OS_ENTER_CRITICAL();
    data  = (INT8U)inp(0x61);                                    /* Disable the timer                  */
    data &= 0xFE;
    outp(0x61, data);
    outp(TICK_T0_8254_CWR, TICK_T0_8254_CTR2_LATCH);             /* Latch the timer value              */
    low  = inp(TICK_T0_8254_CTR2);
    high = inp(TICK_T0_8254_CTR2);
    cnts = (INT16U)0xFFFF - (((INT16U)high << 8) + (INT16U)low); /* Compute time it took for operation */
    OS_EXIT_CRITICAL();
    return ((INT16U)((INT32U)cnts * 54926L >> 16) - PC_ElapsedOverhead);
}
/*$PAGE*/
/*
*********************************************************************************************************
*                                       GET THE CURRENT DATE AND TIME
*
* Description: This function obtains the current date and time from the PC.調(diào)用PC_GetDateTime()函數(shù)，可得到PC中的當(dāng)前日期
和時間，并且以ASCII碼的形式返回，格式是"YYYY-MM-DD  HH:MM:SS"。至少需要21個字符(包括NULL字符)存放這些數(shù)據(jù)。注意：日期數(shù)據(jù)
和時間數(shù)據(jù)有2個空格字符，因此需要21個字符(包括NULL字符)存放這些數(shù)據(jù)。該函數(shù)使用了Borland C的庫函數(shù)gettime()和getdate()，
其他DOS環(huán)境下的C編譯器也應(yīng)該有類似的函數(shù)。
*
* Arguments  : s     is a pointer to where the ASCII string of the current date and time will be stored.
*                    You must allocate at least 21 bytes (includes the NUL) of storage in the return 
*                    string.  The date and time will be formatted as follows:
*
*                        "YYYY-MM-DD  HH:MM:SS"
*
* Returns    : none
*********************************************************************************************************
*/
void PC_GetDateTime (char *s)
{
    struct time now;
    struct date today;


    gettime(&now);
    getdate(&today);
    sprintf(s, "%04d-%02d-%02d  %02d:%02d:%02d",
               today.da_year,
               today.da_mon,
               today.da_day,
               now.ti_hour,
               now.ti_min,
               now.ti_sec);
}
/*$PAGE*/
/*
*********************************************************************************************************
*                                        CHECK AND GET KEYBOARD KEY
*
* Description: This function checks to see if a key has been pressed at the keyboard and returns TRUE if
*              so.  Also, if a key is pressed, the key is read and copied where the argument is pointing
*              to.
PC_GetKey()函數(shù)檢查鍵盤是否有鍵被按下。如果有，就得到按鍵值并返回。該函數(shù)調(diào)用了Borland C的庫函數(shù)kbhit()和getch()，其他DOS
環(huán)境下的C編譯器也同樣有類似函數(shù)。
*
* Arguments  : c     is a pointer to where the read key will be stored.
*
* Returns    : TRUE  if a key was pressed
*              FALSE otherwise
key  指向按鍵值存放地址的指針。如果沒有按鍵被放下，則該指針值為0x0000.
如果按鍵被按下，則返回TRUE；否則返回FALSE
*********************************************************************************************************
*/
BOOLEAN PC_GetKey (INT16S *c)
{
    if (kbhit()) {                                         /* See if a key has been pressed            */
        *c = (INT16S)getch();                              /* Get key pressed                          */
        return (TRUE);
    } else {
        *c = 0x00;                                         /* No key pressed                           */
        return (FALSE);
    }
}
/*$PAGE*/
/*
*********************************************************************************************************
*                                      SET THE PC'S TICK FREQUENCY
*
* Description: This function is called to change the tick rate of a PC.
PC_SetTickRate()允許通過指定所需要的頻率，改變時鐘節(jié)拍的頻率值。在DOS下，時鐘節(jié)拍產(chǎn)生18.206 48次/秒，或每隔54.925ms產(chǎn)生一
次。這是因?yàn)?2C54芯片的計(jì)數(shù)器沒有被初始化，而是使用默認(rèn)值65 535.如果芯片被初始化為59 659，那么時鐘節(jié)拍就會是精確的20.000Hz
筆者決定將時鐘節(jié)拍設(shè)置的更快一些，用的是大約200Hz（實(shí)際為199.9966HZ）。OS_CPU_A.ASM文件中的代碼將會每11個時鐘節(jié)拍調(diào)用1次
DOS的時鐘節(jié)拍處理程序，這是為了保證DOS以前，須調(diào)用PC_SetTickRate()函數(shù)，并將頻率設(shè)為18Hz。
*
* Arguments  : freq      is the desired frequency of the ticker (in Hz)
*
* Returns    : none
*
* Notes      : 1) The magic number 2386360 is actually twice the input frequency of the 8254 chip which
*                 is always 1.193180 MHz.
*              2) The equation computes the counts needed to load into the 8254.  The strange equation
*                 is actually used to round the number using integer arithmetic.  This is equivalent to
*                 the floating point equation:
*
*                             1193180.0 Hz
*                     count = ------------ + 0.5
*                                 freq
*********************************************************************************************************
*/
void PC_SetTickRate (INT16U freq)
{
#if OS_CRITICAL_METHOD == 3                      /* Allocate storage for CPU status register           */
    OS_CPU_SR  cpu_sr;
#endif    
    INT16U     count;


