//========================================================================
// CPUSPEED
//
// CPU Timer for the Action, Arcade, Strategy Games Group, a part of
// the Entertainment Business Unit at Microsoft.
//
// (c) Copyright 1999-2000 Microsoft Corporation.
// Written by Michael Lyons
//
// USED WITH PERMISSION
//
//========================================================================

//========================================================================
// Content References in Game Coding Complete 2nd Edition
//
// GetCPUSpeed - Chapter 5, page 135
//========================================================================


#include "GameCodeStd.h"

#define SLEEPTIME 0


//========================================================================
// define static variables
//========================================================================
static int s_milliseconds;
static __int64 s_ticks;

static int s_milliseconds0;
static __int64 s_ticks0;

//========================================================================
// fabs
//
// floating point absolute value function
//========================================================================
#if 0
#pragma message("Dsiabled local fabs()implementation to prevent collision w/impl in VS.NET 2k3")
float inline fabs(float a)
{
if (a < 0.0f)
return -a;
else
return a;
}
#endif
//========================================================================
// StartTimingCPU
//
// Call this function to start timing the CPU. It takes the CPU tick
// count and the current time and stores it. Then, while you do other
// things, and the OS task switches, the counters continue to count, and
// when you call UpdateCPUTime, the measured speed is accurate.
//
//========================================================================
int StartTimingCPU()
{
//
// detect ability to get info
//
__asm
{
pushfd ; push extended flags
pop eax ; store eflags into eax
mov ebx, eax ; save EBX for testing later
xor eax, (1<<21) ; switch bit 21
push eax ; push eflags
popfd ; pop them again
pushfd ; push extended flags
pop eax ; store eflags into eax
cmp eax, ebx ; see if bit 21 has changed
jz no_cpuid ; make sure it's now on
}

//
// make ourselves high priority just for the time between
// when we measure the time and the CPU ticks
//
DWORD dwPriorityClass = GetPriorityClass(GetCurrentProcess());
int dwThreadPriority = GetThreadPriority(GetCurrentThread());
SetPriorityClass(GetCurrentProcess(), REALTIME_PRIORITY_CLASS);
SetThreadPriority(GetCurrentThread(), THREAD_PRIORITY_TIME_CRITICAL);

//
// start timing
//
s_milliseconds0 = (int)timeGetTime();

__asm
{
lea ecx, s_ticks0 ; get the offset
mov dword ptr [ecx], 0 ; zero the memory
mov dword ptr [ecx+4], 0 ;
rdtsc ; read time-stamp counter
mov [ecx], eax ; store the negative
mov [ecx+4], edx ; in the variable
}

//
// restore thread priority
//
SetThreadPriority(GetCurrentThread(), dwThreadPriority);
SetPriorityClass(GetCurrentProcess(), dwPriorityClass);

return 0;

no_cpuid:
return -1;
}

//========================================================================
// UpdateCPUTime
//
// This function stops timing the CPU by adjusting the timers to account
// for the amount of elapsed time and the number of CPU cycles taked
// during the timing period.
//========================================================================
void UpdateCPUTime()
{
//
// make ourselves high priority just for the time between
// when we measure the time and the CPU ticks
//
DWORD dwPriorityClass = GetPriorityClass(GetCurrentProcess());
int dwThreadPriority = GetThreadPriority(GetCurrentThread());
SetPriorityClass(GetCurrentProcess(), REALTIME_PRIORITY_CLASS);
SetThreadPriority(GetCurrentThread(), THREAD_PRIORITY_TIME_CRITICAL);

//
// get the times
//
s_milliseconds = -s_milliseconds0;
s_ticks = -s_ticks0;

s_milliseconds += (int)timeGetTime();

__asm
{
lea ecx, s_ticks ; get the offset
rdtsc ; read time-stamp counter
add [ecx], eax ; add the tick count
adc [ecx+4], edx ;
}

//
// restore thread priority
//
SetThreadPriority(GetCurrentThread(), dwThreadPriority);
SetPriorityClass(GetCurrentProcess(), dwPriorityClass);

return;
}

//========================================================================
// CalcCPUSpeed
//
// This function takes the measured values and returns a speed that
// represents a common possible CPU speed.
//========================================================================
int CalcCPUSpeed()
{
//
// get the actual cpu speed in MHz, and
// then find the one in the CPU speed list
// that is closest
//
const struct tagCPUSPEEDS
{
float fSpeed;
int iSpeed;
} cpu_speeds[] =
{
//
// valid CPU speeds that are not integrally divisible by
// 16.67 MHz
//
{ 60.00f, 60 },
{ 75.00f, 75 },
{ 90.00f, 90 },
{ 120.00f, 120 },
{ 180.00f, 180 },
};

//
// find the closest one
//
float fSpeed=((float)s_ticks)/((float)s_milliseconds*1000.0f);
int iSpeed=cpu_speeds[0].iSpeed;
float fDiff=(float)fabs(fSpeed-cpu_speeds[0].fSpeed);

for (int i=1 ; i<sizeof(cpu_speeds)/sizeof(cpu_speeds[0]) ; i++)
{
float fTmpDiff = (float)fabs(fSpeed-cpu_speeds[i].fSpeed);

if (fTmpDiff < fDiff)
{
iSpeed=cpu_speeds[i].iSpeed;
fDiff=fTmpDiff;
}
}

//
// now, calculate the nearest multiple of fIncr
// speed
//

//
// now, if the closest one is not within one incr, calculate
// the nearest multiple of fIncr speed and see if that's
// closer
//
const float fIncr=16.66666666666666666666667f;
const int iIncr=4267; // fIncr << 8

//if (fDiff > fIncr)
{
//
// get the number of fIncr quantums the speed is
//
int iQuantums = (int)((fSpeed / fIncr) + 0.5f);
float fQuantumSpeed = (float)iQuantums * fIncr;
float fTmpDiff = (float)fabs(fQuantumSpeed - fSpeed);

if (fTmpDiff < fDiff)
{
iSpeed = (iQuantums * iIncr) >> 8;
fDiff=fTmpDiff;
}
}

return iSpeed;
}


//========================================================================
// GetCPUSpeed
//
// Gets the CPU speed by timing it for 1 second.
//========================================================================
int GetCPUSpeed()
{
static int CPU_SPEED = 0;

if(CPU_SPEED!=0)
{
//This will assure that the 0.5 second delay happens only once
return CPU_SPEED;
}

if (StartTimingCPU())
return 0;

//This will lock the application for 1 second
do
{
UpdateCPUTime();
Sleep(SLEEPTIME);
} while (s_milliseconds < 1000);

CPU_SPEED = CalcCPUSpeed();
return CPU_SPEED;
}


代碼是《Game Coding Complete》上的……