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A processor register is a quickly accessible location available to a computer's
processor Processor may refer to: Computing Hardware * Processor (computing) **Central processing unit (CPU), the hardware within a computer that executes a program *** Microprocessor, a central processing unit contained on a single integrated circuit (I ...
. Registers usually consist of a small amount of fast storage, although some registers have specific hardware functions, and may be read-only or write-only. In computer architecture, registers are typically addressed by mechanisms other than
main memory Computer data storage is a technology consisting of computer components and recording media that are used to retain digital data. It is a core function and fundamental component of computers. The central processing unit (CPU) of a comput ...
, but may in some cases be assigned a memory address e.g. DEC
PDP-10 Digital Equipment Corporation (DEC)'s PDP-10, later marketed as the DECsystem-10, is a mainframe computer family manufactured beginning in 1966 and discontinued in 1983. 1970s models and beyond were marketed under the DECsystem-10 name, espec ...
, ICT 1900. Almost all computers, whether load/store architecture or not, load data from a larger memory into registers where it is used for arithmetic operations and is manipulated or tested by machine instructions. Manipulated data is then often stored back to main memory, either by the same instruction or by a subsequent one. Modern processors use either static or
dynamic Dynamics (from Greek δυναμικός ''dynamikos'' "powerful", from δύναμις ''dynamis'' "power") or dynamic may refer to: Physics and engineering * Dynamics (mechanics) ** Aerodynamics, the study of the motion of air ** Analytical dyn ...
RAM as main memory, with the latter usually accessed via one or more cache levels. 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 Parallel is a geometric term of location which may refer to: Computing * Parallel algorithm * Parallel computing * Parallel metaheuristic * Parallel (software), a UNIX utility for running programs in parallel * Parallel Sysplex, a cluster o ...
and speculative execution. Modern x86 design acquired these techniques around 1995 with the releases of Pentium Pro, Cyrix 6x86, Nx586, and AMD K5. When a
computer program A computer program is a sequence or set of instructions in a programming language for a computer to Execution (computing), execute. Computer programs are one component of software, which also includes software documentation, documentation and oth ...
accesses the same data repeatedly, this is called locality of reference. Holding frequently used values in registers can be critical to a program's performance. Register allocation is performed either by a
compiler In computing, a compiler is a computer program that translates computer code written in one programming language (the ''source'' language) into another language (the ''target'' language). The name "compiler" is primarily used for programs tha ...
in the code generation phase, or manually by an
assembly language In computer programming, assembly language (or assembler language, or symbolic machine code), often referred to simply as Assembly and commonly abbreviated as ASM or asm, is any low-level programming language with a very strong correspondence b ...
programmer.


Size

Registers are normally measured by the number of bits they can hold, for example, an " 8-bit register", " 32-bit register", "
64-bit In computer architecture, 64-bit integers, memory addresses, or other data units are those that are 64 bits wide. Also, 64-bit CPUs and ALUs are those that are based on processor registers, address buses, or data buses of that size. A ...
register", or even more. 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. Typically it is implemented by adding extra registers that map their memory into a larger register. Processors that have the ability to execute single instructions on multiple data are called
vector processor In computing, a vector processor or array processor is a central processing unit (CPU) that implements an instruction set where its instructions are designed to operate efficiently and effectively on large one-dimensional arrays of data calle ...
s.


