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In computing, an arithmetic logic unit (ALU) is a combinational digital circuit that performs arithmetic and bitwise operations on integer binary numbers. [ 1 ] [ 2 ] This is in contrast to a floating-point unit (FPU), which operates on floating point numbers.
The 74S181 4-bit ALU bitslice resting on a page from the datasheet The 74181 is a 4-bit slice arithmetic logic unit (ALU), implemented as a 7400 series TTL integrated circuit . Introduced by Texas Instruments in February 1970, [ 1 ] it was the first complete ALU on a single chip. [ 2 ]
For example, the part of an arithmetic logic unit, or ALU, that does mathematical calculations is constructed using combinational logic. Other circuits used in computers, such as half adders, full adders, half subtractors, full subtractors, multiplexers, demultiplexers, encoders and decoders are also made by using combinational logic.
A data path is the ALU, the set of registers, and the CPU's internal bus(es) that allow data to flow between them. [2] A microarchitecture data path organized around a single bus. The simplest design for a CPU uses one common internal bus. Efficient addition requires a slightly more complicated three-internal-bus structure. [3]
An arithmetic logic unit (ALU) capable of adding and subtracting 8-bit 2's complement integers from registers A and B. This module also has a flags register with two possible flags (Z and C). Z stands for "zero," and is activated if the ALU outputs zero. C stands for "carry," and is activated if the ALU produces a carry-out bit.
For example, the following adder is a 64-bit adder that uses four 16-bit CLAs with two levels of lookahead carry units. Other adder designs include the carry-select adder , conditional sum adder , carry-skip adder , and carry-complete adder.
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In the 1970s and 1980s, a number of research and commercial computers used bit slicing, in which the CPU's arithmetic logic unit (ALU) was built from multiple 4-bit-wide sections, each section including a chip such as an Am2901 or 74181. The Zilog Z80, although it is an 8-bit microprocessor, has a 4-bit ALU. [11] [12]