Processor register
Encyclopedia
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 RAM or dynamic RAM as main memory, the latter often being implicitly accessed via one or more cache-levels. A common property of computer program
s 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.
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:
Hardware register
s are similar, but occur outside CPUs.
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.
of code generated by optimizing compilers. The Strahler number
defines the minimum number of registers required to evaluate an expression tree.
Computer architecture
In computer science and engineering, computer architecture is the practical art of selecting and interconnecting hardware components to create computers that meet functional, performance and cost goals and the formal modelling of those systems....
, a processor register is a small amount of storage
Computer storage
Computer data storage, often called storage or memory, refers to computer components and recording media that retain digital data. Data storage is one of the core functions and fundamental components of computers....
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 RAM or dynamic RAM as main memory, the latter often being implicitly accessed via one or more cache-levels. A common property of computer program
Computer program
A computer program is a sequence of instructions written to perform a specified task with a computer. A computer requires programs to function, typically executing the program's instructions in a central processor. The program has an executable form that the computer can use directly to execute...
s is locality of reference
Locality of reference
In computer science, locality of reference, also known as the principle of locality, is the phenomenon of the same value or related storage locations being frequently accessed. There are two basic types of reference locality. Temporal locality refers to the reuse of specific data and/or resources...
: 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
Memory hierarchy
The term memory hierarchy is used in the theory of computation when discussing performance issues in computer architectural design, algorithm predictions, and the lower level programming constructs such as involving locality of reference. A 'memory hierarchy' in computer storage distinguishes each...
, 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
Instruction set
An instruction set, or instruction set architecture , is the part of the computer architecture related to programming, including the native data types, instructions, registers, addressing modes, memory architecture, interrupt and exception handling, and external I/O...
. However, modern high performance CPUs often have duplicates of these "architectural registers" in order to improve performance via register renaming
Register renaming
In computer architecture, register renaming refers to a technique used to avoid unnecessary serialization of program operations imposed by the reuse of registers by those operations.-Problem definition:...
, allowing parallel and speculative execution
Speculative execution
Speculative execution in computer systems is doing work, the result of which may not be needed. This performance optimization technique is used in pipelined processors and other systems.-Main idea:...
. 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
Register allocation
In compiler optimization, register allocation is the process of assigning a large number of target program variables onto a small number of CPU registers...
is either performed by a compiler
Compiler
A compiler is a computer program that transforms source code written in a programming language into another computer language...
, in the code generation phase, or manually, by an assembly language
Assembly language
An assembly language is a low-level programming language for computers, microprocessors, microcontrollers, and other programmable devices. It implements a symbolic representation of the machine codes and other constants needed to program a given CPU architecture...
programmer.
Categories of registers
Registers are normally measured by the number of bits they can hold, for example, an "8-bit8-bit
The first widely adopted 8-bit microprocessor was the Intel 8080, being used in many hobbyist computers of the late 1970s and early 1980s, often running the CP/M operating system. The Zilog Z80 and the Motorola 6800 were also used in similar computers...
register" or a "32-bit
32-bit
The range of integer values that can be stored in 32 bits is 0 through 4,294,967,295. Hence, a processor with 32-bit memory addresses can directly access 4 GB of byte-addressable memory....
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 are used to hold numeric values such as integerInteger (computer science)In computer science, an integer is a datum of integral data type, a data type which represents some finite subset of the mathematical integers. Integral data types may be of different sizes and may or may not be allowed to contain negative values....
and floating-point values. In some older and low end CPUs, a special data register, known as the accumulatorAccumulator (computing)In a computer's central processing unit , 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 write the result of each calculation to main memory, perhaps only to be read right back again for...
, is used implicitly for many operations. - Address registers hold addresses and are used by instructions that indirectly access memoryComputer memoryIn computing, memory refers to the physical devices used to store programs or data on a temporary or permanent basis for use in a computer or other digital electronic device. The term primary memory is used for the information in physical systems which are fast In computing, memory refers to the...
.- 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 registerIndex registerAn index registerCommonly known as a B-line in early British computers. in a computer's CPU is a processor register used for modifying operand addresses during the run of a program, typically for doing vector/array operations...
