ref: 1d220602ef785d0b3c436123ddb51bd40a976b83
dir: /sys/src/ape/lib/ap/386/cycles.s/
#define RDTSC BYTE $0x0F; BYTE $0x31 TEXT _cycles(SB),1,$0 /* time stamp counter; cycles since power up */ RDTSC MOVL vlong+0(FP), CX /* &vlong */ MOVL AX, 0(CX) /* lo */ MOVL DX, 4(CX) /* hi */ RET