Bit slice

Bit slicing is a technique for constructing a processor from modules of processors of smaller bit width, for the purpose of increasing the word length; in theory to make an arbitrary n-bit CPU. Each of these component modules 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 slicing more or less died out as a way to make computers, because of microprocessors. It's making a comeback to make (so far only ALUs for) quantum computers, and a little earlier has also been used as a software technique for use e.g. in x86 CPUs, for cryptography.

Bit slice processors usually include an arithmetic logic unit (ALU) of 1, 2, 4, 8 or 16 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 (result need not be power of two, e.g. three 1-bit can make a 3-bit ALU, thus 3-bit (or n-bit) CPU, while such hasn't been used in volume). 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 could 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.

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