请教：获得多任务实时操作系统的CPU使用率的原理是什么？

如以下程序，IDLE时间是怎样计算的？

//C++:实时获取CPU使用率的类(源码) 出处：www.csdn.net

//=========================================================
//
// Copyright (c) 2000-2004 iWise Technologies，Co. Ltd.
// All Rights Reserved.
//
// Product: iW988
// File: CpuUsage.h
// Created: 天衣有缝
//
// Description:
// ValueAdded main program for iW988.
// Contact:
// waterpub@mail.csdn.net
//
//=========================================================

#pragma once

#define SystemBasicInformation 0
#define SystemPerformanceInformation 2
#define SystemTimeInformation 3

#define Li2Double(x) ((double)((x).HighPart) * 4.294967296E9 + (double)((x).LowPart))

class CCpuUsage
{
public:
CCpuUsage();
~CCpuUsage();

public:
int GetCpuUsage(); //得到系统cpu利用率
int SetRefreshInterval(int milli_sec); //定时刷新间隔

private:
//类型定义
typedef LONG (WINAPI *PROCNTQSI)(UINT,PVOID,ULONG,PULONG);

typedef struct
{
DWORD dwUnknown1;
ULONG uKeMaximumIncrement;
ULONG uPageSize;
ULONG uMmNumberOfPhysicalPages;
ULONG uMmLowestPhysicalPage;
ULONG uMmHighestPhysicalPage;
ULONG uAllocationGranularity;
PVOID pLowestUserAddress;
PVOID pMmHighestUserAddress;
ULONG uKeActiveProcessors;
BYTE bKeNumberProcessors;
BYTE bUnknown2;
WORD wUnknown3;
} SYSTEM_BASIC_INFORMATION;

typedef struct
{
LARGE_INTEGER liIdleTime;
DWORD dwSpare[76];
} SYSTEM_PERFORMANCE_INFORMATION;

typedef struct
{
LARGE_INTEGER liKeBootTime;
LARGE_INTEGER liKeSystemTime;
LARGE_INTEGER liExpTimeZoneBias;
ULONG uCurrentTimeZoneId;
DWORD dwReserved;
} SYSTEM_TIME_INFORMATION;

//变量定义
SYSTEM_PERFORMANCE_INFORMATION SysPerfInfo;
SYSTEM_TIME_INFORMATION SysTimeInfo;
SYSTEM_BASIC_INFORMATION SysBaseInfo;

double dbIdleTime;
double dbSystemTime;

LONG status;

LARGE_INTEGER liOldIdleTime;
LARGE_INTEGER liOldSystemTime;

PROCNTQSI NtQuerySystemInformation;

int m_nCpuUsage;

//定时
HWND m_hWnd;
int m_nRefreshInterval;//默认为1000毫秒
int m_nTimerID;

private:
static void CalcCpuUsage(HWND hwnd, UINT uMsg, UINT_PTR idEvent, DWORD dwTime);
int OnTimer();
};

inline CCpuUsage::CCpuUsage()
{
//
m_hWnd = NULL;
m_nRefreshInterval = 1000;
m_nTimerID = 1000;
m_nCpuUsage = 0;

//
memset(&liOldIdleTime , 0, sizeof(LARGE_INTEGER));
memset(&liOldSystemTime, 0, sizeof(LARGE_INTEGER));

//
NtQuerySystemInformation = (PROCNTQSI)GetProcAddress(
GetModuleHandle("ntdll") , "NtQuerySystemInformation");
if (!NtQuerySystemInformation)
return;

// get number of processors in the system
status = NtQuerySystemInformation(SystemBasicInformation,&SysBaseInfo,sizeof(SysBaseInfo),NULL);
if (status != NO_ERROR)
return;

// create control for timer
m_hWnd = ::CreateWindow("static", "", 0, 0, 0, 0, 0, NULL, NULL, 0, NULL);
::SetWindowLong(m_hWnd , GWL_USERDATA , (long)(this) );

TIMERPROC tp = (TIMERPROC)CalcCpuUsage;
SetTimer(m_hWnd , m_nTimerID, m_nRefreshInterval, tp);

}

inline CCpuUsage::~CCpuUsage()
{
KillTimer(m_hWnd , m_nTimerID);
DestroyWindow(m_hWnd);
}

inline void CCpuUsage::CalcCpuUsage(
HWND hwnd,
UINT uMsg,
UINT_PTR idEvent,
DWORD dwTime
)
{
CCpuUsage* pCpu = (CCpuUsage*)::GetWindowLong(hwnd , GWL_USERDATA);

if ( pCpu )
{
pCpu->OnTimer();
}

}

inline int CCpuUsage::OnTimer()
{
status = NtQuerySystemInformation(SystemTimeInformation,&SysTimeInfo,sizeof(SysTimeInfo),0);

if (status!=NO_ERROR)
return 0;

// get new CPU's idle time
status = NtQuerySystemInformation(SystemPerformanceInformation,&SysPerfInfo,sizeof(SysPerfInfo),NULL);
if (status != NO_ERROR)
return 0;

// if it's a first call - skip it
if (liOldIdleTime.QuadPart != 0)
{
// CurrentValue = NewValue - OldValue
dbIdleTime = Li2Double(SysPerfInfo.liIdleTime) - Li2Double(liOldIdleTime);
dbSystemTime = Li2Double(SysTimeInfo.liKeSystemTime) - Li2Double(liOldSystemTime);

// CurrentCpuIdle = IdleTime / SystemTime
dbIdleTime = dbIdleTime / dbSystemTime;

// CurrentCpuUsage% = 100 - (CurrentCpuIdle * 100) / NumberOfProcessors
dbIdleTime = 100.0 - dbIdleTime * 100.0 / (double)SysBaseInfo.bKeNumberProcessors + 0.5;

m_nCpuUsage = (UINT)dbIdleTime;
}

// store new CPU's idle and system time
liOldIdleTime = SysPerfInfo.liIdleTime;
liOldSystemTime = SysTimeInfo.liKeSystemTime;

// wait one second

return 0;
}

inline int CCpuUsage::GetCpuUsage()
{
return m_nCpuUsage;
}

inline int CCpuUsage::SetRefreshInterval(int milli_sec)
{
m_nRefreshInterval = milli_sec;

if ( m_hWnd )
{
TIMERPROC tp = (TIMERPROC)CalcCpuUsage;
SetTimer(m_hWnd, m_nTimerID, m_nRefreshInterval ,tp);
}

return 0;

}