Microcode - Writable Control Stores

Writable Control Stores

A few computers were built using "writable microcode" — rather than storing the microcode in ROM or hard-wired logic, the microcode was stored in a RAM called a Writable Control Store or WCS. Such a computer is sometimes called a Writable Instruction Set Computer or WISC. Many experimental prototype computers used writable control stores, and there were also commercial machines that used writable microcode, such as early Xerox workstations, the DEC VAX 8800 ("Nautilus") family, the Symbolics L- and G-machines, a number of IBM System/360 and System/370 implementations, some DEC PDP-10 machines, and the Data General Eclipse MV/8000. Many more machines offered user-programmable writable control stores as an option (including the HP 2100, DEC PDP-11/60 and Varian Data Machines V-70 series minicomputers). The IBM System/370 included a facility called Initial-Microprogram Load (IML or IMPL) that could be invoked from the console, as part of Power On Reset (POR) or from another processor in a tightly coupled multiprocessor complex.

Some commercial machines, e.g., IBM 360/85, had both a Read-only storage and a Writable Control Store for microcode.

WCS offered several advantages including the ease of patching the microprogram and, for certain hardware generations, faster access than ROMs could provide. User-programmable WCS allowed the user to optimize the machine for specific purposes.

Several Intel CPUs in the x86 architecture family have writable microcode. This has allowed bugs in the Intel Core 2 microcode and Intel Xeon microcode to be fixed in software, rather than requiring the entire chip to be replaced.