In computer architecture, a processor register is a small amount of storage available as part of a digital processor, such as a central processing unit (CPU). Such registers are typically addressed by mechanisms other than main memory and can be accessed faster. 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 machine instructions. Manipulated data is then often stored back into main memory, either by the same instruction or a subsequent one. Modern processors use either static or dynamic 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 and speculative execution. Modern x86 design acquired these techniques around 1995 with the releases of Pentium Pro, Cyrix 6x86, Nx586, and AMD K5.
A common property of computer programs is locality of reference, which refers to accessing the same values repeatedly and holding frequently used values in registers to improve performance; this is what makes fast registers and caches meaningful. Allocating frequently used variables to registers can be critical to a program's performance; this register allocation is performed either 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 according to their content or 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.
- Data registers can hold numeric values such as integer and, in some architectures, 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.
- General-purpose registers (GPRs) can store both data and addresses, i.e., they are combined data/address registers and rarely the register file is unified to include floating point as well.
- Status registers hold truth values often used to determine whether some instruction should or should not be executed.
- 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 (SPRs) hold 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 (MTRRs)
- Internal registers – registers not accessible by instructions, used internally for processor operations.
Hardware registers are similar, but occur outside CPUs.
In some architectures, such as SPARC and MIPS, the first or last register in the integer register file is a pseudo-register in a way that it is hardwired to always return zero when read (mostly to simplify indexing modes), and it cannot be overwritten. In Alpha 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.
The table shows the number of registers of several mainstream 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.
|4004||1 accumulator, 16 others||0||Register A is for general purpose, while r0–r15 registers are for the address and segment.|
|8008||1 accumulator, 6 others||0||The A register is an accumulator to which all arithmetic is done; the H and L registers can be used in combination as an address register; all registers can be used as operands in load/store/move/increment/decrement instructions and as the other operand in arithmetic instructions. There is no FP unit available.|
|8080||1 accumulator, 6 others||0||Plus a stack pointer. The A register is an accumulator to which all arithmetic is done; the register pairs B+C, D+E, and H+L, can be used as address registers in some instructions; all registers can be used as operands in load/store/move/increment/decrement instructions and as the other operand in arithmetic instructions. Some instructions only use H+L; another instruction swaps H+L and D+E. There is no FP unit available.|
|16-bit x86||8||stack of 8 (if FP present)||8086/8088, 80186/80188, 80286, with 8087, 80187 or 80287 for floating-point, with an 80-bit wide, 8 deep register stack with some instructions able to use registers relative to the top of the stack as operands; without 8087/80187/80287, no floating-point registers|
|IA-32||8||stack of 8 (if FP present), 8 (if SSE/MMX present)||80386 required 80387 for floating-point, later processors had built-in floating point, with both having an 80-bit wide, 8 deep register stack with some instructions able to use registers relative to the top of the stack as operands. The Pentium III and later had the SSE with additional 128-bit XMM registers.|
|x86-64||16||16||FP registers are 128-bit XMM registers, later extended to 256-bit YMM registers with AVX.|
|Xeon Phi||16||32||Including 32 512-bit ZMM registers with AVX-512|
|Motorola 6800||2 data, 1 index||0||Plus a stack pointer|
|Motorola 68k||8 data, 7 address||8 (if FP present)||Address register 7 is the stack pointer. 68000, 68010, 68012, 68020, and 68030 require an FPU for floating point; 68040 had FPU built in. FP registers are 80-bit.|
|Emotion Engine||4||1 + 32||The Emotion Engine's main core contains four 32-bit general-purpose registers for integer computation and one register for general floating-point computation. The processor is also connected via 32 128-bit built-in vector registers to a vector coprocessor based on MIPS architecture.|
|CUDA||1||8/16/32/64/128||Each CUDA core contains single 32/64-bit integer data register while floating point unit holding much larger number of register, G8x contains eight 128-bit HDR vector register. GT200 increased the count to 16. Fermi extended the register width to 256 bits and increased the register count to 32, while Kepler increased it to 64. Maxwell contains a massive amount of 128 512-bit vector registers.|
|IBM/360||16||4 (if FP present)||This applies to S/360's successors, System/370 through System/390; FP was optional in System/360, and always present in S/370 and later. In processors with the Vector Facility, there are 16 vector registers containing a machine-dependent number of 32-bit elements.|
|z/Architecture||16||16||64-bit version of S/360 and successors|
|Itanium||128||128||And 64 1-bit predicate registers and 8 branch registers. The FP registers are 82-bit.|
|SPARC||31||32||Global register 0 is hardwired to 0. Uses register windows.|
|IBM POWER||32||32||And 1 link and 1 count register.|
|Power Architecture||32||32||And 1 link and 1 count register. Processors supporting the Vector facility also have 32 128-bit vector registers,|
|IBM Cell SPE||0||1||Each SPE contains a 128-bit, 128-entry unified register file, and the register is a pushdown stack.|
|Alpha||31||31||Registers R31 (integer) and F31 (floating-point) are hardwired to zero.|
|6502||1||0||6502's content A (Accumulator) register for main purpose data store and memory address (8-bit data/16-bit address), X,Y are condition register and SP register are specific index only.|
|W65C816S||1||0||65C816 is the 16-bit successor of the 6502. X,Y, D (Direct Page register) are condition register and SP register are specific index only. main accumulator extended to 16-bit (B) while keep 8-bit (A) for compatibility and main register can now address up to 24-bit (16-bit wide data instruction/24-bit memory address).|
|65002||1||0||Direct successor of 6502, 65002 only content A (Accumulator) register for main purpose data store and extend data wide to 32-bit and 64-bit instruction wide, support 48-bit virtual address in software mode, X,Y are still condition register and remain 8-bit and SP register are specific index but increase to 16-bit wide.|
|ARM 32/26-bit||14||Varies (up to 32)||r15 is the program counter, and not usable as a GPR; r13 is the stack pointer; r8-r13 can be switched out for others (banked) on a processor mode switch. Older versions had 26-bit addressing.|
|ARM 64/32-bit||31||32||Register r31 is the stack pointer or hardwired to 0, depending on the context.|
|MIPS||31||32||Register 0 is hardwired to 0.|
|Epiphany||64 (per core)||Each instruction controls whether registers are interpreted as integers or single precision floating point. Architecture is scalable to 4096 cores with 16 and 64 core implementations currently available.|
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.
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