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 modeAddressing modeAddressing modes are an aspect of the instruction set architecture in most central processing unit designs. The various addressing modes that are defined in a given instruction set architecture define how machine language instructions in that architecture identify the operand of each instruction...
s, used to specify the effective address of an operandOperandIn mathematics, an operand is the object of a mathematical operation, a quantity on which an operation is performed.-Example :The following arithmetic expression shows an example of operators and operands:3 + 6 = 9\;...
, exist. - A stack pointer, sometimes called a stack registerStack registerA stack register is a computer central processor register whose purpose is to keep track of a call stack. On an accumulator-based architecture machine, this may be a dedicated register such as SP on an Intel x86 machine. On a general register machine, it may be a register which is reserved by...
, is the name given to a register that can be used by some instructions to maintain a stackStack (data structure)In computer science, a stack is a last in, first out abstract data type and linear data structure. A stack can have any abstract data type as an element, but is characterized by only three fundamental operations: push, pop and stack top. The push operation adds a new item to the top of the stack,...
.
- 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
- 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 pointFloating pointIn computing, floating point describes a method of representing real numbers in a way that can support a wide range of values. Numbers are, in general, represented approximately to a fixed number of significant digits and scaled using an exponent. The base for the scaling is normally 2, 10 or 16...
numbers in many architectures. - Constant registers hold read-only values such as zero, one, or piPi' is a mathematical constant that is the ratio of any circle's circumference to its diameter. is approximately equal to 3.14. Many formulae in mathematics, science, and engineering involve , which makes it one of the most important mathematical constants...
. - Vector registers hold data for vector processingVector processorA vector processor, or array processor, is a central processing unit that implements an instruction set containing instructions that operate on one-dimensional arrays of data called vectors. This is in contrast to a scalar processor, whose instructions operate on single data items...
done by SIMDSIMDSingle instruction, multiple data , is a class of parallel computers in Flynn's taxonomy. It describes computers with multiple processing elements that perform the same operation on multiple data simultaneously...
instructions (Single Instruction, Multiple Data). - Special purpose registers ( SPR ) hold program state; they usually include the program counterProgram counterThe program counter , commonly called the instruction pointer in Intel x86 microprocessors, and sometimes called the instruction address register, or just part of the instruction sequencer in some computers, is a processor register that indicates where the computer is in its instruction sequence...
(aka instruction pointer), stack pointer, and status registerStatus registerA status register or flag register is a collection of flag bits for a processor. An example is the FLAGS register of the x86 architecture....
(aka processor status word). In embedded microprocessors, they can also correspond to specialized hardware elements.- Instruction registerInstruction registerIn computing, an instruction register is the part of a CPU's control unit that stores the instruction currently being executed or decoded. In simple processors each instruction to be executed is loaded into the instruction register which holds it while it is decoded, prepared and ultimately...
s store the instruction currently being executed.
- Instruction register
- 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 (PSWProgram status wordThe Program status word is an IBM System/360 architecture and successors control register which performs the function of a Status register in other architectures, and more....
). - 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 registerMemory buffer registerA Memory Buffer Register is the register in a computer's processor, or central processing unit, CPU, that stores the data being transferred to and from the immediate access store. It acts as a buffer allowing the processor and memory units to act independently without being affected by minor...
- Memory data registerMemory data registerThe Memory Data Register is the register of a computer's control unit that contains the data to be stored in the computer storage , or the data after a fetch from the computer storage...
- Memory address registerMemory address registerThe Memory Address Register is a CPU register that either stores the memory address from which data will be fetched to the CPU or the address to which data will be sent and stored....
- Memory Type Range RegistersMemory Type Range RegistersMemory type range registers are a set of processor supplementary capabilities control registers that provide system software with control of how accesses to memory ranges by the CPU are cached...
(MTRR)
- Memory buffer register
Hardware register
Hardware register
In digital electronics, especially computing, a hardware register stores bits of information, in a way that all the bits can be written to or read out simultaneously.The hardware registers inside a central processing unit are called processor registers....
s are similar, but occur outside CPUs.
