sys/src/libc/amd64/cycles.s

   1 TEXT cycles(SB),1,$0                            /* time stamp counter; cycles since power up */
   2         RDTSC
   3         MOVL    AX, 0(RARG)                     /* lo */
   4         MOVL    DX, 4(RARG)                     /* hi */
   5         RET