*               screen followed by a video attribute.  An attribute of 0x07 displays the character in 
*               WHITE with a black background.
用于顯示一個ASCII碼字符串，實際上，可以顯示由255個ASCII字符所組成的任意字符串數組，只要該數組以NULL結束即可。
*
* Arguments   : x      corresponds to the desired column on the screen.  Valid columns numbers are from
*                      0 to 79.  Column 0 corresponds to the leftmost column.
*               y      corresponds to the desired row on the screen.  Valid row numbers are from 0 to 24.
*                      Line 0 corresponds to the topmost row.
x和y  指定所顯示字符串中第1個字符的坐標(列和行)。行的值為0~DISP_MAX_Y-1，而列的值為0~DISP_MAX_X-1，可參看文件PC.C
*               s      Is the ASCII string to display.  You can also specify a string containing 
*                      characters with numeric values higher than 128.  In this case, special character 
*                      based graphics will be displayed.
指向所需要顯示字符串的指針，字符串必須以NULL結束。可顯示0x00~0xFF的任何字符
*               color  specifies the foreground/background color to use (see PC.H for available choices)
*                      and whether the characters will blink or not.
指向所顯示字符的屬性，即字符的色彩組合。可添加一個DISP_FGND_???常數和一個DISP_BGND_???常數，以得到所需要的色彩組合。
這2個常數的定義見PC.H文件。
*
* Returns     : None
注意：字符串數組中的所有字符都以相同的顏色屬性顯示。
*********************************************************************************************************
*/
void PC_DispStr (INT8U x, INT8U y, INT8U *s, INT8U color)
{
    INT8U  far *pscr;
    INT16U      offset;


    offset  = (INT16U)y * DISP_MAX_X * 2 + (INT16U)x * 2;   /* Calculate position of 1st character     */
    pscr    = (INT8U far *)MK_FP(DISP_BASE, offset);
    while (*s) {
        *pscr++ = *s++;                                     /* Put character in video RAM              */
        *pscr++ = color;                                    /* Put video attribute in video RAM        */
    }
}
/*$PAGE*/
/*
*********************************************************************************************************
*                                             RETURN TO DOS
*
* Description : This functions returns control back to DOS by doing a 'long jump' back to the saved
*               location stored in 'PC_JumpBuf'.  The saved location was established by the function
*               'PC_DOSSaveReturn()'.  After execution of the long jump, execution will resume at the 
*               line following the 'set jump' back in 'PC_DOSSaveReturn()'.  Setting the flag 
該函數讓程序返回DOS。前提是已經調用過PC_DOSSaveReturn()保存重要寄存器內容以保證正常返回DOS。
*               'PC_ExitFlag' to TRUE ensures that the 'if' statement in 'PC_DOSSaveReturn()' executes.
*
* Arguments   : None
*
* Returns     : None
注意：用戶必須保證：在調用PC_DOSReturn()函數前，已經調用過PC_DOSSaveReturn()函數。
*********************************************************************************************************
*/
void PC_DOSReturn (void)
{
    PC_ExitFlag = TRUE;                                    /* Indicate we are returning to DOS         */
    longjmp(PC_JumpBuf, 1);                                /* Jump back to saved environment希望可以返回DOS，只需調用
    PC_DOSReturn()函數即可。在這個函數中，PC_ExitFlag被設定為TRUE，并且執行了跳轉函數longjmp()。*/
}
/*$PAGE*/
/*
*********************************************************************************************************
*                                        SAVE DOS RETURN LOCATION
函數PC_DOSSaveReturn()用于保存當前DOS環境，主程序main()通過調用該函數完成一下功能：1 設置ucos的環境軟中斷向量，以實現任務
切換。2 設置時鐘中斷服務子程序ISR的中斷向量。3 保存DOS環境設置，使得在需要終止一個基于UCOS的應用程序時，可以返回DOS。
PC_DOSSaveReturn()函數的源代碼如下
*
* Description : This function saves the location of where we are in DOS so that it can be recovered.
*               This allows us to abort multitasking under uC/OS-II and return back to DOS as if we had
*               never left.  When this function is called by 'main()', it sets 'PC_ExitFlag' to FALSE
*               so that we don't take the 'if' branch.  Instead, the CPU registers are saved in the
*               long jump buffer 'PC_JumpBuf' and we simply return to the caller.  If a 'long jump' is
*               performed using the jump buffer then, execution would resume at the 'if' statement and
*               this time, if 'PC_ExitFlag' is set to TRUE then we would execute the 'if' statements and
*               restore the DOS environment.
*
* Arguments   : None
*
* Returns     : None
*********************************************************************************************************
*/
void PC_DOSSaveReturn (void)
{
#if OS_CRITICAL_METHOD == 3                      /* Allocate storage for CPU status register           */
    OS_CPU_SR  cpu_sr;
#endif    


    PC_ExitFlag  = FALSE;                                  /* Indicate that we are not exiting yet!PC_DOSSaveReturn()從設
定PC_ExitFlag標志位FALSE開始，標志著現在不希望返回DOS。*/
    OSTickDOSCtr =     1;                                  /* Initialize the DOS tick counter函數將OSTickDOSCtr初始化為1，
這個變量將在函數OSTickISR()中遞減。當其值為0時，下次遞減將會翻轉為255.*/
    PC_TickISR   = PC_VectGet(VECT_TICK);                  /* Get MS-DOS's tick vector                 */
    
