逆向BC3.1的程序后得到DOS下面关于精确延时1ms的函数（理论上可以非常精确延时到10微妙级别）

对于DOS下的编译器还没有delay函数(比如MSVC1.52编译器)的同仁来说，它很有用！

word Get8253Count(void)
{
    typedef union {
        word wData;
        struct {
            byte byLow;
            byte byHigh;
        } byData;
    } WORD_BYTE_UNION;

    #define     IoDelay()     outportb(0xeb, 0)     // 自定义一个空函数, 这样的语句肯定不会被编译器优化

    WORD_BYTE_UNION wCount;

    disable();
    outportb(0x43, 0);      // Timer 8253-5, 0 - 计数器0，计数器锁存操作，工作在方式0，二进制计数
    IoDelay();
    wCount.byData.byLow = inportb(0x40);
    IoDelay();
    wCount.byData.byHigh = inportb(0x40);
    enable();
    wCount.wData = ~wCount.wData;

    return wCount.wData;
}

/***
*void xdelay(unsigned int milliseconds)
*
*Purpose:
*    Make an exactly millisecond delay
*
*Entry:
*    unsigned int milliseconds  - The delay time (millisecond)
*
*Exit:
*   void
*
*Exceptions:
*    Note:
*       .This function was created by Denny at 10/26/2008
*       .Dongguan Siliten Electronics CO., LTD.
*       .The timer frequency is 1193180Hz
*       .The timer counter is 16bits, so only elapsed 55ms for/
*        counter a circle.
*       .E-mail: [email protected]
*
*******************************************************************************/
void xdelay(unsigned int milliseconds)
{
    typedef union {
        dword dwData;
        struct {
            word wLow;
            word wHigh;
        } wData;
    } DWORD_WORD_UNION;

    word wBase;
    DWORD_WORD_UNION dwTotal;
    DWORD_WORD_UNION dwCurr;

    wBase = Get8253Count();
    dwTotal.dwData = (dword)wBase + (dword)milliseconds*1193;

    while (TRUE)
    {
        dwCurr.dwData = Get8253Count();
        if (dwCurr.dwData >= dwTotal.dwData)
            break;
        if (dwCurr.wData.wLow >= wBase)
            goto delay_Continue;
        if (dwTotal.wData.wHigh == 0)
            break;
        dwTotal.wData.wHigh--;

            delay_Continue:
        wBase = dwCurr.wData.wLow;
    }
}