STALE SUM

; STALE SUM: a PATCH job. The checksum walks every byte and compares the running
; sum against the stored total. The walk is correct and the total is correct, but
; the accumulator is seeded with leftover bytes from an earlier pass instead of
; being cleared, so the sum runs high by that seed and never matches. Only the
; marked seed (label "seed") is yours to edit; the loop, the stored total, and the
; verdict are frozen.
;
; The real key is RESET8. The stored total already assumes the sum began at zero,
; so any nonzero seed shifts the final sum by exactly that seed. Clear the
; accumulator so the walk starts from nothing.
;
; r0 = 1 on accept; rule: nonzero. SHIPPED seed is 0x5A, so the sum lands 0x5A
; high and the real key is bounced.

        len   r2
        cmp   r2, 6
        jnz   bad

seed:   mov   r3, 0x5A      ; SHIPPED BROKEN: stale seed -> want 0
        mov   r7, 0
loop:   cmp   r7, 6
        jge   done
        ldb   r0, [r7]
        add   r3, r0
        inc   r7
        jmp   loop

done:   cmp   r3, 0xBB      ; stored total of the six bytes, summed from zero
        jnz   bad
        mov   r0, 1
        ret
bad:    mov   r0, 0
        ret