busted checksum routine (thanks MJ)

    tst.b   (Checksum_OK).w
    bne.w   CS_Skip
; MJ's Modular Checksum
CheckSumCheck:
    movea.w #ErrorTrap,a0   ; prepare start address
    move.l  (ROMEndLoc).l,d7; load size
    sub.l   a0,d7       ; minus start address
    move.b  d7,d5       ; copy end nybble
    andi.w  #$F,d5      ; get only the remaining nybble
    lsr.l   #4,d7       ; divide the size by 20
    move.w  d7,d6       ; load lower word size
    swap    d7      ; get upper word size
    moveq   #0,d0       ; clear d0

; loc_338: ChecksumLoop:
CS_MainBlock:
    add.w   (a0)+,d0    ; modular checksum (8 words)
    add.w   (a0)+,d0    ; ''
    add.w   (a0)+,d0    ; ''
    add.w   (a0)+,d0    ; ''
    add.w   (a0)+,d0    ; ''
    add.w   (a0)+,d0    ; ''
    add.w   (a0)+,d0    ; ''
    add.w   (a0)+,d0    ; ''
    dbf d6,CS_MainBlock ; repeat until all main block sections are done
    dbf d7,CS_MainBlock ; ''
    subq.w  #1,d5       ; decrease remaining nybble for dbf
    bpl.s   CS_Finish   ; if there is no remaining nybble, branch

CS_Remains:
    add.w   (a0)+,d0    ; add remaining words
    dbf d5,CS_Remains   ; repeat until the remaining words are done

CS_Finish:
    cmp.w   (Checksum).l,d0 ; does the checksum match?
    bne.w   ChecksumError   ; if not, branch
    move.b  #1,(Checksum_OK).w

CS_Skip: