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Starting with the [[Pentium Pro]] in 1995, several [[x86]] CPUs have writable [[Intel Microcode]].<ref name="Stiller_1996">{{cite journal |last1=Stiller |first1=Andreas |last2=Paul |first2=Matthias R.<!-- info contributor on processor internals --> |date=1996-05-12 |title=Prozessorgeflüster |series=Trends & News |language=de |journal=[[c't|c't – magazin für computertechnik]] |publisher=[[Heise Verlag]] |url=https://www.heise.de/ct/artikel/Prozessorgefluester-284546.html |access-date=2017-08-28 |url-status=live |archive-url=https://web.archive.org/web/20170828172141/https://www.heise.de/ct/artikel/Prozessorgefluester-284546.html |archive-date=2017-08-28}}</ref><ref>{{cite book |url=http://www.intel.com/Assets/PDF/manual/253668.pdf |title=Intel 64 and IA-32 Architectures Software Developer's Manual, Volume 3A: System Programming Guide, Part 1 |chapter=9.11: Microcode update facilities |publisher=[[Intel]] |date=September 2016}}</ref> This, for example, has allowed bugs in the [[Intel Core 2]] and Intel [[Xeon]] microcodes to be fixed by patching their microprograms, rather than requiring the entire chips to be replaced. A second prominent example is the set of microcode patches that Intel offered for some of their processor architectures of up to 10 years in age, in a bid to counter the security vulnerabilities discovered in their designs – [[Spectre (security vulnerability)|Spectre]] and [[Meltdown (security vulnerability)|Meltdown]] – which went public at the start of 2018.<ref>[http://www.tomshardware.com/news/intel-meltdown-spectre-patch-silicon,36672.html Intel Patches All Recent CPUs, Promises Hardware Fixes For Upcoming 8th Gen Chips] by Paul Alcorn on March 15, 2018</ref><ref>{{cite web |url=https://downloadcenter.intel.com/download/27591/Linux-Processor-Microcode-Data-File |title=Download Linux* Processor Microcode Data File |access-date=2018-03-21 |url-status=dead |archive-url=https://web.archive.org/web/20180319202103/https://downloadcenter.intel.com/download/27591/Linux-Processor-Microcode-Data-File |archive-date=2018-03-19}}</ref> A microcode update can be installed by Linux,<ref>{{cite web |url=http://urbanmyth.org/microcode/ |title=Intel Microcode Update Utility for Linux |archive-url=https://web.archive.org/web/20120226174302/http://urbanmyth.org/microcode/ |archive-date=2012-02-26 |url-status=dead}}</ref> [[FreeBSD]],<ref>{{cite web |url=https://svnweb.freebsd.org/ports/head/sysutils/cpupdate/ |title=[ports] Index of /head/sysutils/cpupdate |publisher=Freebsd.org |access-date=2020-01-16 |url-status=live |archive-url=https://web.archive.org/web/20200401215701/https://svnweb.freebsd.org/ports/head/sysutils/cpupdate/ |archive-date=2020-04-01}}</ref> Microsoft Windows,<ref>{{Cite news |url=http://support.microsoft.com/kb/936357 |title=A microcode reliability update is available that improves the reliability of systems that use Intel processors |access-date=2008-02-25 |url-status=live |archive-url=https://web.archive.org/web/20080223074207/http://support.microsoft.com/kb/936357 |archive-date=2008-02-23}}</ref> or the motherboard BIOS.<ref>{{cite web |url=http://www.intel.com/support/motherboards/server/sb/cs-021619.htm |title=Server Products - BIOS Update required when Missing Microcode message is seen during POST |date=January 24, 2013 |website=Intel |archive-url=https://web.archive.org/web/20140901063251/http://www.intel.com/support/motherboards/server/sb/cs-021619.htm |archive-date=September 1, 2014}}</ref>
Some machines offer user-programmable writable control stores as an option, including the [[HP 2100]], DEC [[PDP-11|PDP-11/60]], [[TI-990]]/12,<ref>{{cite web |title=Model 990/12 LR Computer Depot Maintenance and Repair Manual |url=http://www.bitsavers.org/pdf/ti/990/990-12/2268241_990-12CPU_DepoRepair_Feb83.pdf |website=Bitsavers.org |publisher=Texas Instruments |access-date=15 February 2024}}</ref><ref>{{cite book |title=Texas Instruments Model 990 Computer MDS-990 Microcode Development System Programmer's Guide |___location=Texas Instruments Archives, RG-20 accession 94-08, Box 10, 45C. DeGolyer Library, Southern Methodist University, Dallas, TX USA |edition=15 August 1979}}</ref> and [[Varian Data Machines]] V-70 series [[minicomputer]]s. WCS options extended down to microprocessors too. The [[PDP-11#LSI-11|DEC LSI-11]] has an option to allow programming of the internal 8-bit micromachine to create application-specific extensions to the instruction set.<ref>{{cite book |title=LSI-11 WCS user's guide |date=June 1978 |publisher=Digital Equipment Corporation |edition=1st |url=http://www.bitsavers.org/pdf/dec/pdp11/1103/EK-KUV11-TM_LSI11_WCS.pdf |access-date=7 January 2023 |archive-date=23 February 2023 |archive-url=https://web.archive.org/web/20230223212303/http://www.bitsavers.org/pdf/dec/pdp11/1103/EK-KUV11-TM_LSI11_WCS.pdf |url-status=live }}</ref>
==Comparison to VLIW and RISC==
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