LARGE_INTEGER posixEpoch;	/* Posix epoch expressed as 100-ns ticks since
				 * the windows epoch. */

    /*
     * Data used in developing the estimate of performance counter frequency
     */

    Tcl_WideUInt fileTimeSample[SAMPLES];
				/* Last 64 samples of system time. */
    Tcl_WideInt perfCounterSample[SAMPLES];
				/* Last 64 samples of performance counter. */
    int sampleNo;		/* Current sample number. */
} TimeInfo;

static TimeInfo timeInfo = {
    { NULL, 0, 0, NULL, NULL, 0 },
    0,
    0,
    1,
    (HANDLE) NULL,
    (HANDLE) NULL,
    (HANDLE) NULL,
#if defined(HAVE_CAST_TO_UNION) && !defined(__cplusplus)
    (LARGE_INTEGER) (Tcl_WideInt) 0,
    (ULARGE_INTEGER) (DWORDLONG) 0,
    (LARGE_INTEGER) (Tcl_WideInt) 0,
    (LARGE_INTEGER) (Tcl_WideInt) 0,
    (LARGE_INTEGER) (Tcl_WideInt) 0,
#else
    {0, 0},
    {0, 0},
    {0, 0},
    {0, 0},
    {0, 0},
#endif

/*
 * Declarations for functions defined later in this file.
 */

static void		StopCalibration(ClientData clientData);
static DWORD WINAPI	CalibrationThread(LPVOID arg);
static void 		UpdateTimeEachSecond(void);
static void		ResetCounterSamples(Tcl_WideUInt fileTime,
			    Tcl_WideInt perfCounter, Tcl_WideInt perfFreq);
static Tcl_WideInt	AccumulateSample(Tcl_WideInt perfCounter,
			    Tcl_WideUInt fileTime);
static void		NativeScaleTime(Tcl_Time* timebuf,
			    ClientData clientData);
static Tcl_WideInt	NativeGetMicroseconds(void);
static void		NativeGetTime(Tcl_Time* timebuf,
			    ClientData clientData);

/*
 * TIP #233 (Virtualized Time): Data for the time hooks, if any.
 */

 *
 *----------------------------------------------------------------------
 */

Tcl_WideUInt
TclpGetSeconds(void)
{
    Tcl_WideInt usecSincePosixEpoch;

    /* Try to use high resolution timer */
    if ( tclGetTimeProcPtr == NativeGetTime
      && (usecSincePosixEpoch = NativeGetMicroseconds())
    ) {
	return usecSincePosixEpoch / 1000000;
    } else {

 *
 *----------------------------------------------------------------------
 */

Tcl_WideUInt
TclpGetClicks(void)
{
    Tcl_WideInt usecSincePosixEpoch;

    /* Try to use high resolution timer */
    if ( tclGetTimeProcPtr == NativeGetTime
      && (usecSincePosixEpoch = NativeGetMicroseconds())
    ) {
	return (Tcl_WideUInt)usecSincePosixEpoch;
    } else {

 *	This should be used for time-delta resp. for measurement purposes
 *	only, because on some platforms can return microseconds from some
 *	start time (not from the epoch).
 *
 *----------------------------------------------------------------------
 */

Tcl_WideInt
TclpGetWideClicks(void)
{
    LARGE_INTEGER curCounter;

    if (!wideClick.initialized) {
	LARGE_INTEGER perfCounterFreq;

	    wideClick.microsecsScale = 1;
	}

	wideClick.initialized = 1;
    }
    if (wideClick.perfCounter) {
	if (QueryPerformanceCounter(&curCounter)) {
	    return (Tcl_WideInt)curCounter.QuadPart;
	}
	/* fallback using microseconds */
	wideClick.perfCounter = 0;
	wideClick.microsecsScale = 1;
	return TclpGetMicroseconds();
    } else {
    	return TclpGetMicroseconds();

 *
 * Side effects:
 *	None.
 *
 *----------------------------------------------------------------------
 */

