68k initialization

Basic init code

The initial PC value in the 68k binary must be $C00402 (Init BIOS call), the initial SP value must be $10F300 (end of the user RAM).

The entrypoints for the 68k code start at $000122. There's usually 4 JMP instructions there (why ?).

    lea     $10F300, sp
    lea     $10F300, a7       ; Depends on assemblers...
    move.b  d0,REG_DIPSW      ; Kick watchdog
    move.w  #7,REG_IRQACK     ; Acknowledge any pending interrupts
    move.w  #$2700,sr         ; Disable all interrupts

See 68k interrupts and Watchdog for more informations.

Clearing user RAM:

    lea     RAMSTART,a0
    move.l  #($F300/4)-1,d0   ; User RAM's size is $F300, and we clear longwords (4 bytes)
clram:
    clr.l   (a0)+
    dbra    d0,clram

Note that this function also clears the stack. Don't make a subroutine out of it unless you're sure that the return address won't be cleared.

BIOS calls BIOSF_CLEARFIX ($C004C2) and BIOSF_CLEARSPR ($C004C8) can be used to clear sprites and the fix layer.

Don't forget to kick the watchdog regularly in your init code if the VBL interrupt isn't enabled or doesn't kick it itself.