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Computer Internals

Glossary

Register

A tiny, very fast storage slot inside the CPU that holds one value (8 to 64 bits) an instruction is working on.

Registers are the CPU's working memory. On x86-64 the general-purpose ones are rax, rbx, rcx, rdx, rsi, rdi, rbp, rsp and r8–r15; their lower halves have their own names (eax, ax, al). Reading a register takes no bus cycle, which is why compilers try hard to keep hot values in them.