Processor register

In computer architecture, a processor register is a small amount of storage available as part of a CPU or other digital processor. Such registers are (typically) addressed by mechanisms other than main memory and can be accessed more quickly. Almost all computers, load-store architecture or not, load data from a larger memory into registers where it is used for arithmetic, manipulated, or tested, by some machine instruction. Manipulated data is then often stored back in main memory, either by the same instruction or a subsequent one. Modern processors use either static or dynamic RAM as main memory, the latter often being implicitly accessed via one or more cache levels. A common property of computer programs is locality of reference: the same values are often accessed repeatedly and frequently used values held in registers improves performance. This is what makes fast registers (and caches) meaningful.

Processor registers are normally at the top of the memory hierarchy, and provide the fastest way to access data. The term normally refers only to the group of registers that are directly encoded as part of an instruction, as defined by the instruction set. However, modern high performance CPUs often have duplicates of these "architectural registers" in order to improve performance via register renaming, allowing parallel and speculative execution. Modern x86 is perhaps the most well known example of this technique.

Allocating frequently used variables to registers can be critical to a program's performance. This register allocation is either performed by a compiler, in the code generation phase, or manually, by an assembly language programmer.

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". A processor often contains several kinds of registers, that can be classified accordingly 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 can hold numeric values such as integer and floating-point values, as well as characters, small bit arrays and other data. In some older and low end CPUs, a special data register, known as the accumulator, is used implicitly for many operations.
• Address registers hold addresses and are used by instructions that indirectly access primary memory.
  • Some processors contain registers that may only be used to hold an address or only to hold numeric values (in some cases used as an index register whose value is added as an offset from some address); others allow registers to hold either kind of quantity. A wide variety of possible addressing modes, used to specify the effective address of an operand, exist.
  • The stack pointer is used to manage the run-time stack. Rarely, other data stacks are addressed by dedicated address registers, see stack machine.
• 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 processing done by SIMD instructions (Single Instruction, Multiple Data).
• Special purpose registers ( SPR ) hold program state; they usually include the program counter (aka instruction pointer) and status register (aka processor status word). The aforementioned stack pointer is sometimes also included in this group. Embedded microprocessors can also have registers corresponding to specialized hardware elements.
  • Instruction register store the instruction currently being executed.
• 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 and program status word (PSW).
• Registers related to fetching information from RAM, 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 (MTRR)

Hardware registers are similar, but occur outside CPUs.

Some examples

Architecture	Integer registers	FP registers
x86	8	8
IA-32e	16	16
AMD64	16	16
EM64T	24	16
IBM/360	16	4
Z/Architecture	16	16
Itanium	128	128
UltraSPARC	32	32
IBM Cell	4~16	1~4
IBM POWER	32	32
Alpha	32	32
6502	3	0
PIC microcontroller	1	0
AVR microcontroller	32	0
ARM	4	4
ARM 64bit	8	8

The table shows the number of registers of several mainstream architectures. 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. Similar caveats apply to most architectures. x86 FPUs have 8 80-bit stack levels in legacy mode, and at least 8 128-bit XMM registers in SSE modes.

Although all of the shown architectures are different, 95% of the computer architectures are based on a basic architecture known as Von Neumann architecture proposed by Von Neumann.

Register usage

The number of registers available on a processor and the operations that can be performed using those registers has a significant impact on the efficiency of code generated by optimizing compilers. The Strahler number defines the minimum number of registers required to evaluate an expression tree.

See also

• Shift register
• Register allocation
• CPU cache