Register file
The collection of the processor's registers — the fastest storage of all, where the data the CPU is working with right now is held. Physically a small, extremely fast memory bank with several read and write ports so many execution units can reach it at once.
Role: registers are the top of the memory hierarchy (faster than the L1 cache) and the only layer the ALU and other units can operate on directly. An instruction reads its operands from the register file and writes its result back there. A key detail in modern CPUs: the difference between architectural registers (the ~16-32 that the instruction set exposes, e.g. RAX, RBX) and physical registers (the many more — often hundreds — that the hardware actually has). Register renaming maps the few architectural ones to the many physical ones to allow out-of-order execution without false dependencies. The number of read/write ports (how many operands can be read/written per cycle) is a costly design trade-off that limits the issue width. Fundamental CPU storage. Related to register renaming and ALU.