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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 ]
Many relatively simple CPUs have a 2-read, 1-write register file connected to the 2 inputs and 1 output of the ALU. During the late 1990s, there was growing research in the area of reconfigurable data paths—data paths that may be re-purposed at run-time using programmable fabric —as such designs may allow for more efficient processing as ...
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]
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.
The mutual goal of IEEE Std 91-1984 and IEC 617-12 was to provide a uniform method of describing the complex logic functions of digital circuits with schematic symbols. These functions were more complex than simple AND and OR gates. They could be medium-scale circuits such as a 4-bit counter to a large-scale circuit such as a microprocessor.
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.
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]