Editing Processor register (section)

==Types==
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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 a division into data registers and address registers.
** '''''{{vanchor|Data register}}s''''' can hold [[Data (computer science)|numeric data value]]s such as [[integer (computer science)|integers]] and, in some architectures, [[floating-point numbers]], as well as [[character (computing)|characters]], small [[bit array]]s 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-5]]/[[PDP-8]], and the [[HP 2100]], a special data register known as the [[accumulator (computing)|accumulator]] is used implicitly for many operations.
** '''''{{vanchor|Address register}}s''''' hold [[Memory address|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 mode]]s, used to specify the effective address of an operand, exist.
*** The '''''[[Call stack#Stack and frame pointers|stack and frame pointers]]''''' are used to manage the [[call stack]]. Rarely, other [[stack (abstract data type)|data stacks]] are addressed by dedicated address registers (see [[stack machine]]).
** '''''General-purpose registers''''' (''{{Vanchor|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.
** '''''{{vanchor|Floating-point register|FLOATING_POINT_REGISTER|floating point register}}s''''' (''FPR''s) store floating-point numbers in many architectures.
** '''''[[Constant (computer programming)|Constant]] registers''''' hold read-only values such as zero, one, or [[pi]].
** '''''{{visible anchor|Vector registers}}''''' hold data for [[vector processing]] done by [[Single instruction, multiple data|SIMD]] instructions (Single Instruction, Multiple Data).
** '''''[[Status register]]s''''' hold [[truth value]]s often used to determine whether some instruction should or should not be executed.
** '''''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, such as [[microcontrollers]], can also have ''[[special function register]]s'' corresponding to specialized hardware elements.
** '''''[[Control register]]s''''' are used to set the behaviour of system components such as the [[CPU]].
*** '''''[[Model-specific register]]s''''' (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 are not expected to remain standard between processor generations.
*** '''''[[Memory type range register]]s''''' (''MTRR''s)
* '''''{{Vanchor|Internal register}}s''''' are not accessible by instructions and are used internally for processor operations.
** The '''''[[instruction register]]''''' holds the instruction currently being executed.
** Registers related to fetching information from [[random-access memory|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'''''{{anchor|ARCHITECTURAL}} 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 register]]s''' are similar, but occur outside CPUs.

In some architectures (such as [[SPARC]] and [[MIPS architecture|MIPS]]), the first or last register in the integer [[register file]] is a ''[[zero register|pseudo-register]]'' in that it is hardwired to always return zero when read (mostly to simplify indexing modes), and it cannot be overwritten. In [[DEC Alpha|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.