Why are CPU registers fast to access?

Question

Register variables are a well-known way to get fast access (register int i). But why are registers on the top of hierarchy (registers, cache, main memory, secondary memory)? What are all the things that make accessing registers so fast?

Answer 1

Registers are a core part of the CPU, and much of the instruction set of a CPU will be tailored for working against registers rather than memory locations. Accessing a register's value will typically require very few clock cycles (likely just 1), as soon as memory is accessed, things get more complex and cache controllers / memory buses get involved and the operation is going to take considerably more time.

Answer 2

Registers are circuits which are literally wired directly to the ALU, which contains the circuits for arithmetic. Every clock cycle, the register unit of the CPU core can feed a half-dozen or so variables into the other circuits. Actually, the units within the datapath (ALU, etc.) can feed data to each other directly, via the bypass network, which in a way forms a hierarchy level above registers — but they still use register-numbers to address each other. (The control section of a fully pipelined CPU dynamically maps datapath units to register numbers.)

The register keyword in C does nothing useful and you shouldn't use it. The compiler decides what variables should be in registers and when.

Answer 3

Registers are essentially internal CPU memory. So accesses to registers are easier and quicker than any other kind of memory accesses.

Answer 4

Smaller memories are generally faster than larger ones; they can also require fewer bits to address. A 32-bit instruction word can hold three four-bit register addresses and have lots of room for the opcode and other things; one 32-bit memory address would completely fill up an instruction word leaving no room for anything else. Further, the time required to address a memory increases at a rate more than proportional to the log of the memory size. Accessing a word from a 4 gig memory space will take dozens if not hundreds of times longer than accessing one from a 16-word register file.

A machine that can handle most information requests from a small fast register file will be faster than one which uses a slower memory for everything.

Answer 5

Every microcontroller has a CPU as Bill mentioned, that has the basic components of ALU, some RAM as well as other forms of memory to assist with its operations. The RAM is what you are referring to as Main memory.

The ALU handles all of the arthimetic logical operations and to operate on any operands to perform these calculations, it loads the operands into registers, performs the operations on these, and then your program accesses the stored result in these registers directly or indirectly.

Since registers are closest to the heart of the CPU (a.k.a the brain of your processor), they are higher up in the chain and ofcourse operations performed directly on registers take the least amount of clock cycles.