ref: 646062da1c4248e5410f29bec14cd1efea5beb45 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