Two pointer/counter FIFO implementation

Two pointer/counter FIFO implementation
The other method to determine if a FIFO is empty or full is to implement a counter. In the following code, Size contains the number of bytes currently stored in the FIFO.
The advantage of implementing the counter is that FIFO 1/4 full and 3/4 full conditions are easier to implement. If you were studying the behavior of a system it might be informative to measure the current Size as a function of time.


/* Pointer,counter implementation of the FIFO */
#define FifoSize 10 /* Number of 8 bit data in the Fifo */
char *PutPt; /* Pointer of where to put next */
char *GetPt; /* Pointer of where to get next */
unsigned char Size; /* Number of elements currently in the FIFO */
/* FIFO is empty if Size=0 */
/* FIFO is full if Size=FifoSize */
char Fifo[FifoSize]; /* The statically allocated fifo data */
void InitFifo(void) { char SaveSP;
asm(" tpa\n staa %SaveSP\n sei"); /* make atomic, entering critical*/
PutPt=GetPt=&Fifo[0]; /* Empty when Size==0 */
Size=0;
asm(" ldaa %SaveSP\n tap"); /* end critical section */
}
int PutFifo (char data) { char SaveSP;
if (Size == FifoSize ) {
return(0);} /* Failed, fifo was full */
else{
asm(" tpa\n staa %SaveSP\n sei"); /* make atomic, entering critical*/
Size++;
*(PutPt++)=data; /* put data into fifo */
if (PutPt == &Fifo[FifoSize]) PutPt = &Fifo[0]; /* Wrap */
asm(" ldaa %SaveSP\n tap"); /* end critical section */
return(-1); /* Successful */
}
}
int GetFifo (char *datapt) { char SaveSP;
if (Size == 0 ){
return(0);} /* Empty if Size=0 */
else{
asm(" tpa\n staa %SaveSP\n sei"); /* make atomic, entering critical*/
*datapt=*(GetPt++); Size--;
if (GetPt == &Fifo[FifoSize]) GetPt = &Fifo[0];
asm(" ldaa %SaveSP\n tap"); /* end critical section */
return(-1); }
}
Program 5.18. C language routines to implement a two pointer with counter FIFO.
To check for FIFO full, the above PutFifo routine simply compares Size to the maximum allowed
value. If the FIFO is already full then the routine is exited without saving the data. With this
implementation a FIFO with 10 allocated bytes can actually hold 10 data points.
To check for FIFO empty, the following GetFifo routine simply checks to see if Size equals 0.
If Size is zero at the start of the routine, then GetFifo returns with the "empty" condition signified.