CPUregisters

CPU registers are small, fast storage locations inside a processor that hold data, addresses, and control information during instruction execution. They are the quickest storage available to the CPU, operating far faster than caches or main memory. The exact set and organization of registers vary by architecture, and access is controlled by the processor's register file and instruction set.

General-purpose registers hold operands and intermediate results for arithmetic and data movement. Many CPUs also include dedicated data or index registers to help with addressing. Special-purpose registers perform control-related tasks, such as the program counter (instruction pointer) that marks the next instruction to fetch, the stack pointer that points to the top of the current call stack, and the status or flags register that stores condition codes produced by computations. In addition, floating-point or SIMD registers store non-integer data for specialized math and vector operations.

Examples: x86-64 provides RAX, RBX, RCX, RDX and R8–R15 as general-purpose registers, with RSP as the stack pointer, RBP as the base pointer, RIP as the instruction pointer, and RFLAGS as the flags register. ARM typically uses R0–R12 as general-purpose registers, with SP, LR, and PC, and separate floating-point or NEON registers in supported modes. Other architectures differ in number and naming.

In execution, registers hold operands, results, and addresses used by instructions. The compiler maps frequently used variables to registers through register allocation; when there are not enough registers, values are spilled to memory. Modern CPUs employ register renaming to avoid false dependencies and to support out-of-order execution.

Register usage is subject to calling conventions that specify which registers must be preserved across subroutine calls. Because of their small size, registers are volatile across interrupts or context switches, making them a scarce resource that influences program performance and compiler design.