 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, computer vendors would often sell writable control store as an option, allowing the customers to customize the machine's microprogram.