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bit slice processor

An earlier logic chip used as a building block for CPUs. Bit slice processors used arithmetic logic units (ALUs) that typically came in 4-bit increments, although 1- and 2-bit devices were also made. Connected to a control unit, the ALU slices were strung together to make larger processors (8-bit, 16-bit, etc.). They included inputs and outputs for borrow and carry bits (addition and subtraction require carrying to and borrowing from the digit on the left).

In the early days of microprocessors, bit slice processors enabled larger CPUs to be built from off-the-shelf components, and products were made by AMD, Intel and National Semiconductor in the mid-1970s. Most notable was AMD's 2900 family of integrated circuits used in CPUs from Digital and others, which included the Am2901 4-bit slice ALU. See ALU.



ALU

(Arithmetic Logic Unit) The high-speed circuit in the CPU that does the calculating and comparing. Numbers are transferred from RAM (memory) into the ALU for calculation, and the results are sent back to RAM. Alphanumeric data are sent from RAM into the ALU for comparing. The results of the compare are tested and may cause the computer to go to another part of the program; for example, If ItemA equals ItemB GoTo UpdateRoutine.

Floating Point Operations
A division may result in a fraction, and while some ALUs handle floating point operations, which support fractions, others do not and require a separate circuit (see math coprocessor). See DSP chip.

Multiple ALUs
Some chips have multiple ALUs that allow for simultaneous calculations. In an extreme case, Chromatic Research's MPACT media processor had 450 ALUs. It allowed audio, video and other multimedia processes to be performed simultaneously (see MPACT chip). See computer, control unit and half adder.




An ALU in 1957
An arithmetic logic unit you have to sit back and admire. This floor-standing ALU was part of Honeywell's Datamatic 1000 computer. (Image courtesy of Honeywell, Inc.)






Thirty Years Later
In 1987, the ALU embedded within this 386 chip would fit on the end of a pencil eraser with room to spare. Today, an ALU takes up less space than the tip of the pencil. (Image courtesy of Intel Corporation.)






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