Microcode - History

History

In 1947, the design of the 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 was a diode matrix: a two-dimensional lattice, where one dimension accepted "control time pulses" from the CPU's internal clock, and the other connected to control signals on gates and other circuits. A "pulse distributor" would take the pulses generated by the CPU clock and break them up into eight separate time pulses, each of which would activate a different row of the lattice. When the row was activated, it would activate the control signals connected to it.

Described another way, the signals transmitted by the control store are being played much like a player piano roll. That is, they are controlled by a sequence of very wide words constructed of bits, and they are "played" sequentially. In a control store, however, the "song" is short and repeated continuously.

In 1951 Maurice Wilkes enhanced this concept by adding conditional execution, a concept akin to a 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, as it were) 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.