ref: 00f7f2ceb6a1818e2975baf6f5c55c1d2d4ecb6e 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