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Subtractor

In electronics, a subtractor is a digital circuit that performs subtraction of numbers, and it can be designed using the same approach as that of an adder. The binary subtraction process is summarized below. As with an adder, in the general case of calculations on multi-bit numbers, three bits are involved in performing the subtraction for each bit of the difference: the minuend, subtrahend, and a borrow in from the previous bit order position. The outputs are the difference bit and borrow bit . The subtractor is best understood by considering that the subtrahend and both borrow bits have negative weights, whereas the X and D bits are positive. The operation performed by the subtractor is to rewrite as the sum . ,

Half subtractor
The half subtractors can be designed through the combinational Boolean logic circuits [2] as shown in Figure 1 and 2. The half subtractor is a combinational circuit which is used to perform subtraction of two bits. It has two inputs, the minuend X and subtrahend Y and two outputs the difference D and borrow out B_\text{out}. The borrow out signal is set when the subtractor needs to borrow from the next digit in a multi-digit subtraction. That is, B_{\text{out}} = 1 when X . Since X and Y are bits, B_\text{out} = 1 if and only if X = 0 and Y = 1. An important point worth mentioning is that the half subtractor diagram aside implements X - Y and not Y-X since B_\text{out} on the diagram is given by :B_{\text{out}} = \overline{X} \cdot Y. This is an important distinction to make since subtraction itself is not commutative, but the difference bit D is calculated using an XOR gate which is commutative. The truth table for the half subtractor is: Using the table above and a Karnaugh map, we find the following logic equations for D and B_\text{out}: :D = X \oplus Y :B_\text{out} = \overline X \cdot Y. Consequently, a simplified half-subtract circuit, advantageously avoiding crossed traces in particular as well as a negate gate is: X ── XOR ─┬─────── |X-Y|, is 0 if X equals Y, 1 otherwise ┌──┘ └──┐ Y ─┴─────── AND ── borrow, is 1 if Y > X, 0 otherwise where lines to the right are outputs and others (from the top, bottom or left) are inputs. ==Full subtractor==
Full subtractor
The full subtractor is a combinational circuit which is used to perform subtraction of three input bits: the minuend X, subtrahend Y, and borrow in B_\text{in}. The full subtractor generates two output bits: the difference D and borrow out B_\text{out}. B_\text{in} is set when the previous digit is borrowed from X. Thus, B_\text{in} is also subtracted from X as well as the subtrahend Y. Or in symbols: X - Y - B_\text{in}. Like the half subtractor, the full subtractor generates a borrow out when it needs to borrow from the next digit. Since we are subtracting Y and B_\text{in} from X, a borrow out needs to be generated when X . When a borrow out is generated, 2 is added in the current digit. (This is similar to the subtraction algorithm in decimal. Instead of adding 2, we add 10 when we borrow.) Therefore, D = X - Y - B_\text{in} + 2B_\text{out}. The truth table for the full subtractor is: Therefore the equation is: D=X\oplus Y\oplus B_{in} B_{out}=\bar{X}B_{in}+\bar{X}Y+YB_{in} ==See also==
Source: Wikipedia ↗
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