processor register
is a small amount of storage available on the CPU whose contents can be accessed more quickly than storage available elsewhere. Most, but not all, modern computer architectures operate on the principle of moving data from main memory into registers, operating on them, then moving the result back into main memory—a so-called load-store architecture. A common property of computer programs is locality ofreference, that is the same values are often accessed repeatedly; and holding these frequently used values in registers improves program execution performance.
Processor registers are at the top of the memory hierarchy, and provide the fastest way for a CPU to access data. The term is often used to refer only to the group of registers that are directly encoded as part of an instruction, as defined by the instruction set. More properly, these are called the "architectural 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 registers than just these eight.
Allocating frequently used variables to registers can be critical to a program's performance. This action, namely register allocation is performed by a compiler in the code generation phase.
