A register is a high-speed memory location within a CPU.
Common Types of CPU Registers
Various types of registers are used in combination in computer architecture:
General-purpose registers
Used to temporarily store values. Most modern architectures have several dozen general-purpose registers.
Accumulator
A special designation of general-purpose register, used to hold the results of mathematical operations (originally add/subtracts, but now any numerical operation).
Status Register
A status register (or Flag register, or Condition code register) contain flag bits, 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 non-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
Control registers alter the operation of the processor, such as by enabling binary coded decimal (BCD) math or toggling little-endian/big-endian mode.
Stack Pointer
A stack pointer is a register used for indirect access to the stack, and is automatically incremented or decremented on each access.
Program Counter
The program counter 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 and then writing the subroutine address to the program counter. Upon completion of the subroutine, the return address is popped from the stack and written to the program counter.
Relative addressing modes add a signed value to the program counter to calculate the effective address.
Other Uses of the Term "Register"
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.