Patent ID: 8606842

Claim:
An N-digit subtraction unit comprising: a first-digit subtraction circuit including: an input of a first-digit minuend; an input of a first-digit subtrahend; a logic circuit to output a first upper borrowing output to an upper-digit subtraction circuit for combinations of the first-digit minuend and the first-digit subtrahend; and a first-digit subtraction output corresponding to the combinations of the first-digit minuend and the first-digit subtrahend; at least one upper-digit subtraction circuit including: an input of an upper-digit minuend; an input of an upper-digit subtrahend; an input of a borrowing bit corresponding to the first upper borrowing output or a second upper borrowing output; a borrowing circuit to output an upper-digit borrowing output corresponding to combinations of the upper-digit minuend, the upper-digit subtrahend, and the borrowing bit, wherein the upper-digit borrowing output is generated by a first 3-input exclusive OR circuit that outputs exclusive OR combinations of the upper-digit minuend, the upper-digit subtrahend, and the borrowing bit, and wherein prior to being input to the first 3-input exclusive OR circuit: the upper-digit minuend is inverted prior to being input to a first 2-input AND circuit for combinations of the upper-digit minuend and the upper-digit subtrahend; the upper-digit subtrahend is inverted prior to being input to a 3-input AND circuit for the combinations of the upper-digit minuend, the upper-digit subtrahend, and the borrowing bit; and the borrowing bit is input to a second 2-input AND circuit for combinations of the upper-digit subtrahend and the borrowing bit; and an upper-digit subtraction output corresponding to the combinations of the upper-digit minuend, the upper-digit subtrahend, and the borrowing bit, wherein the upper-digit subtraction output is generated by a second 3-input exclusive OR circuit that outputs exclusive OR combinations of the upper-digit minuend, the upper-digit subtrahend, and the borrowing bit.