Tcl_WideInt
TclpGetMicroseconds(void)
{
    Tcl_WideInt usecSincePosixEpoch;

    /* Try to use high resolution timer */
    if ( tclGetTimeProcPtr == NativeGetTime
      && (usecSincePosixEpoch = NativeGetMicroseconds())
    ) {
	return usecSincePosixEpoch;
    } else {
	/*
	* Use the Tcl_GetTime abstraction to get the time in microseconds, as
	* nearly as we can, and return it.
	*/

	Tcl_Time now;

	tclGetTimeProcPtr(&now, tclTimeClientData);	/* Tcl_GetTime inlined */
	return (((Tcl_WideInt)now.sec) * 1000000) + now.usec;
    }
}

/*
 *----------------------------------------------------------------------
 *
 * Tcl_GetTime --

 *----------------------------------------------------------------------
 */

void
Tcl_GetTime(
    Tcl_Time *timePtr)		/* Location to store time information. */
{
    Tcl_WideInt usecSincePosixEpoch;

    /* Try to use high resolution timer */
    if ( tclGetTimeProcPtr == NativeGetTime
      && (usecSincePosixEpoch = NativeGetMicroseconds())
    ) {
	timePtr->sec = (long) (usecSincePosixEpoch / 1000000);
	timePtr->usec = (unsigned long) (usecSincePosixEpoch % 1000000);

 *	counter. Also spins a thread whose function is to wake up periodically
 *	and monitor these values, adjusting them as necessary to correct for
 *	drift in the performance counter's oscillator.
 *
 *----------------------------------------------------------------------
 */

static inline Tcl_WideInt
NativeCalc100NsTicks(
    ULONGLONG fileTimeLastCall,
    LONGLONG perfCounterLastCall,
    LONGLONG curCounterFreq,
    LONGLONG curCounter
) {
    return fileTimeLastCall +
	((curCounter - perfCounterLastCall) * 10000000 / curCounterFreq);
}

static Tcl_WideInt
NativeGetMicroseconds(void)
{
    /*
     * Initialize static storage on the first trip through.
     *
     * Note: Outer check for 'initialized' is a performance win since it
     * avoids an extra mutex lock in the common case.

	     */

#if !defined(_WIN64)
	    if (timeInfo.perfCounterAvailable
		    /*
		     * The following lines would do an exact match on crystal
		     * frequency:
		     * && timeInfo.nominalFreq.QuadPart != (Tcl_WideInt)1193182
		     * && timeInfo.nominalFreq.QuadPart != (Tcl_WideInt)3579545
		     */
		    && timeInfo.nominalFreq.QuadPart > (Tcl_WideInt) 15000000){
		/*
		 * As an exception, if every logical processor on the system
		 * is on the same chip, we use the performance counter anyway,
		 * presuming that everyone's TSC is locked to the same
		 * oscillator.
		 */

 */

static void
NativeGetTime(
    Tcl_Time *timePtr,
    TCL_UNUSED(ClientData))
{
    Tcl_WideInt usecSincePosixEpoch;

    /*
     * Try to use high resolution timer.
     */
    if ( (usecSincePosixEpoch = NativeGetMicroseconds()) ) {
	timePtr->sec = (long) (usecSincePosixEpoch / 1000000);
	timePtr->usec = (unsigned long) (usecSincePosixEpoch % 1000000);

    LARGE_INTEGER curPerfCounter;
				/* Current value returned from
				 * QueryPerformanceCounter. */
    FILETIME curSysTime;	/* Current system time. */
    static LARGE_INTEGER lastFileTime; /* File time of the previous calibration */
    LARGE_INTEGER curFileTime;	/* File time at the time this callback was
				 * scheduled. */
    Tcl_WideInt estFreq;	/* Estimated perf counter frequency. */
    Tcl_WideInt vt0;		/* Tcl time right now. */
    Tcl_WideInt vt1;		/* Tcl time one second from now. */
    Tcl_WideInt tdiff;		/* Difference between system clock and Tcl
				 * time. */
    Tcl_WideInt driftFreq;	/* Frequency needed to drift virtual time into
				 * step over 1 second. */

