The subtraction is executed bit by bit, starting with the least significant bit.
Subtracting a bit from another bit can lead to a carry-over. This carry-over has to be considered when subtracting the next digits. There are two ways of "remembering" the carry-over:
![]() World before (left) and after (right) the execution of the first transition of state "sub -1" (the green circle marks the carry). |