In computer architecture, millicode is a higher level of microcode used to implement the instruction set of a computer. Millicode runs on top of the microcoded instructions and uses those instructions to implement more complex instructions visible to the user of the system. Implementation of millicode requires a special processor mode called millimode that provides its own set of registers, and possibly its own special instructions invisible to the user.[1]

IBM invented both the concept and the term millicode for the System/390 9672-G4 processor in 1997.[1] The following are cited as advantages of millicode:[1]

  • More complex instructions can easily be constructed from several millicode instructions.
  • Construction of a compatible line of computer models with different performance is simplified.
  • Millicode instructions can bypass CPU cache to improve performance.
  • Instructions can update multiple storage locations without concern for being interrupted.
  • Millicode can execute instructions at a higher privilege level without involving the operating system.
  • Millicode can provide a complex instruction as if it were a subroutine, making user code smaller.

The "i370" code for the "Capitol" chipset used in some ES/9370 models was similar to millicode, in that it was written as a combination of System/370 instructions and code that had access to special hardware features.[2]

See also


  1. Rogers, Bob (Sep–Oct 2012). "The What and Why of zEnterprise Millicode". IBM Systems Magazine.
  2. Wilhelm Spruth (ed.). "7.2 High Level Microprogramming in I370". The Design of a Microprocessor. Springer-Verlag. ISBN 978-3-642-74918-6.
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