First checksum:
- clear 2 byte-wide registers
- omit the leading 16h
- from the first byte (length) subtract 1
- repeat for all bytes in body:
  - add byte to first register, ignore overflows
  - add first register to second register, ignore overflows
- done.

Second checksum:
- clear 2 byte-wide registers
- omit the leading 16h
- repeat for all bytes in body:
  - add byte to first register, ignore overflows
  - add first register to second register, ignore overflows
- subtract from zero both first and second register 
  and place it back into first register
- done.