Processor register

Processor registers are normally at the top of the memory hierarchy, and provide the fastest way to access data.

Modern x86 design acquired these techniques around 1995 with the releases of Pentium Pro, Cyrix 6x86, Nx586, and AMD K5.

Register allocation is performed either by a compiler in the code generation phase, or manually by an assembly language programmer.

In some instruction sets, the registers can operate in various modes, breaking down their storage memory into smaller parts (32-bit into four 8-bit ones, for instance) to which multiple data (vector, or one-dimensional array of data) can be loaded and operated upon at the same time.

Note that in x86-compatible processors, the stack pointer (ESP) is counted as an integer register, even though there are a limited number of instructions that may be used to operate on its contents.