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Processor register

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In computer architecture, a processor register is a small amount of very fast computer memory used to speed the execution of computer programs by providing quick access to commonly used values—typically, the values being calculated at a given point in time. 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.

Processor registers are the top of the memory hierarchy, and provide the fastest way for the system to access data. 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 "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.

Putting frequently used variables into registers is critical to the program's performance. This action, namely register allocation is usually done by a compiler in the code generation phase.

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Categories of registers

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.

There are several classes of registers according to the content:

Data registers are used to store integer numbers (see also Floating Point Registers, below). In some older and simple current CPUs, a special data register is the accumulator, used implicitly for many operations.
Address registers hold memory addresses and are used to access memory. In some CPUs, a special address register is an index register, although often these hold numbers used to modify addresses rather than holding addresses.
General Purpose registers (GPRs) can store both data and addresses, i.e., they are combined Data/Address registers.
Floating Point registers (FPRs) are used to store floating point numbers in many architectures.
Constant registers hold read-only values (e.g., zero, one, pi, ...).
Vector registers hold data for vector processing done by SIMD instructions (Single Instruction, Multiple Data).
Special Purpose registers hold program state; they usually include the program counter (aka instruction pointer), stack pointer, and status register (aka processor status word).
- Instruction registers store the instruction currently being executed.
- Index registers are used for modifying operand addresses during the run of a program.
In some architectures, model-specific registers (also called machine-specific registers) store data and settings related to the processor itself. Because their meanings are attached to the design of a specific processor, they cannot be expected to remain standard between processor generations.
Registers related to fetching information from random access memory, a collection of storage registers located on separate chips from the CPU (unlike most of the above, these are generally not architectural registers):