ref: 8ea31ae7b21e9f0c8991b2b861bc51ea3d67c3cd
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