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These elements could often be bought altogether in one chip. This chip came in a fixed width which would form a 'slice' through the execution unit. These were known a 'bit slice' chips.
The parts of the execution units, and the execution units themselves are interconnected by a bundle of wires called a [[Computer bus|bus]].
Programmers develop microprograms. The basic tools are software: A [[microassembler]] allows a programmmer to define the table of bits symbolically. A [[simulator]] program executes the bits in the same way as the electronics (hopefully), and allows much more freedom to debug the microprogram.
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A few computers were built using "writable microcode" -- rather that storing the microcode in ROM or hard-wired logic, the microcode was stored in a RAM. Most of them were experimental laboratory prototypes, but there were a few commercial machines that used writable microcode, such as early [[Xerox PARC|Xerox]] workstations.
A CPU that uses [[microcode]] generally takes several clock cycles to execute a single instruction, one clock cycle for each step in the microprogram for that instruction. Some [[CISC]] processors include instructions that can take a very long time to execute.
Such variations in instruction length interfere with [[pipelining]] and [[interrupt]] [[latency]].
Many [[RISC]] and [[VLIW]] processors are designed to execute every instruction (as long as it is in the cache) in a single cycle. This is very similar to the way CPUs with microcode execute one microinstruction per cycle. [[VLIW]] processors have instructions that look like very wide horizontal microcode, while [[RISC]] processors generally have instructions that look more like narrow vertical microcode.
==See also==
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[[Category:Computer hardware]][[Category:Computer terminology]]
== external links ==
* [http://c2.com/cgi/wiki?WritableInstructionSetComputer Writable Instruction Set Computer]
[[de:Mikroprogrammsteuerwerk]]
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