Bit slice

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A bit slice is a prefabricated building block in the form of an integrated circuit that was used in microelectronics for the individual construction of a processor . The bit-slice components, which were primarily used in the 1970s to 1980s, were a development step on the way from the computer made up of individual transistors to the microprocessor used exclusively today , i.e. the processor on a single die . Bit-slicing was the term used to describe a method from computer architecture in which larger arithmetic units were assembled from several individual components, which were often all designed for relatively small words - the bit slices - (mostly 4 bits long) .

Bit slices for arithmetic and logic units , registers and jump and branch units were combined in order to achieve the desired word length of the processor. In addition, there were memories for the microprogram , which mostly consisted of fast ROM components , a microprogram control unit and various smaller components such as bus drivers.


In the 1970s and 1980s, bit slice and microprocessor coexisted for a long time, as bit slice chips were still superior to microprocessors. The first microprocessors only offered word widths of 4 or 8 bits; on the other hand, 16 and 32 bit wide processors were built with bit slice or even unusual word widths such as 24 bit and high performance processors with 64 bit.

Increasing integration density enabled both the microprocessors and the bit slices to perform better, so that soon microprocessors with 32 bits were available and also bit slices with 32 bits on a single chip, which due to higher clock rates and more complex arithmetic units initially still belong to the microprocessor were superior.

In the second half of the 1980s it was then possible to integrate the entire CPU for a mainframe computer on a single chip, and the division into several components increasingly set limits to the clock frequencies that could be exceeded by processors on just one chip. The higher development and production costs for processors from bit slices no longer brought any performance advantages. As a result, the bit slices in general computer construction lost their right to exist and only existed for a certain time in special applications.

A bit-slice family is to be understood as a modular system for processors, but requires higher computer architecture and electronics knowledge than the use of a finished microprocessor module. However, this plays a subordinate role when you consider that almost every imaginable processor can be implemented using bit slice components. This advantage has been used many times to build an inexpensive prototype of a microprocessor that is easy to use - e.g. B. by changing the microprogram - can test, modify and correct. Another area of ​​application was the copying of old processor architectures for miniaturization or replacement.

With the advent of programmable logic components that were complex enough to accommodate complete processors (or at least large parts, distributed over several components), bit-slice components gradually disappeared from the market in the late 1980s. Programmable logic modules are more flexible, and some types can be completely reprogrammed even if they are already built into a circuit.

Bit-slice processors / building blocks

  • 1 bit
    • ...
  • 4 bit
    • National IMP-4
    • National IMP-16
    • AMD Am2900
    • Monolithic Memories MMI 5700/6700
    • Texas Instruments SBP0400
    • TI SN74181
    • TI SN74S281 + SN74S282
    • TI SN74S481 + SN74S482
    • Fairchild 9400 (MACROLOGIC), 4700
    • Motorola MC10800
  • 8 bit
  • 16 bit
    • AMD 29100
    • Synopsys 49C402

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