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===Writable stores===
Some computers were built using "writable microcode" — rather than storing the microcode in ROM or hard-wired logic, the microcode was stored in a RAM called a ''writable control store'' or ''WCS''. Such a computer is sometimes called a ''Writable Instruction Set Computer'' or ''WISC''.<ref>
The original [[IBM System/360|System/360]] models had read-only control store, but later System/360, [[IBM System/370|System/370]] and successor models loaded part or all of their microprograms from floppy disks or other [[Direct access storage device|DASD]] into a writable control store consisting of ultra-high speed [[random-access memory|random-access]] [[read-write memory]]. The System/370 architecture included a facility called '''Initial-Microprogram Load''' ('''IML''' or '''IMPL''')<ref>{{cite manual
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Several Intel CPUs in the [[x86]] architecture family have writable microcode,<ref>
This has allowed bugs in the [[Intel Core 2]] microcode and Intel [[Xeon]] microcode to be fixed in software, rather than requiring the entire chip to be replaced.
Such fixes 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}}</ref> [[FreeBSD]],<ref>{{cite mailing list |url=https://lists.freebsd.org/pipermail/freebsd-hackers/2018-March/052359.html |title=New microcode updating tool for FreeBSD |author=Stefan Blachmann |mailing-list=freebsd-hackers |date=2018-03-02 |access-date=2019-07-09}}</ref> Microsoft Windows,<ref>{{cite web| url = http://support.microsoft.com/kb/936357| title = A microcode reliability update is available that improves the reliability of systems that use Intel processors}}{{dead link}}</ref> or the motherboard BIOS.<ref>{{cite web| url = https://www.intel.com/content/www/us/en/support/articles/000007784/server-products.html | title = BIOS Update required when Missing Microcode message is seen during POST | website = [[Intel]] | access-date = 2022-01-13}}</ref>
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