ref: 94e39c482eb80039bd64e2bcc24da8fe61b25530 dir: /sys/src/ape/lib/ap/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