Bit slicing

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This article is about processor construction technique. For bit slicing as bit plane separation used in computer graphics and image processing, see bit-plane.

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 consist of an arithmetic logic unit (ALU) of 1, 2, 4 or 8 bits and control lines (including carry or overflow signals usually internal to the processor). For example, two 4-bit ALUs could be arranged side by side, with control lines between them, to form an 8-bit ALU. 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's Am2900 family the National Semiconductor IMP-16 and IMP-8 family, and the 74181.

During the era this technique was most common (mid-1970's through 1980's), there was some debate over how much bus width was necessary in a given computer system, and silicon chip technology and parts were generally much more expensive than today. Using multiple simpler (and cheaper) ALUs was seen as a way to increase computing power in a cost effective manner. 32-bit architectures were being discussed but few were in production. 16-bit processors were common but expensive, and the 8-bit processors, such as the Z80 were widely used in the nascent home computer market. Combining components to produce bit slice products allowed engineers and students to create more powerful and complex computers at a more reasonable cost, using off-the-shelf components that could be custom-configured. The complexities of creating a new computer architecture were greatly reduced when the details of the ALU were already specified (and debugged).

Bit slicing (although it was not called that) was also used in computers before integrated circuits. One example was the Storage Address Registers (StARs) of the IBM 1401, which were built on IBM Standard Modular System cards containing a one bit slice of the four registers in the basic machine.

This article was originally based on material from the Free On-line Dictionary of Computing, which is licensed under the GFDL.