    /*
     * Sample performance counter and system time (from posix epoch).
     */

    GetSystemTimeAsFileTime(&curSysTime);

     * the first case). Our estimated frequency will be the nominal frequency.
     *
     * Store the current sample into the circular buffer of samples, and
     * estimate the performance counter frequency.
     */

    estFreq = AccumulateSample(curPerfCounter.QuadPart,
	    (Tcl_WideUInt) curFileTime.QuadPart);

    /*
     * We want to adjust things so that time appears to be continuous.
     * Virtual file time, right now, is
     *
     * vt0 = 10000000 * (curPerfCounter - perfCounterLastCall)
     *	     / curCounterFreq

    } else if (vt0 != curFileTime.QuadPart) {
	/*
	 * Be sure the clock ticks never backwards (avoid it by negative drifting)
	 * just compare native time (in 100-ns) before and hereafter using
	 * new calibrated values) and do a small adjustment (short time freeze)
	 */
	LARGE_INTEGER newPerfCounter;
	Tcl_WideInt nt0, nt1;

	QueryPerformanceCounter(&newPerfCounter);
	nt0 = NativeCalc100NsTicks(timeInfo.fileTimeLastCall.QuadPart,
		timeInfo.perfCounterLastCall.QuadPart, timeInfo.curCounterFreq.QuadPart,
		newPerfCounter.QuadPart);
	nt1 = NativeCalc100NsTicks(vt0,
		curPerfCounter.QuadPart, estFreq,

 *	given frequency.
 *
 *----------------------------------------------------------------------
 */

static void
ResetCounterSamples(
    Tcl_WideUInt fileTime,	/* Current file time */
    Tcl_WideInt perfCounter,	/* Current performance counter */
    Tcl_WideInt perfFreq)	/* Target performance frequency */
{
    int i;
    for (i=SAMPLES-1 ; i>=0 ; --i) {
	timeInfo.perfCounterSample[i] = perfCounter;
	timeInfo.fileTimeSample[i] = fileTime;
	perfCounter -= perfFreq;
	fileTime -= 10000000;

 *
 * In either case, we'll need to reinitialize the circular buffer with samples
 * relative to the current system time and the NOMINAL performance frequency
 * (not the actual, because the actual has probably run slow in the first
 * case).
 */

static Tcl_WideInt
AccumulateSample(
    Tcl_WideInt perfCounter,
    Tcl_WideUInt fileTime)
{
    Tcl_WideUInt workFTSample;	/* File time sample being removed from or
				 * added to the circular buffer. */
    Tcl_WideInt workPCSample;	/* Performance counter sample being removed
				 * from or added to the circular buffer. */
    Tcl_WideUInt lastFTSample;	/* Last file time sample recorded */
    Tcl_WideInt lastPCSample;	/* Last performance counter sample recorded */
    Tcl_WideInt FTdiff;		/* Difference between last FT and current */
    Tcl_WideInt PCdiff;		/* Difference between last PC and current */
    Tcl_WideInt estFreq;	/* Estimated performance counter frequency */

    /*
     * Test for jumps and reset the samples if we have one.
     */

    if (timeInfo.sampleNo == 0) {
	lastPCSample =

	 */

	workPCSample = timeInfo.perfCounterSample[timeInfo.sampleNo];
	workFTSample = timeInfo.fileTimeSample[timeInfo.sampleNo];
	estFreq = 10000000 * (perfCounter - workPCSample)
		/ (fileTime - workFTSample);
	timeInfo.perfCounterSample[timeInfo.sampleNo] = perfCounter;
	timeInfo.fileTimeSample[timeInfo.sampleNo] = (Tcl_WideInt) fileTime;

	/*
	 * Advance the sample number.
	 */

	if (++timeInfo.sampleNo >= SAMPLES) {
	    timeInfo.sampleNo = 0;