有关sys/bios中的Semaphore信号量

大家好 我对下面程序的工作流程不太清楚 尤其是：

sem = Semaphore_create(1, NULL, NULL);

sem = Semaphore_create(0, NULL, NULL);

这两个有什么区别？程序中把1改为0后 程序运行结果区别很大！

Void task1(UArg arg0, UArg arg1);
Void task2(UArg arg0, UArg arg1);

Int resource = 0;

Semaphore_Handle sem;
Task_Handle tsk1;
Task_Handle tsk2;

Int finishCount = 0;

/*
* ======== main ========
*/
Int main()
{
Task_Params taskParams;

/* Create a Semaphore object to be use as a resource lock */
sem = Semaphore_create(1, NULL, NULL);

/* Create two tasks that share a resource*/
Task_Params_init(&taskParams);
taskParams.priority = 1;
tsk1 = Task_create (task1, &taskParams, NULL);

Task_Params_init(&taskParams);
taskParams.priority = 2;
tsk2 = Task_create (task2, &taskParams, NULL);

BIOS_start(); /* does not return */
return(0);
}

/*
* ======== task1 ========
*/
Void task1(UArg arg0, UArg arg1)
{
UInt32 time;

for (;;)
{
System_printf("Running task1 function\n");

if (Semaphore_getCount(sem) == 0)
{
System_printf("Sem blocked in task1\n");
}

/* Get access to resource */
Semaphore_pend(sem, BIOS_WAIT_FOREVER);

/* do work by waiting for 2 system ticks to pass */
time = Clock_getTicks();
while (Clock_getTicks() <= (time + 1))
{
;
}

/* do work on locked resource */
resource += 1;
/* unlock resource */

Semaphore_post(sem);

Task_sleep(10);
}
}

/*
* ======== task2 ========
*/
Void task2(UArg arg0, UArg arg1)
{
for (;;)
{
System_printf("Running task2 function\n");

if (Semaphore_getCount(sem) == 0)
{
System_printf("Sem blocked in task2\n");
}

/* Get access to resource */
Semaphore_pend(sem, BIOS_WAIT_FOREVER);

/* do work on locked resource */
resource += 1;
/* unlock resource */

Semaphore_post(sem);

Task_sleep(10);

finishCount++;
if (finishCount == 5)
{
System_printf("Calling BIOS_exit from task2\n");
BIOS_exit(0);
}
}
}