gistfile1.txt

// OS X comm page time offsets
// http://www.opensource.apple.com/source/xnu/xnu-1699.26.8/osfmk/i386/cpu_capabilities.h
#define	nt_tsc_base	0x50
#define	nt_scale	0x58
#define	nt_shift	0x5c
#define	nt_ns_base	0x60
#define	nt_generation	0x68
#define	gtod_generation	0x6c
#define	gtod_ns_base	0x70
#define	gtod_sec_base	0x78

TEXT nanotime<>(SB), NOSPLIT, $32
	MOVQ	$0x7fffffe00000, BP	/* comm page base */
	// Loop trying to take a consistent snapshot
	// of the time parameters.
timeloop:
	MOVL	gtod_generation(BP), R8
	TESTL	R8, R8
	JZ	systime
	MOVL	nt_generation(BP), R9
	TESTL	R9, R9
	JZ	timeloop
	RDTSC
	MOVQ	nt_tsc_base(BP), R10
	MOVL	nt_scale(BP), R11
	MOVQ	nt_ns_base(BP), R12
	CMPL	nt_generation(BP), R9
	JNE	timeloop
	MOVQ	gtod_ns_base(BP), R13
	MOVQ	gtod_sec_base(BP), R14
	CMPL	gtod_generation(BP), R8
	JNE	timeloop

	// Gathered all the data we need. Compute time.
	//	((tsc - nt_tsc_base) * nt_scale) >> 32 + nt_ns_base - gtod_ns_base + gtod_sec_base*1e9
	// The multiply and shift extracts the top 64 bits of the 96-bit product.
	SHLQ	$32, DX
	ADDQ	DX, AX
	SUBQ	R10, AX
	MULQ	R11
	SHRQ	$32, AX:DX
	ADDQ	R12, AX
	SUBQ	R13, AX
	IMULQ	$1000000000, R14
	ADDQ	R14, AX
	RET

systime:
	// Fall back to system call (usually first call in this thread).
	MOVQ	SP, DI	// must be non-nil, unused
	MOVQ	$0, SI
	MOVL	$(0x2000000+116), AX
	SYSCALL
	// sec is in AX, usec in DX
	// return nsec in AX
	IMULQ	$1000000000, AX
	IMULQ	$1000, DX
	ADDQ	DX, AX
	RET