ROM Patches

Known ROM Versions

1.1
1.2.1
1.3b2

Startup Patching

The Outbound contains EEPROMs placed in Apple's "Test Software" address space.

If the stock Macintosh ROM finds the value 0x55AAAA55 there, it will execute that code instead of continuing with normal startup. This check is one of the first few instructions that the computer executes.

Here is an example of the check from the v1 Macintosh Plus ROM:

    movem.l $F80000,D0/A0
    cmpi.l  #$55AAAA55,D0
    bne.b   NormalStartup
    lea     NormalStartup,A1
    jmp     (A0)

This ROM code then does setup for the Outbound hardware, and enables the trace bit on the 68000. This causes code to run after every instruction that checks the address of the instruction and jumps to patched code if it matches.