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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.
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.
4-bit computing is the use of computer architectures in which integers and other data units are 4 bits wide. 4-bit central processing unit (CPU) and arithmetic logic unit (ALU) architectures are those that are based on registers or data buses of that size.
An adder, or summer, [1] is a digital circuit that performs addition of numbers. In many computers and other kinds of processors, adders are used in the arithmetic logic units (ALUs).
A common example is the Data General Nova, which was a 16-bit design that performed 16-bit math as a series of four 4-bit operations. 4-bits was the word size of a widely available single-chip ALU and thus allowed for inexpensive implementation. Using the definition being applied to the 68000, the Nova would be a 4-bit computer, or 4/16.
A serial computer processes data a single bit at a time. For example, the PDP-8/S was a 12-bit computer using a 1-bit ALU, processing the 12 bits serially. [2]An example of a 1-bit computer built from discrete logic SSI chips is the Wang 500 (1970/1971) calculator [3] [4] as well as the Wang 1200 (1971/1972) [5] word processor series developed by Wang Laboratories.
Also, 128-bit central processing unit (CPU) and arithmetic logic unit (ALU) architectures are those that are based on registers, address buses, or data buses of that size. General home computing and gaming utility emerged at 8-bit word sizes, as 2 8 =256 words , a natural unit of data, became possible.
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]