Page "Control store" Paragraph 6

The original System / 360 models of IBM mainframe had read-only control store, but later System / 360, System / 370 and successor models loaded part or all of their microprograms from floppy disks or other DASD into a writable control store consisting of ultra-high speed random-access read-write memory.

The System / 370 architecture 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.

This permitted IBM to easily repair microprogramming defects in the field.

Even when the majority of the control store is stored in ROM of some sort, computer vendors often sell writeable control store as an option, allowing the customers to customize the machine's microprogram.

Other vendors, e. g., IBM, use the WCS to run microcode for emulator features and hardware diagnostics.