Types

A processor often contains several kinds of registers, which can be classified according to the types of values they can store or the instructions that operate on them: * ''User-accessible registers'' can be read or written by machine instructions. The most common division of user-accessible registers is into data registers and address registers. ** ''s'' can hold numeric data values such as
integer An integer is the number zero (), a positive natural number (, , , etc.) or a negative integer with a minus sign ( −1, −2, −3, etc.). The negative numbers are the additive inverses of the corresponding positive numbers. In the languag ...
and, in some architectures, floating-point values, as well as characters, small bit arrays and other data. In some older architectures, such as the IBM 704, the IBM 709 and successors, the PDP-1, the PDP-4/ PDP-7/ PDP-9/
PDP-15 The PDP-15 was the fifth and last of the 18-bit minicomputers produced by Digital Equipment Corporation. The PDP-1 was first delivered in December 1959 and the first PDP-15 was delivered in February 1970. More than 400 of these successors to ...
, the PDP-5/ PDP-8, and the HP 2100, a special data register known as the accumulator is used implicitly for many operations. ** ''s'' 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 In computer science, computer engineering and programming language implementations, a stack machine is a computer processor or a virtual machine in which the primary interaction is moving short-lived temporary values to and from a push down ...
). ** ''General-purpose registers'' (''GPR''s) can store both data and addresses, i.e., they are combined data/address registers; in some architectures, the register file is ''unified'' so that the GPRs can store floating-point numbers as well. ** '' Status registers'' hold
truth value In logic and mathematics, a truth value, sometimes called a logical value, is a value indicating the relation of a proposition to truth, which in classical logic has only two possible values ('' true'' or '' false''). Computing In some pro ...
s often used to determine whether some instruction should or should not be executed. ** ''s'' (''FPR''s) store floating-point numbers in many architectures. ** '' Constant registers'' hold read-only values such as zero, one, or pi. ** ' hold data for vector processing done by SIMD instructions (Single Instruction, Multiple Data). ** ''Special-purpose registers'' (''SPR''s) hold some elements of the program state; they usually include the program counter, also called the instruction pointer, and the status register; the program counter and status register might be combined in a program status word (PSW) register. The aforementioned stack pointer is sometimes also included in this group. Embedded microprocessors can also have registers corresponding to specialized hardware elements. ** 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. ** '' Memory type range registers'' (''MTRR''s) * ''Internal registers'' are not accessible by instructions and are used internally for processor operations. ** The ''
instruction register In computing, the instruction register (IR) or current instruction register (CIR) is the part of a CPU's control unit that holds the instruction currently being executed or decoded. In simple processors, each instruction to be executed is loaded ...
'' holds the instruction currently being executed. ** Registers related to fetching information from RAM, a collection of storage registers located on separate chips from the CPU: *** '' Memory buffer register'' (''MBR''), also known as ''memory data register'' (''MDR'') *** '' Memory address register'' (''MAR'') * ''Architectural registers'' are the registers visible to software and are defined by an architecture. They may not correspond to the physical hardware if register renaming is being performed by the underlying hardware. Hardware registers are similar, but occur outside CPUs. In some architectures (such as
SPARC SPARC (Scalable Processor Architecture) is a reduced instruction set computer (RISC) instruction set architecture originally developed by Sun Microsystems. Its design was strongly influenced by the experimental Berkeley RISC system develope ...
and MIPS), the first or last register in the integer register file is a ''pseudo-register'' in that it is hardwired to always return zero when read (mostly to simplify indexing modes), and it cannot be overwritten. In
Alpha Alpha (uppercase , lowercase ; grc, ἄλφα, ''álpha'', or ell, άλφα, álfa) is the first letter of the Greek alphabet. In the system of Greek numerals, it has a value of one. Alpha is derived from the Phoenician letter aleph , whi ...
, this is also done for the floating-point register file. As a result of this, register files are commonly quoted as having one register more than how many of them are actually usable; for example, 32 registers are quoted when only 31 of them fit within the above definition of a register.


Examples

The following table shows the number of registers in several mainstream CPU architectures. 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. Similar caveats apply to most architectures. Although all of the above-listed architectures are different, almost all are in a basic arrangement known as the von Neumann architecture, first proposed by the Hungarian-American
mathematician A mathematician is someone who uses an extensive knowledge of mathematics in their work, typically to solve mathematical problems. Mathematicians are concerned with numbers, data, quantity, structure, space, models, and change. History On ...
John von Neumann John von Neumann (; hu, Neumann János Lajos, ; December 28, 1903 – February 8, 1957) was a Hungarian-American mathematician, physicist, computer scientist, engineer and polymath. He was regarded as having perhaps the widest c ...
. It is also noteworthy that the number of registers on GPUs is much higher than that on CPUs.


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 of an expression tree gives the minimum number of registers required to evaluate that expression tree.


See also

*
CPU cache A CPU cache is a hardware cache used by the central processing unit (CPU) of a computer to reduce the average cost (time or energy) to access data from the main memory. A cache is a smaller, faster memory, located closer to a processor core, whi ...
* Register allocation * Register file * Shift register


References

{{Authority control Computer architecture Digital registers Central processing unit