The instruction-set family Intel introduced with the 8086 (1978) and progressively extended (IA-32, x86-64). A CISC ISA: variable-length encoding, two-operand instructions, many addressing modes, dense binary code.
rax, rbx, …, r8-r15.rdi, rsi, rdx, rcx, r8, r9; return in rax.mov dst, src (Intel syntax) / mov src, dst (AT&T syntax).