Editing Microcode (section)

{{Short description|Layer of hardware-level instructions or data structures}}
{{For|the CAD software vendor|MicroCode Engineering, Inc.}}
{{Program execution}}

In [[processor design]], '''microcode''' serves as an intermediary layer situated between the [[central processing unit]] (CPU) hardware and the programmer-visible [[instruction set architecture]] of a computer, also known as its [[machine code]].<ref name="Kent2813">{{cite book |last1=Kent |first1=Allen |url=https://books.google.com/books?id=EjWV8J8CQEYC |title=Encyclopedia of Computer Science and Technology: Volume 28 - Supplement 13 |last2=Williams |first2=James G. |date=April 5, 1993 |publisher=Marcel Dekker, Inc |isbn=0-8247-2281-7 |location=New York |access-date=Jan 17, 2016 |archive-url=https://web.archive.org/web/20161120161636/https://books.google.com/books?id=EjWV8J8CQEYC |archive-date=November 20, 2016 |url-status=live}}</ref>{{Page needed|date=July 2022}} It consists of a set of hardware-level instructions that implement the higher-level machine code instructions or control internal [[finite-state machine]] sequencing in many [[digital processing]] components. While microcode is utilized in [[Intel]] and [[AMD]] general-purpose CPUs in contemporary desktops and laptops, it functions only as a fallback path for scenarios that the faster [[hardwired control unit]] is unable to manage.<ref name="FogMicro">{{cite report |url=https://www.agner.org/optimize/microarchitecture.pdf |title=The microarchitecture of Intel, AMD and VIA CPUs |last1=Fog |first1=Agner |date=2017-05-02 |publisher=Technical University of Denmark |access-date=2024-08-21 |archive-url= https://web.archive.org/web/20170328065929/https://agner.org/optimize/microarchitecture.pdf |archive-date=2017-03-28 |url-status=live}}</ref>

Housed in special high-speed memory, microcode translates machine instructions, [[state machine]] data, or other input into sequences of detailed circuit-level operations. It separates the machine instructions from the underlying [[electronics]], thereby enabling greater flexibility in designing and altering instructions. Moreover, it facilitates the construction of complex multi-step instructions, while simultaneously reducing the complexity of computer circuits. The act of writing microcode is often referred to as ''microprogramming'', and the microcode in a specific processor implementation is sometimes termed a ''microprogram''.

Through extensive microprogramming, [[microarchitecture]]s of smaller scale and simplicity can [[Emulator|emulate]] more robust architectures with wider [[Word (computer architecture)|word]] lengths, additional [[execution unit]]s, and so forth. This approach provides a relatively straightforward method of ensuring software compatibility between different products within a processor family.

Some hardware vendors, notably [[IBM]] and [[Lenovo]], use the term ''microcode'' interchangeably with ''[[firmware]]''. In this context, all code within a device is termed microcode, whether it is microcode or machine code. For instance, updates to a [[hard disk drive]]'s microcode often encompass updates to both its microcode and firmware.<ref>{{cite web |title=IBM pSeries Servers - Microcode Update for Ultrastar 73LZX (US73) 18/36 GB |url=http://download.boulder.ibm.com/ibmdl/pub/software/server/firmware/73lzx.html |url-status=dead |archive-url=https://web.archive.org/web/20190419105117/http://download.boulder.ibm.com/ibmdl/pub/software/server/firmware/73lzx.html |archive-date=April 19, 2019 |access-date=January 22, 2015 |website=IBM}}</ref>