ref: f0ff0fb0544841d73bc29872d8e2fb4a6a8a5e06
dir: /sys/src/libc/386/cycles.s/
#define RDTSC BYTE $0x0F; BYTE $0x31 TEXT cycles(SB),1,$0 /* time stamp counter; cycles since power up */ RDTSC MOVL vlong+0(FP), CX /* &vlong */ MOVL AX, 0(CX) /* lo */ MOVL DX, 4(CX) /* hi */ RET