Some examples
| Architecture | Integer registers |
FP registers |
|---|---|---|
| x86 | 16 | 16 |
| x86-64 X86-64 x86-64 is an extension of the x86 instruction set. It supports vastly larger virtual and physical address spaces than are possible on x86, thereby allowing programmers to conveniently work with much larger data sets. x86-64 also provides 64-bit general purpose registers and numerous other... |
32 | 32 |
| IBM/360 | 16 | 4 |
| Z/Architecture Z/Architecture z/Architecture, initially and briefly called ESA Modal Extensions , refers to IBM's 64-bit computing architecture for IBM mainframe computers. IBM introduced its first z/Architecture-based system, the zSeries Model 900, in late 2000. Later z/Architecture systems include the IBM z800, z990, z890,... |
16 | 16 |
| Itanium Itanium Itanium is a family of 64-bit Intel microprocessors that implement the Intel Itanium architecture . Intel markets the processors for enterprise servers and high-performance computing systems... |
128 | 128 |
| UltraSPARC UltraSPARC The UltraSPARC is a microprocessor developed by Sun Microsystems who is now a part of Oracle Corporation and fabricated by Texas Instruments that implements the SPARC V9 instruction set architecture . It was introduced in mid-1995. It was the first microprocessor from Sun Microsystems to implement... |
32 | 32 |
| IBM POWER IBM POWER POWER is a reduced instruction set computer instruction set architecture developed by IBM. The name is an acronym for Performance Optimization With Enhanced RISC.... |
32 | 32 |
| Alpha DEC Alpha Alpha, originally known as Alpha AXP, is a 64-bit reduced instruction set computer instruction set architecture developed by Digital Equipment Corporation , designed to replace the 32-bit VAX complex instruction set computer ISA and its implementations. Alpha was implemented in microprocessors... |
32 | 32 |
| 6502 MOS Technology 6502 The MOS Technology 6502 is an 8-bit microprocessor that was designed by Chuck Peddle and Bill Mensch for MOS Technology in 1975. When it was introduced, it was the least expensive full-featured microprocessor on the market by a considerable margin, costing less than one-sixth the price of... |
3 | 0 |
| PIC microcontroller PIC microcontroller PIC is a family of Harvard architecture microcontrollers made by Microchip Technology, derived from the PIC1650 originally developed by General Instrument's Microelectronics Division... |
1 | 0 |
| AVR microcontroller Atmel AVR The AVR is a modified Harvard architecture 8-bit RISC single chip microcontroller which was developed by Atmel in 1996. The AVR was one of the first microcontroller families to use on-chip flash memory for program storage, as opposed to one-time programmable ROM, EPROM, or EEPROM used by other... |
32 | 0 |
| ARM ARM architecture ARM is a 32-bit reduced instruction set computer instruction set architecture developed by ARM Holdings. It was named the Advanced RISC Machine, and before that, the Acorn RISC Machine. The ARM architecture is the most widely used 32-bit ISA in numbers produced... |
16 | 16 |
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
Von Neumann architecture
The term Von Neumann architecture, aka the Von Neumann model, derives from a computer architecture proposal by the mathematician and early computer scientist John von Neumann and others, dated June 30, 1945, entitled First Draft of a Report on the EDVAC...
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 efficiencyAlgorithmic efficiency
In computer science, efficiency is used to describe properties of an algorithm relating to how much of various types of resources it consumes. Algorithmic efficiency can be thought of as analogous to engineering productivity for a repeating or continuous process, where the goal is to reduce...
of code generated by optimizing compilers. The Strahler number
Strahler number
In mathematics, the Strahler number or Horton–Strahler number of a mathematical tree is a numerical measure of its branching complexity....
defines the minimum number of registers required to evaluate an expression tree.
See also
- Shift registerShift registerIn digital circuits, a shift register is a cascade of flip flops, sharing the same clock, which has the output of any one but the last flip-flop connected to the "data" input of the next one in the chain, resulting in a circuit that shifts by one position the one-dimensional "bit array" stored in...
- Register allocationRegister allocationIn compiler optimization, register allocation is the process of assigning a large number of target program variables onto a small number of CPU registers...
- CPU cacheCPU cacheA CPU cache is a cache used by the central processing unit of a computer to reduce the average time to access memory. The cache is a smaller, faster memory which stores copies of the data from the most frequently used main memory locations...