Control store

A control store is the part of a CPU's control unit that stores the CPU's microprogram. It is usually accessed by a microsequencer. Early types of control store took the form of diode-arrays that were accessed via address decoders, but were later implemented as writable microcode that was stored in a form of read-only memory called a writable control store. The outputs generally had to go through a register to prevent a race condition from occurring. The register was clocked by the clock signal of the system it was running on. A control store is the part of a CPU's control unit that stores the CPU's microprogram. It is usually accessed by a microsequencer. Early types of control store took the form of diode-arrays that were accessed via address decoders, but were later implemented as writable microcode that was stored in a form of read-only memory called a writable control store. The outputs generally had to go through a register to prevent a race condition from occurring. The register was clocked by the clock signal of the system it was running on. Early control stores were implemented as a diode-array accessed via address decoders, a form of read-only memory. This tradition dates back to the program timing matrix on the MIT Whirlwind, first described in 1947. Modern VLSI processors instead use matrices of field-effect transistors to build the ROM and/or PLA structures used to control the processor as well as its internal sequencer in a microcoded implementation. IBM System/360 used a variety of techniques: CCROS (Card Capacitor Read-Only Storage) on the Model 30, TROS (Transformer Read-Only Storage) on the Model 40, and BCROS (Balanced Capacitor Read-Only Storage) on the Model 50. Some 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 of these machines were experimental laboratory prototypes, such as the WISC CPU/16 and the RTX 32P. 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, computer vendors would often sell writable 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. Other commercial machines that used 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 stored their microcode in SRAM chips (about 80 bits wide x 2 Kwords), which was typically loaded on power-on through some other front-end CPU. 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 Mentec M11 and Mentec M1 stored its microcode in SRAM chips, loaded on power-on through another CPU.The Data General Eclipse MV/8000 ('Eagle') had a SRAM writable control store, loaded on power-on through another CPU.