The processor, or CPU (central processing unit), is the part that runs programs. It only understands very simple instructions, called machine code: load a number from memory, add two numbers, compare them, store the result, jump to a different instruction. Every program, however complicated, comes down to long lists of these.
It keeps the values it's working on in registers, a handful of tiny, extremely fast storage slots inside the chip. The arithmetic logic unit (ALU) does the sums and comparisons with circuits of logic gates. A special register, the program counter, holds the address of the next instruction.
The CPU runs a loop called the fetch–decode–execute cycle. Fetch: read the instruction at the program counter's address from memory. Decode: work out what it means. Execute: do it, then move the program counter on to the next instruction (or somewhere else, for a jump).
LOAD R1, [100] ; R1 = the number at address 100
LOAD R2, [104] ; R2 = the number at address 104
ADD R3, R1, R2 ; R3 = R1 + R2
STORE R3, [108] ; save R3 at address 108A clock keeps it in step. A 3 GHz processor's clock ticks 3 billion times a second, and modern CPUs finish several instructions per tick. They also have several cores, each a processor of its own, so they can run several things at once. Languages like Python are translated into these instructions (by a compiler or an interpreter) before the CPU can run them.
Put the steps of the instruction cycle in order.
| CPU | Runs programs, one simple instruction at a time (per core). |
|---|---|
| Machine code | The CPU's own instructions: load, add, compare, store, jump. |
| Register | A tiny, very fast slot inside the CPU. |
| Program counter | Where the next instruction is. |
| Fetch–decode–execute | The cycle every instruction goes through. |
| GHz | Billions of clock ticks per second. |