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The lowest layer in a computer's software stack is traditionally raw [[machine code]] instructions for the processor. In microcoded processors, fetching and decoding those instructions, and executing them, may be done by microcode. To avoid confusion, each microprogram-related element is differentiated by the ''micro'' prefix: microinstruction, microassembler, microprogrammer, etc.{{Citation needed|date=July 2022}}
Complex digital processors may also employ more than one (possibly microcode-based) [[control unit]] in order to delegate sub-tasks that must be performed essentially asynchronously in parallel. For example, the [[VAX 9000]] has an hardwired IBox unit to fetch and decode instructions, which it hands to a microcoded EBox unit to be executed,<ref>{{cite book|url=http://www.bitsavers.org/pdf/dec/vax/9000/EK-KA90S-TD-001_VAX_9000_System_Technical_Description_May90.pdf|title=VAX 9000 System Technical Description|publisher=[[Digital Equipment Corporation]]|date=May 1990|id=EK-KA90S-TD-001|pages=3{{hyp}}5-3{{hyp}}32}}</ref> and the [[VAX 8800]] has both a microcoded IBox and a microcoded EBox.<ref>{{cite book|url=http://bitsavers.org/pdf/dec/vax/8800/EK-KA882_8800sysTech2_Jul86.pdf|title=VAX 8800 System Technical Description Volume 2|publisher=[[Digital Equipment Corporation]]|date=July 1986|id=EK-KA882-TD-PRE}}</ref>
A high-level programmer, or even an [[assembly language]] programmer, does not normally see or change microcode. Unlike machine code, which often retains some [[backward compatibility]] among different processors in a family, microcode only runs on the exact [[electronic circuit]]ry for which it is designed, as it constitutes an inherent part of the particular processor design itself.
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