Checksum:
    ld     a,e          ; add (hl) to de
    add    (hl)
    ld     e,a
    ld     a,d
    adc    $00
    ld     d,a
    inc    hl           ; move pointer on and decrement counter
    dec    bc
    ld     a,b          ; repeat until counter is zero
    or     c
    jr     nz,Checksum
    ret