A control store is the part of a CPU's

Central processing unit

The central processing unit is the portion of a computer system that carries out the instructions of a computer program, to perform the basic arithmetical, logical, and input/output operations of the system. The CPU plays a role somewhat analogous to the brain in the computer. The term has been in...

 control unit

Control unit

A control unit in general is a central part of the machinery that controls its operation, provided that a piece of machinery is complex and organized enough to contain any such unit. One domain in which the term is specifically used is the area of computer design...

 that stores the CPU's microprogram. It is usually accessed by a microsequencer

Microsequencer

In computer architecture and engineering, a sequencer or microsequencer is a part of the control unit of a CPU. It generates the addresses used to step through the microprogram of a control store....

. Early control stores were implemented as a diode

Diode

In electronics, a diode is a type of two-terminal electronic component with a nonlinear current–voltage characteristic. A semiconductor diode, the most common type today, is a crystalline piece of semiconductor material connected to two electrical terminals...

-array accessed via address decoders, a form of read-only memory

Read-only memory

Read-only memory is a class of storage medium used in computers and other electronic devices. Data stored in ROM cannot be modified, or can be modified only slowly or with difficulty, so it is mainly used to distribute firmware .In its strictest sense, ROM refers only...

. 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

Rom

ROM, Rom, or rom is an abbreviation and name that may refer to:-In computers and mathematics:* Read-only memory, a type of storage media which is used in computers and other electronic devices....

 and/or PLA

Programmable logic array

A programmable logic array is a kind of programmable logic device used to implement combinational logic circuits. The PLA has a set of programmable AND gate planes, which link to a set of programmable OR gate planes, which can then be conditionally complemented to produce an output...

 structures used to control the processor as well as its internal sequencer in a microcoded implementation.

Writable stores

The original System/360

System/360

The IBM System/360 was a mainframe computer system family first announced by IBM on April 7, 1964, and sold between 1964 and 1978. It was the first family of computers designed to cover the complete range of applications, from small to large, both commercial and scientific...

 models of IBM mainframe

IBM mainframe

IBM mainframes are large computer systems produced by IBM from 1952 to the present. During the 1960s and 1970s, the term mainframe computer was almost synonymous with IBM products due to their marketshare...

 had read-only control store, but later System/370

System/370

The IBM System/370 was a model range of IBM mainframes announced on June 30, 1970 as the successors to the System/360 family. The series maintained backward compatibility with the S/360, allowing an easy migration path for customers; this, plus improved performance, were the dominant themes of the...

 and successor models loaded their microprograms from floppy disks into a writeable control store consisting of ultra-high speed random-access

Random-access memory

Random access memory is a form of computer data storage. Today, it takes the form of integrated circuits that allow stored data to be accessed in any order with a worst case performance of constant time. Strictly speaking, modern types of DRAM are therefore not random access, as data is read in...

 read-write memory

Read-write memory

Read-write memory is a type of computer memory that may be relatively easily written to as well as read from . The term RAM is often used to describe writable memory. RAM actually referring to memory that can be accessed at any "location"....

. 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.

Timing, latching and avoiding a race condition

The control store usually has a register on its outputs. The outputs that go back into the sequencer to determine the next address have to go through some sort of register to prevent the creation of a race condition. In most designs all of the other bits also go through a register. This is because the machine will work faster if the execution of the next microinstruction is delayed by one cycle. This register is known as a pipeline register. Very often the execution of the next microinstruction is dependent on the result of the current microinstruction, which will not be stable until the end of the current microcycle. It can be seen that either way, all of the outputs of the control store go into one big register. Historically it used to be possible to buy EPROMs with these register bits on the same chip.

The clock signal determining the cycle time of the system primarily clocks this register.