In computer architecture, a processor register is a very fast computer memory used to speed the execution of computer programs by providing quick access to commonly used values-typically, the values being in the midst of a calculation at a given point in time.
These registers are the top of the memory hierarchy, and are the fastest way for the system to manipulate data. In a very simple microprocessor, it consists of a single memory location, usually called an accumulator. Registers are built from fast multi-ported memory cell. They must be able to drive its data onto an internal bus in a single clock cycle. The result of ALU operation is stored here and could be re-used in a subsequent operation or saved into memory.
Registers are normally measured by the number of bits they can hold, for example, an “8-bit register” or a “32-bit register”. Registers are now usually implemented as a register file, but they have also been implemented using individual flip-flops, high speed core memory, thin film memory, and other ways in various machines.
The term is often used to refer only to the group of registers that can be directly indexed for input or output of an instruction, as defined by the instruction set. More properly, these are called the “architected registers“. For instance, the x86 instruction set defines a set of eight 32-bit registers, but a CPU that implements the X86 instruction set will contain many more hardware registers than just these eight.
There are several other classes of registers:
(a) Accumulator: It is most frequently used register used to store data taken from memory. Its number varies from microprocessor to microprocessor.
(b) General Purpose registers: General purpose registers are used to store data and intermediate results during program execution. Its contents can be accessed through assembly programming.
(c) Special purpose Registers: Users do not access these registers. These are used by computer system at the time of program execution. Some types of special purpose registers are given below: