ref: 6e6beff85d3f2d71c4e1d92d1f2091d01c7d5c34 dir: /sys/src/libc/amd64/cycles.s/
TEXT cycles(SB),1,$0 /* time stamp counter; cycles since power up */ RDTSC MOVL AX, 0(RARG) /* lo */ MOVL DX, 4(RARG) /* hi */ RET