Early control stores were implemented as a diode-array accessed via address decoders, a form of read-only memory.
The System/370 architecture includes a facility called Initial-Microprogram Load (IML or IMPL)[4] that can be invoked from the console, as part of Power On Reset (POR) or from another processor in a tightly coupled multiprocessor complex.
[7] Other commercial machines that use writable microcode include the Burroughs Small Systems (1970s and 1980s), the Xerox processors in their Lisp machines and Xerox Star workstations, the DEC VAX 8800 ("Nautilus") family, and the Symbolics L- and G-machines (1980s).
Some DEC PDP-10 machines store their microcode in SRAM chips (about 80 bits wide x 2 Kwords), which is typically loaded on power-on through some other front-end CPU.
The Data General Eclipse MV/8000 ("Eagle") has a SRAM writable control store, loaded on power-on through another CPU.
[9] WCS offers several advantages including the ease of patching the microprogram and, for certain hardware generations, faster access than ROMs could provide.
The outputs that go back into the sequencer to determine the next address have to go through some sort of register to prevent the creation of a race condition.