ref: a8361bc1b4b71a2ee495b23cfca1f03b5a844f28
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