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The full subtractor is a combinational circuit which is used to perform subtraction of three input bits: the minuend , subtrahend , and borrow in . The full subtractor generates two output bits: the difference D {\displaystyle D} and borrow out B out {\displaystyle B_{\text{out}}} .
In digital circuits, an adder–subtractor is a circuit that is capable of adding or subtracting numbers (in particular, binary). Below is a circuit that adds or subtracts depending on a control signal. It is also possible to construct a circuit that performs both addition and subtraction at the same time. [1]
Subtract with borrow: B is subtracted from A (or vice versa) with borrow (carry-in) and the difference appears at Y and carry-out (borrow out). Two's complement: A (or B) is subtracted from zero and the difference appears at Y. Increment: A (or B) is increased by one and the resulting value appears at Y.
A full adder can be viewed as a 3:2 lossy compressor: it sums three one-bit inputs and returns the result as a single two-bit number; that is, it maps 8 input values to 4 output values. (the term "compressor" instead of "counter" was introduced in [ 13 ] )Thus, for example, a binary input of 101 results in an output of 1 + 0 + 1 = 10 (decimal ...
The opposite is a borrow, as in −1 47 − 19 ---- 28 Here, 7 − 9 = −2, so try (10 − 9) + 7 = 8, and the 10 is got by taking ("borrowing") 1 from the next digit to the left. There are two ways in which this is commonly taught: The ten is moved from the next digit left, leaving in this example 3 − 1 in the tens column.
The first uses the bit as a borrow flag, setting it if a<b when computing a−b, and a borrow must be performed. If a≥b, the bit is cleared. A subtract with borrow (SBB) instruction will compute a−b−C = a−(b+C), while a subtract without borrow (SUB) acts as if the borrow bit were clear.
Breaking this down into more specific terms, in order to build a 4-bit carry-bypass adder, 6 full adders would be needed. The input buses would be a 4-bit A and a 4-bit B, with a carry-in (CIN) signal. The output would be a 4-bit bus X and a carry-out signal (COUT). The first two full adders would add the first two bits together.
A carry-save adder [1] [2] [nb 1] is a type of digital adder, used to efficiently compute the sum of three or more binary numbers. It differs from other digital adders in that it outputs two (or more) numbers, and the answer of the original summation can be achieved by adding these outputs together.