OS_ENTER_CRITICAL();
    PC_VectSet(VECT_DOS_CHAIN, PC_TickISR);                /* Store MS-DOS's tick to chain             */
    OS_EXIT_CRITICAL();
    
    setjmp(PC_JumpBuf);                                    /* Capture where we are in DOS              */
    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             */
        PC_VectSet(VECT_TICK, PC_TickISR);                 /* Restore DOS's tick vector                */
        OS_EXIT_CRITICAL();
        PC_DispClrScr(DISP_FGND_WHITE + DISP_BGND_BLACK);  /* Clear the display                        */
        exit(0);                                           /* Return to DOS                            */
    }
}
/*$PAGE*/
/*
*********************************************************************************************************
*                                       ELAPSED TIME INITIALIZATION
*
* Description : This function initialize the elapsed time module by determining how long the START and
*               STOP functions take to execute.  In other words, this function calibrates this module
*               to account for the processing time of the START and STOP functions.
*
* Arguments   : None.
*
* Returns     : None.
*********************************************************************************************************
*/
void PC_ElapsedInit(void)
{
    PC_ElapsedOverhead = 0;
    PC_ElapsedStart();
    PC_ElapsedOverhead = PC_ElapsedStop();
}
/*$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.
*********************************************************************************************************
*/
void PC_ElapsedStart(void)
{
    INT8U data;


    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);
}
/*$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)
{
    INT8U  data;
    INT8U  low;
    INT8U  high;
    INT16U cnts;


    data  = 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 */
    return ((INT16U)((ULONG)cnts * 54926L >> 16) - PC_ElapsedOverhead);
}
/*$PAGE*/
/*
*********************************************************************************************************
*                                       GET THE CURRENT DATE AND TIME
*
* Description: This function obtains the current date and time from the PC.
*
* Arguments  : s     is a pointer to where the ASCII string of the current date and time will be stored.
*                    You must allocate at least 19 bytes (includes the NUL) of storage in the return 
*                    string.
*
* Returns    : none
*********************************************************************************************************
*/
void PC_GetDateTime (char *s)
{
    struct time now;
    struct date today;


    gettime(&now);
    getdate(&today);
    sprintf(s, "%02d-%02d-%02d  %02d:%02d:%02d",
               today.da_mon,
               today.da_day,
               today.da_year,
               now.ti_hour,
               now.ti_min,
               now.ti_sec);
}
/*$PAGE*/
/*
*********************************************************************************************************
*                                        CHECK AND GET KEYBOARD KEY
*
* Description: This function checks to see if a key has been pressed at the keyboard and returns TRUE if
*              so.  Also, if a key is pressed, the key is read and copied where the argument is pointing
*              to.
*
* Arguments  : c     is a pointer to where the read key will be stored.
*
* Returns    : TRUE  if a key was pressed
*              FALSE otherwise
*********************************************************************************************************
*/
BOOLEAN PC_GetKey (INT16S *c)
{
    if (kbhit()) {                                         /* See if a key has been pressed            */
        *c = getch();                                      /* Get key pressed                          */
        return (TRUE);
    } else {
        *c = 0x00;                                         /* No key pressed                           */
        return (FALSE);
    }
}
/*$PAGE*/
/*
*********************************************************************************************************
*                                      SET THE PC'S TICK FREQUENCY
*
* Description: This function is called to change the tick rate of a PC.
*
* 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)
{
    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;
    }
    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                                     */
}
/*$PAGE*/
/*
*********************************************************************************************************
*                                        OBTAIN INTERRUPT VECTOR
*
* Description: This function reads the pointer stored at the specified vector.
*
* Arguments  : vect  is the desired interrupt vector number, a number between 0 and 255.
*
* Returns    : none
*********************************************************************************************************
*/
void *PC_VectGet (INT8U vect)
{
    return (getvect(vect));
}

/*
*********************************************************************************************************
*                                        INSTALL INTERRUPT VECTOR
*
* Description: This function sets an interrupt vector in the interrupt vector table.
*
* Arguments  : vect  is the desired interrupt vector number, a number between 0 and 255.
*              isr   is a pointer to a function to execute when the interrupt or exception occurs.
*
* Returns    : none
*********************************************************************************************************
*/
void PC_VectSet (INT8U vect, void (*isr)(void))
{
    setvect(vect, (void interrupt (*)(void))isr);
}