    if (freq == 18) {                            /* See if we need to restore the DOS frequency        */
        count = 0;
    } else if (freq > 0) {                        
                                                 /* Compute 8254 counts for desired frequency and ...  */
                                                 /* ... round to nearest count                         */
        count = (INT16U)(((INT32U)2386360L / freq + 1) >> 1); 
    } else {
        count = 0;
    }
    OS_ENTER_CRITICAL();
    outp(TICK_T0_8254_CWR,  TICK_T0_8254_CTR0_MODE3); /* Load the 8254 with desired frequency          */  
    outp(TICK_T0_8254_CTR0, count & 0xFF);            /* Low  byte                                     */
    outp(TICK_T0_8254_CTR0, (count >> 8) & 0xFF);     /* High byte                                     */
    OS_EXIT_CRITICAL();
}
/*$PAGE*/
/*
*********************************************************************************************************
*                                        OBTAIN INTERRUPT VECTOR
PC_VectGet()和PC_VectSet()這2個函數(shù)應(yīng)該是與編譯器無關(guān)的，是需要編譯器支持一下3個宏即可
MK_FP()定義長指針
FP_OFF()得到一個長指針的偏移量
FP_SEG()得到一個長指針的段地址。
*
* Description: This function reads the pointer stored at the specified vector.
這個函數(shù)用于獲取由中斷向量值vect指定的中斷服務(wù)子程序的地址。80x86處理器可支持多達(dá)256個中斷/異常處理程序
*
* Arguments  : vect  is the desired interrupt vector number, a number between 0 and 255.
vect  中斷向量值，是一個介于0~255的數(shù)值
*
* Returns    : The address of the Interrupt handler stored at the desired vector location.
由中斷向量值指定的中斷/異常處理程序的地址
注意：中斷向量值指定的中斷/異常處理程序
      假定80x86的代碼在編譯時選擇大模式，即所有的返回指針都是長指針。
      假定80x86運(yùn)行在實(shí)時模式下
*********************************************************************************************************
*/
void *PC_VectGet (INT8U vect)
{
#if OS_CRITICAL_METHOD == 3                      /* Allocate storage for CPU status register           */
    OS_CPU_SR  cpu_sr;
#endif    
    INT16U    *pvect;
    INT16U     off;
    INT16U     seg;
    
    
    pvect = (INT16U *)MK_FP(0x0000, vect * 4);        /* Point into IVT at desired vector location     */
    OS_ENTER_CRITICAL();
    off   = *pvect++;                                 /* Obtain the vector's OFFSET                    */
    seg   = *pvect;                                   /* Obtain the vector's SEGMENT                   */
    OS_EXIT_CRITICAL();
    return (MK_FP(seg, off));
}

/*
*********************************************************************************************************
*                                        INSTALL INTERRUPT VECTOR
*
* Description: This function sets an interrupt vector in the interrupt vector table.
用于設(shè)定中斷向量表的內(nèi)容。80x86處理器可支持多達(dá)256個中斷/異常處理程序
*
* Arguments  : vect  is the desired interrupt vector number, a number between 0 and 255.
中斷向量值，是一個介于0~255的數(shù)值
*              isr   is a pointer to a function to execute when the interrupt or exception occurs.
中斷/異常處理程序的地址
*
* Returns    : none
注意：設(shè)定中斷向量時必須小心，因?yàn)橛袑懼袛嘞蛄渴潜徊僮飨到y(tǒng)(DOS和/或UCOS_II)使用的
      假定80x86的代碼在編譯時選擇大模式，即所有的返回指針都是長指針。
*********************************************************************************************************
*/
void PC_VectSet (INT8U vect, void (*isr)(void))
{
#if OS_CRITICAL_METHOD == 3                      /* Allocate storage for CPU status register           */
    OS_CPU_SR  cpu_sr;
#endif    
    INT16U    *pvect;
    
    
    pvect    = (INT16U *)MK_FP(0x0000, vect * 4);     /* Point into IVT at desired vector location     */
    OS_ENTER_CRITICAL();
    *pvect++ = (INT16U)FP_OFF(isr);                   /* Store ISR offset                              */
    *pvect   = (INT16U)FP_SEG(isr);                   /* Store ISR segment                             */
    OS_EXIT_CRITICAL();
}