Page 413 - Programming Microcontrollers in C

398 Chapter 8 MCORE, A RISC Machine

read the contents of the semaphore and then, if it is in the proper
condition, it will be set. Interrupts can come asynchronously at any
time. Suppose that in the process of attaching a semaphore, the status
of the interrupt is read to be tested, and, prior to marking the semaphore
as used, an interrupt occurs. Within this interrupt routine, it also could
require an interrupt. In this case, the interrupt that was being processed
by the earlier routine will seem to be available, but it is not. In fact
when control is returned to the initial portion of the program, the
semaphore will be again marked as busy, and now two completely
different processes will both assume control over the same semaphore.

/* A call to attach_semaphore(‘a’) will attempt to attach
safely a semaphore. If a semaphore can be attached, a
semaphore number will be returned. Otherwise, a -1 is
returned and no semaphore can now be attached. When
attach_semaphore(n), where 0<=n<10, is executed, an
attempt is made to attach the semaphore specified by the
number. If it is not available, a –1 is returned. After
the semaphore is attached, use the semaphore number as a
parameter on all other semaphore function calls. Function
release_semaphore() returns the semaphore to the avail­
able semaphore pool. The function semaphore_status()
returns TRUE when the semaphore is NOT available, and the
function wait_for_semaphore() waits in a tight loop until
the semaphore becomes available */

#include “mmc2001.h”

#define Number_of_semaphores 10
#define Minus_one -1

/* function prototypes */
int attach_semaphore(void);
void release_semaphore(int);
int semaphore_status(int);
void wait_for_semaphore(int);

static volatile int semaphore[Number_of_semaphores];
int save;

int attach_semaphore(int r)
{
int i;