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A 4-bit ripple-carry adder–subtractor based on a 4-bit adder that performs two's complement on A when D = 1 to yield S = B − A. Having an n-bit adder for A and B, then S = A + B. Then, assume the numbers are in two's complement. Then to perform B − A, two's complement theory says to invert each bit of A with a NOT gate then add one.
An asynchronous (ripple) counter is a "chain" of toggle (T) flip-flops in which the least-significant flip-flop (bit 0) is clocked by an external signal (the counter input clock), and all other flip-flops are clocked by the output of the nearest, less significant flip-flop (e.g., bit 0 clocks the bit 1 flip-flop, bit 1 clocks the bit 2 flip ...
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 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 ...
It can be contrasted with the simpler, but usually slower, ripple-carry adder (RCA), for which the carry bit is calculated alongside the sum bit, and each stage must wait until the previous carry bit has been calculated to begin calculating its own sum bit and carry bit. The carry-lookahead adder calculates one or more carry bits before the sum ...
A 16-bit carry-select adder with a uniform block size of 4 can be created with three of these blocks and a 4-bit ripple-carry adder. Since carry-in is known at the beginning of computation, a carry select block is not needed for the first four bits. The delay of this adder will be four full adder delays, plus three MUX delays.
An example of a 4-bit Kogge–Stone adder is shown in the diagram. Each vertical stage produces a "propagate" and a "generate" bit, as shown. The culminating generate bits (the carries) are produced in the last stage (vertically), and these bits are XOR'd with the initial propagate after the input (the red boxes) to produce the sum bits. E.g., the first (least-significant) sum bit is ...
synchronous presettable up/down 4-bit decade counter 16 SN74LS190: 74x191 1 synchronous presettable up/down 4-bit binary counter 16 SN74LS191: 74x192 1 synchronous presettable up/down 4-bit decade counter, clear 16 SN74LS192: 74x193 1 synchronous presettable up/down 4-bit binary counter, clear 16 SN74LS193: 74x194 1