 A control store implementation whose contents are unalterable is known as a Read Only Memory (ROM) or Read Only Storage (ROS); one whose contents are alterable is known as a Writable Control Store (WCS).Early control stores were implemented as a diode-array accessed via address decoders, a form of read-only memory.This tradition dates back to the "program timing matrix" on the MIT Whirlwind, first described in 1947.