This document is based largely on lecture notes and materials from the 0603-352 Computer Organization course, which in turn are based largely on M. Morris Mano's textbook Computer System Architecture, Third Edition, Prentice-Hall 1993.
DEC's VAX-11/780 had a writable control store (WCS). This was part of the microcode memory; at hard boot time, the WCS was reloaded from an 8" floppy located in the main system cabinet. Customers could extend the instruction set or modify those instructions whose microprograms were loaded into the WCS.
DEC and VAX are trademarks of Digital Equipment Corporation.