Editing Microcode (section)

===Early examples===
In 1947, the design of the [[Whirlwind (computer)|MIT Whirlwind]] introduced the concept of a control store as a way to simplify computer design and move beyond ''[[ad hoc]]'' methods. The control store is a [[diode matrix]]: a two-dimensional lattice, where one dimension accepts "control time pulses" from the CPU's internal clock, and the other connects to control signals on gates and other circuits. A "pulse distributor" takes the pulses generated by the [[CPU clock]] and breaks them up into eight separate time pulses, each of which activates a different row of the lattice. When the row is activated, it activates the control signals connected to it.<ref>{{Cite tech report |last1=Everett |first1=R.R. |last2=Swain |first2=F.E. |year=1947 |title=Whirlwind I Computer Block Diagrams |publisher=MIT Servomechanisms Laboratory |id=R-127 |url=http://www.cryptosmith.com/wp-content/uploads/2009/05/whirlwindr-127.pdf |access-date=June 21, 2006 |url-status=dead |archive-url=https://web.archive.org/web/20120617112919/http://www.cryptosmith.com/wp-content/uploads/2009/05/whirlwindr-127.pdf |archive-date=June 17, 2012}}</ref>

In 1951, [[Maurice Wilkes]]<ref>{{multiref|{{cite tech report |last=Wilkes |first=Maurice |year=1951 |title=The Best Way to Design an Automatic Calculating Machine |institution=[[University of Manchester]]}}|{{cite book |last=Wilkes |first=Maurice |chapter=The Best Way to Design an Automatic Calculating Machine |chapter-url=https://www.cs.princeton.edu/courses/archive/fall09/cos375/BestWay.pdf |editor-first=M. |editor-last=Campbell-Kelly |title=The early British computer conferences |publisher=MIT Press |date=1989 |isbn=978-0-262-23136-7 |pages=182–4 }}}}</ref> enhanced this concept by adding ''conditional execution'', a concept akin to a [[Conditional (programming)|conditional]] in computer software. His initial implementation consisted of a pair of matrices: the first one generated signals in the manner of the Whirlwind control store, while the second matrix selected which row of signals (the microprogram instruction word, so to speak) to invoke on the next cycle. Conditionals were implemented by providing a way that a single line in the control store could choose from alternatives in the second matrix. This made the control signals conditional on the detected internal signal. Wilkes coined the term ''microprogramming'' to describe this feature and distinguish it from a simple control store.