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The subtract function operates on a 2-byte array. It sets the first byte to the difference of the first byte and the second byte:

f( [ A, B ] ) = [ Q, B ], where A,B,Q

Q = A − B

B passes through, unchanged.

The subtraction is transformed into a sum by taking advantage of the fact that A, B, and Q are two’s complement numbers:

A − B = A + -B = A + ~B + 1, where ~ means bitwise inversion.

Its circuit is a modification of the add circuit:

8-bit Subtractor Schematic

The initial carry-in (far right) serves as the + 1 term of the sum.

The circuit bitwise inverts B twice to retain its value.

The Tetris realization follows.

8-bit Subtractor

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