A register is a high-speed memory location within a CPU.
Various types of registers are used in combination in computer architecture. The set of all registers present in a processor is called the Register Set or Register File.
The types of registers present will vary by processor family, but in general consist of some combination of:
Used to temporarily store values. Most modern architectures have several dozen general-purpose registers.
A special designation of general-purpose register, used to hold the results of mathematical operations (originally add/subtracts, but now any numerical operation).
A special designation of general-purpose register, used to hold an index (offset) into an array or memory space. The x86_64 Source Index (SI) and Destination Index (DI) registers are examples of this. Similarly, the 6502 Y and X registers are Index registers.
A status register (or Flag register, or Condition code register) contains bits holding individual flags, which are set/cleared/tested either explicitly (by instructions) or implicitly (as the result of other operations). For example, the ARM AArch32 “Z” flag is set (“1”) if an operation has a zero result, and cleared (“0”) if an operation has a non-zero result. This flag is one bit within the Application Processor Status Register (APSR).
Control registers contain control flags which alter some aspect of the operation of the processor, such as enabling binary coded decimal (BCD) math or toggling little-endian/big-endian mode.
A stack pointer is a register used for indirect access to the stack, and is automatically incremented or decremented on each access.
The program counter or instruction pointer keeps track of the address of the currently-executing instruction.
A jump is performed by writing the destination address to the program counter.
A subroutine (function) call is performed by pushing the program counter onto the stack or saving it in a designated Link Register and then writing the subroutine address to the program counter. Upon completion of the subroutine, the return address is popped from the stack or retrieved from the Link Register and written to the program counter.
Relative addressing modes add a signed value to the program counter to calculate the effective address.
The term register may also be used to refer to an IO port or a memory address within a memory-mapped input/output device, used to set/read device status and parameters or send/receive small amounts of data.