Mac ROM Checksum Computation? — #2

Post #2 by Dennis Nedry

Source Forum	68kMLA
Category	68k
Post Date	Wed, 24 Feb 2010 - 22:10
Original URL	https://68kmla.org/bb/threads/mac-rom-checksum-computation.19981/

Post

I believe I found the checksum subroutine in the Mac SE ROM, with the help of FDisasm.

Code:

                      P_ChecksumRom:
26C8   7000                      MoveQ.L   $0, D0
26CA   7200                      MoveQ.L   $0, D1
26CC   41F9 0040 0000            Lea.L     ($400000), A0
26D2   2818                      Move.L    (A0)+, D4
26D4   263C 0001 FFFE            Move.L    $1FFFE, D3
26DA   3018           L335:      Move      (A0)+, D0
26DC   D280                      Add.L     D0, D1
26DE   5383                      SubQ.L    $1, D3
26E0   66F8                      BNE       L335
26E2   4E71                      Nop
26E4   4E71                      Nop
26E6   B981                      Eor.L     D4, D1
26E8   6704                      BEQ       L336
26EA   3C3C FFFF                 Move      $-1, D6
26EE   4ED6           L336:      Jmp       (A6)

Is anyone fluent enough in 68000 ASM to describe what's going on here? I have some knowledge of ASM but it pertains mostly to much more simple processors/microcontrollers.