3.3 Hooking into the reset vector

The recipe for hooking your own routines into the reset-vector runs as follows:

Set the system variable resvalid to the value 0x31415926, as therwise resvector will be completely ignored by the BIOS ignore.
Write the address of your own function in the resvector vector.
Jump back from the function via jmp(a6).

Important: A jump back from the installed function via rts is not possible, since no stack has been initialised yet at this time. In practice one could proceed as follows, for instance:

RESMAGIC    equ       $31415926
_resvalid   equ       $426
_resvector  equ       $42a
            .text
install:    move.l    _resvalid,oldvalid
            move.l    #RESMAGIC,resvalid
            move.l    _resvector,oldreset
            move.l    #newreset,_resvector
            rts

            dc.b      "XBRARESV"
oldreset:   dc.l      0

newreset:   move.l    oldreset,_resvector
            move.l    oldvalid,_resvalid
            jmp       (a6)

            .bss
oldvalid:   .ds.l     1

Warning: So that multiple programs may install themselves, a clean de-installation must follow the handling of the function.

BIOS

The input/output channels of the BIOS

The system variables