Editing Microcode (section)

===Design===
Engineers normally write the microcode during the design phase of a processor, storing it in a [[read-only memory]] (ROM) or [[programmable logic array]] (PLA)<ref>{{cite journal |last1=Manning |first1=B.M. |last2=Mitby |first2=J.S |last3=Nicholson |first3=J.O. |title=Microprogrammed Processor Having PLA Control Store |journal=IBM Technical Disclosure Bulletin |volume=22 |issue=6 |date=November 1979 |url=http://www.computerhistory.org/collections/accession/102660026 |access-date=2011-07-10 |url-status=live |archive-url=https://web.archive.org/web/20121001165413/http://www.computerhistory.org/collections/accession/102660026 |archive-date=2012-10-01}}</ref> structure, or in a combination of both.<ref>Often denoted a ROM/PLA control store in the context of usage in a CPU; {{cite web |last=Supnik |first=Bob |date=24 February 2008 |title=J-11: DEC's fourth and last PDP-11 microprocessor design ... features ... ROM/PLA control store |url=http://simh.trailing-edge.com/semi/j11.html |access-date=2011-07-10 |url-status=live |archive-url=https://web.archive.org/web/20110709032923/http://simh.trailing-edge.com/semi/j11.html |archive-date=2011-07-09}}</ref> However, machines also exist that have some or all microcode stored in [[static random-access memory]] (SRAM) or [[flash memory]]. This is traditionally denoted as ''writable [[control store]]'' in the context of computers, which can be either read-only or [[read–write memory]]. In the latter case, the CPU initialization process loads microcode into the control store from another storage medium, with the possibility of altering the microcode to correct bugs in the instruction set, or to implement new machine instructions.