    PC_VectSet(VECT_DOS_CHAIN, PC_TickISR);                /* Store MS-DOS's tick to chain；PC_DOSSaveReturn()把DOS中的時
 鐘處理程序保存在一個空的中斷向量入口處，這樣UCOS的時鐘處理程序就可以調用它了   成為向量的轉移。*/
    
    setjmp(PC_JumpBuf);                                    /* Capture where we are in DOS；PC_DOSSaveReturn()調用函數
    setjmp()。這個函數將得到處理器的狀態(各個主要寄存器的內容)，并把它們保存在一個結構體PC_JumpBuf中。獲取處理器的狀態后，
    setjmp()將返回PC_DOSSaveReturn()函數，然后從下一條語句繼續執行。因為PC_ExitFlag被初始化為FALSE，PC_DOSSaveReturn()將
    會跳過if語句，返回到主函數main()。跳轉函數longjmp()使處理器返回到函數PC_DOSSaveReturn()(調用setjmp()函數的下一條語句)*/
    if (PC_ExitFlag == TRUE) {                             /* See if we are exiting back to DOS        */
        OS_ENTER_CRITICAL();
        PC_SetTickRate(18);                                /* Restore tick rate to 18.2 Hz因為PC_ExitFlag為TRUE，將執行if
        語句后的代碼。*/
        OS_EXIT_CRITICAL();
        PC_VectSet(VECT_TICK, PC_TickISR);                 /* Restore DOS's tick vector                */
        PC_DispClrScr(DISP_FGND_WHITE + DISP_BGND_BLACK);  /* Clear the display                        */
        exit(0);                                           /* Return to DOS；PC_DOSSaveReturn()把時鐘節拍改回18.2hz，恢復
        PC原來的時鐘中斷服務子程序并清屏，通過exit(0)函數返回DOS命令行狀態。*/
    }
}
/*$PAGE*/
/*
*********************************************************************************************************
*                                       ELAPSED TIME INITIALIZATION
程序運行時間測量函數用于確定函數的運行時間，這是通過PC機的82C54定時器#2完成的。在實際測量時，只需把被測量的代碼放在
PC_ElapsedStart()和PC_ElapsedStop()2個函數之間即可；但在使用這2個函數之前，必須先調用初始化函數PC_ElapsedInit()，以測量上
面2個函數本身執行所需要的時間。這樣，PC_ElapsedStop()函數返回的時間就是執行測量代碼所需要的實際時間。
注意：上面3個函數都不具備可重入性，所以，必須小心：不得有果個任務同時調用這些函數。
*
* Description : This function initialize the elapsed time module by determining how long the START and
*               STOP functions take to execute.  In other words, this function calibrates this module
*               to account for the processing time of the START and STOP functions.
*
* Arguments   : None.
*
* Returns     : None.
*********************************************************************************************************
*/
void PC_ElapsedInit(void)
{
    PC_ElapsedOverhead = 0;
    PC_ElapsedStart();
    PC_ElapsedOverhead = PC_ElapsedStop();
}
/*$PAGE*/
/*
*********************************************************************************************************
*                                         INITIALIZE PC'S TIMER #2
*
* Description : This function initialize the PC's Timer #2 to be used to measure the time between events.
*               Timer #2 will be running when the function returns.
*
* Arguments   : None.
*
* Returns     : None.
注意：在調用PC_ElapsedStart()或PC_ElapsedStop()之前，一定要調用PC_ElapsedInit()函數。
      本函數不具備可重入性，不能在沒有適當保護機制(信號量和調度鎖定等)的情況下，由多個任務同時調用。
      用戶代碼的執行時間必須短于54.93ms，以保證時間測量函數能夠正常工作。
*********************************************************************************************************
*/
void PC_ElapsedStart(void)
{
#if OS_CRITICAL_METHOD == 3                      /* Allocate storage for CPU status register           */
    OS_CPU_SR  cpu_sr;
#endif    
    INT8U      data;


    OS_ENTER_CRITICAL();
    data  = (INT8U)inp(0x61);                              /* Disable timer #2                         */
    data &= 0xFE;
    outp(0x61, data);
    outp(TICK_T0_8254_CWR,  TICK_T0_8254_CTR2_MODE0);      /* Program timer #2 for Mode 0              */
    outp(TICK_T0_8254_CTR2, 0xFF);
    outp(TICK_T0_8254_CTR2, 0xFF);
    data |= 0x01;                                          /* Start the timer                          */
    outp(0x61, data);
    OS_EXIT_CRITICAL();
}
/*$PAGE*/
/*
*********************************************************************************************************
*                                 STOP THE PC'S TIMER #2 AND GET ELAPSED TIME
*
* Description : This function stops the PC's Timer #2, obtains the elapsed counts from when it was
*               started and converts the elapsed counts to micro-seconds.
*
* Arguments   : None.
*
* Returns     : The number of micro-seconds since the timer was last started.
*
* Notes       : - The returned time accounts for the processing time of the START and STOP functions.
*               - 54926 represents 54926S-16 or, 0.838097 which is used to convert timer counts to
*                 micro-seconds.  The clock source for the PC's timer #2 is 1.19318 MHz (or 0.838097 uS)
*********************************************************************************************************
*/
INT16U PC_ElapsedStop(void)
{
#if OS_CRITICAL_METHOD == 3                      /* Allocate storage for CPU status register           */
    OS_CPU_SR  cpu_sr;
#endif    
    INT8U      data;
    INT8U      low;
    INT8U      high;
    INT16U     cnts;