1. Example 16 Circular Queue
Lecture L7.2

2. A Circular Queue
Empty
Containing 2 values

3. Listing 16.2 Function prototypes (queue.h)
// queue.h A character queue
void initq(void); // initialize the queue
void qstore(char); // store character in queue
int qempty(void); // return 0 if queue is not empty
char getq(void); // read character from queue

4. Listing 16.1 A character queue (queue.c)
// queue.c A character queue
#include "queue.h" // prototype definitions
#define QMAX // size of queue
static char qbuff[QMAX]; // the queue
static int front;
static int rear; // queue pointers
static int min = 0; // start of queue
static int max = QMAX-1; // end of queue
void initq(void){
min = 0;
front = 0;
rear = 0;
max = QMAX-1; }

5. void qstore(char c){
rear++; // inc rear
if(rear > max)
rear = min;
if(rear == front){
rear--; // queue is full
if(rear < min) // rewind rear
rear = max;
}else
qbuff[rear] = c; // store c at rear }

6. int qempty(void){
int flag;
if(front == rear)
flag = 1;
else
flag = 0;
return (flag); }

7. char getq(void){
front++; // inc front
if(front > max)
front = 0;
return qbuff[front]; // return value at front }

8. // Example 16: Example of using a queue
#include <hidef.h> /* common defines and macros */
#include <mc9s12dp256.h> /* derivative information */
#include "queue.h"
#include "main_asm.h" /* interface to the assembly module */
#pragma LINK_INFO DERIVATIVE "mc9s12dp256b"
void main(void) {
char* blanks;
char c, a;
blanks = " ";
PLL_init(); // set system clock frequency to 24 MHz
lcd_init(); // enable lcd
initq(); // initialize the queue
keypad_enable(); // enable keypad
set_lcd_addr(0x00); // display on 1st line
while(1) {
c = getkey(); // read keypad
a = hex2asc(c); // convert to ascii
qstore(a); // and store in queue
data8(a); // display on LCD
wait_keyup(); // wait to release key
switch(c){

9. case 0xE: // if enter (*) key
set_lcd_addr(0x40); // display on 2nd line
while(qempty() != 0){ // empty the queue
data8(getq()); // and display on lcd }
set_lcd_addr(0x00); // clear 1st line
type_lcd(blanks);
wait_keyup(); // wait to release key
set_lcd_addr(0x00); // display on 1st line
break;
case 0xF: // if clear (#) key
clear_lcd(); // clear lcd display
default: