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. A control store implementation whose contents are unalterable is known as a Read Only Memory (ROM) or Read Only Storage (ROS); one whose contents are alterable is known as a Writable Control Store (WCS).
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 Models 50, 65 and 67.
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 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.
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La microprogrammation est une technique de réalisation du séquenceur d'un processeur, utilisé dans la technologie CISC, dans laquelle le comportement du séquenceur est décrit par le contenu d'une mémoire. Ce contenu est appelé microprogramme ou microcode. Il peut être vu comme un programme destiné à une machine très simple appelée micro-séquenceur ; le processeur final est alors considéré comme une machine virtuelle simulée par le micro-séquenceur. Par opposition la technologie RISC ne contient pas de microcode.
thumb|Logo du Digital VAX 11. La famille d'ordinateurs VAX-11 (ou simplement VAX) conçue au sein de la société Digital Equipment Corporation (DEC) par Gordon Bell, Bill Demmer, Richie Lary, Steve Rothman et Bill Strecker à partir de 1975 connut un très grand succès. Cette gamme se positionnait comme les successeurs des PDP-11 dont les premiers modèles gardent un mode compatible. D'ailleurs le nom VAX signifie Virtual Address eXtension (Extension de l'adressage virtuel), faisant référence au faible espace d'adressage mémoire disponible sur le PDP-11 ( de base).
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. A control store implementation whose contents are unalterable is known as a Read Only Memory (ROM) or Read Only Storage (ROS); one whose contents are alterable is known as a Writable Control Store (WCS). 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.