Bit slicing

Bit slicing is a technique for constructing a processor from modules of smaller bit width. Each of these components processes one bit field or "slice" of an operand. The grouped processing components would then have the capability to process the chosen full word-length of a particular software design.

Bit slice processors usually include an arithmetic logic unit (ALU) of 1, 2, 4 or 8 bits and control lines (including carry or overflow signals that are internal to the processor in non-bitsliced CPU designs).

For example, two 4-bit ALU chips could be arranged side by side, with control lines between them, to form an 8-bit ALU. Four 4-bit ALU chips could be used to build a 16-bit ALU. It would take eight chips to build a 32-bit word ALU. The designer can add as many slices as required to manipulate increasingly longer word lengths.

A microsequencer or Control ROM would be used to execute logic to provide data and control signals to regulate function of the component ALUs. Examples of bit-slice microprocessor modules can be seen in the Intel 3000 family, the AMD Am2900 family, the National Semiconductor IMP-16, and IMP-8 family, and the 74181.

Bit slicing, although not called that at the time, was also used in computers before large scale integrated circuits (LSI, the predecessor to today's VLSI, or very-large-scale integration circuits). The first bit-sliced machine was EDSAC 2, built at the University of Cambridge Mathematical Laboratory in 1956–58.

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