另外再跟黑暗執行緒大文中討論的另一篇網址為：
http://blog.darkthread.net/blogs/darkthreadtw/archive/2007/03/29/673.aspx

新版的 msdn 內容有做調整，根本不寫 Win9x 了：

http://msdn.microsoft.com/en-us/library/ms725496(VS.85).aspx

Windows Time


Windows Time

Windows time is the number of milliseconds elapsed since the system was last started. This format exists primarily for backward compatibility with 16-bit Windows. To ensure that applications designed for 16-bit Windows continue to run successfully, the GetTickCount function returns the current Windows time.

You typically use the GetTickCount or GetTickCount64 function to compare the current Windows time with the time returned by the GetMessageTime function. GetMessageTime returns the Windows time when the specified message was created. GetTickCount and GetTickCount64 are limited to the resolution of the system timer, which is approximately 10 milliseconds to 16 milliseconds. The elapsed time retrieved by GetTickCount or GetTickCount64 includes time the system spends in sleep or hibernation.

If you need a higher resolution timer, use the QueryUnbiasedInterruptTime function, a multimedia timer, or a high-resolution timer. The elapsed time retrieved by the QueryUnbiasedInterruptTime function includes only time that the system spends in the working state.

Windows Server 2008, Windows Vista, Windows Server 2003, and Windows XP/2000: The QueryUnbiasedInterruptTime function is available starting with Windows 7 and Windows Server 2008 R2.
You can use the System Up Time performance counter to obtain the number of seconds elapsed since the computer was started. This performance counter can be retrieved from the performance data in the registry key HKEY_PERFORMANCE_DATA. The value returned is an 8-byte value. For more information, see Performance Counters.

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Build date: 10/8/2009

在 VS2005 線上手冊有：
ms-help://MS.MSDNQTR.v80.cht/MS.MSDN.v80/MS.WIN32COM.v10.en/sysinfo/base/windows_time.htm

程式碼區塊

Windows Time
Windows time is the number of milliseconds elapsed since the system started running. This format exists primarily for backward compatibility with 16-bit Windows. To ensure that applications designed for 16-bit Windows continue to run successfully, the GetTickCount function returns the current Windows time.


You typically use the GetTickCount or GetTickCount64 function to compare the current Windows time with the time returned by the GetMessageTime function. GetMessageTime returns the Windows time when the specified message was created. GetTickCount is limited to the resolution of the system timer.

The system timer runs at approximately 10ms.

Windows Me/98/95: The system timer runs at approximately 55ms.


If you need a higher resolution timer, use a multimedia timer or a high-resolution timer.

You can use the System Up Time performance counter to obtain the time elapsed since the computer was started.




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Build date: 2/1/2007

# 璉璉 said on March 28, 2007 9:20 PM:

補充：

所以說 MSDN 在很早就有揭露這個訊息，並不需要特別去測試，在一般電腦上時間最小解析度是 10ms/ 54ms (WinNT/Win9x)，但是若在硬體有更精確的時脈支援下，就可以達到 1ms，所以 Sleep 的精確度是 目標值+- 1ms (+- 1ms 是解析度限制造成誤差) ，若需要使用更高解析度的時間處理，就要改用 QueryPerformanceFrequency / QueryPerformanceCounter 。

註：VBNET 版我也有類似函數，不過懶的整理，所以沒上線。

# 璉璉 said on March 28, 2007 9:27 PM:

再補充：

54 ms 是以前 386/486 主機板每秒震盪 18.4 Hz 算出來的：

1 / 18.4 = 0.0543478260 = 0.054 s = 54 ms

所以以前在 Win3.x 的年代最小也是 54 ms ，但是那時候的 WinNT 3.5/3.51 已經支援到 10 ms，以前要高解析度的時脈要用 WinNT 來跑。

註：WinNT 是 MSDN 定義的縮寫，表示 NT3.x/4/Win2k(5.0)/WinXP(5.1)/Win2003(5.2)/Vista(6.0)

# 璉璉 said on March 28, 2007 9:04 PM:

剛剛從我的 live spaces 那邊看到推薦來源是這邊，就過來喵了一下...

關於這篇，我有幾篇相關的文章：

VB6/VBNET CPU 資源釋出：http://tlcheng.spaces.live.com/blog/cns!145419920BFD55A7!1640.entry

MyWait 函數：http://tlcheng.twbbs.org/TLCheng/Net/NetList.aspx?Action=Function&Module=31&Function=220

Sleep 每次睡，基本上在 WinNT 是 10ms，Win9x 是 54ms，這個在MSDN有提，可以直接搜尋 10 AND 54。

但部份硬體可以低於這個值，讓系統可以睡的更精確，是否能睡的更精確，可以用 API QueryPerformanceFrequency 看看頻率是否與本機 CPU 頻率接近。

我曾經有一台電腦 P4 3G 的電腦，那時用的是華碩主機板，用上面 API 測出來的 QueryPerformanceFrequency 約是 2.997 G，所以那時做上面那個 MyWait 函數時，可以精確到秒以下 5 位，也就是 0.01 ms，而誤差小於 5% ，也就是誤差小於 0.0005 ms (測試範例詳見上面網址說明) ，後來我不小心把主機板燒了，換了一塊技嘉主機板，南北橋雖然都相同，但是卻沒有那麼高的解析度，用 QueryPerformanceFrequency 跑就只有 1193180 (可用 Google 查，有的人乾脆當常數，關鍵字 QueryPerformanceFrequency 1193180) ，我現在這台 C2D E6400 的 CPU 用華碩 P5B 跑，QueryPerformanceFrequency ，得到的值是  2135090000 ，跟系統、內容的 CPU 時脈 2.14GHz 差不多，而跑上面 MyWait 得到：

? MyWait(0.001, False), MyWait(0.001, True) , MyWait(0.001, True, 1)

1.00014519294269E-03        1.00118683521538E-03        1.70647232669349E-03

第二、三個會呼叫 Sleep 睡 0, 1 ms，可以看得出來睡覺不會睡到 10 ms。

所以我覺得是硬體的問題，而硬體是否允許 Sleep 睡覺小於 10 ms ，可以比對 QueryPerformanceFrequency 回傳值是否大於 1193180 來判斷。