (talk) 19:03, 25 August 2019 (CEST)