Processor Register

Processor register - Wikipedia, the free encyclopedia
Processor registers are at the top of the memory hierarchy, and provide the ... Registers are now usually implemented as a register file, but they have also ...
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In computer architecture, a processor register is a small amount of Computer 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 of reference, 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 central processing unit 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 (compiler) phase.

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-flop (electronics)s, high speed core memory, thin film memory, and other ways in various machines.

A processor often contains several kinds of registers, that can be classified according to their content or instructions that operate on them:

User-accessible Registers - The most common division of user-accessible registers is into data registers and address registers.
Data registers are used to hold numeric values such as integer (computer science) and floating-point values. In some older and low end CPUs, a special data register, known as the Accumulator_(computing), is used implicitly for many operations.
Address registers hold addresses and are used by instructions that indirectly access computer 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.
Conditional registers hold truth values often used to determine whether some instruction should or should not be executed.
General purpose registers (GPRs) can store both data and addresses, i.e., they are combined Data/Address registers.
Floating point registers (FPRs) store floating point numbers in many architectures.
Constant registers hold read-only values such as zero, one, or pi.
Vector registers hold data for vector processor done by SIMD instructions (Single Instruction, Multiple Data).
Special purpose registers hold program state; they usually include the program counter (aka instruction pointer), Call stack#Structure, 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.
Control and status registers - It has three types. Program counter, instruction registers, Program status word (Program status word).
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):

Memory buffer register
Memory data register
Memory address register
Memory Type Range Registers

Hardware registers are similar, but occur outside CPUs.

Some examples The table below shows the number of registers of several mainstream processors. Note that the stack pointer (ESP) is counted as an integer register on x86-compatible processors, even though there are a limited number of instructions that may be used to operate on its contents.{| class="wikitable"!Processors!Integer registers!Double FP registers|-|Intel Core (CPU)||8||8|-|Intel Core 2||8||8|-|[Pentium 4 with EM64T]||16||16|-|Athlon MP] 240||16||16|-|[Itanium 2 IIIi||32||32|-|[POWER3||3||0|}

See also

Processor register - Wikipedia, the free encyclopedia
In computer architecture, a processor register is a small amount of storage available on the CPU whose contents can be accessed more quickly than storage available elsewhere.

Accumulator (computing) - Wikipedia, the free encyclopedia
In a computer CPU, an accumulator is a register in which intermediate arithmetic and logic results are stored. Without a register like an accumulator, it would be necessary to ...

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Processor register - Hutchinson encyclopedia article about Processor ...
Hutchinson encyclopedia article about Processor register. Processor register. Information about Processor register in the Hutchinson encyclopedia.

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register set from FOLDOC
register set. All a processor's registers. The size and arrangement of a processor's register set is one of the crucial factors in its performance.

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