μC/OS-II支援對信号量的3種操作：建立信号量OSSemCreate()、發送信号量OSSemPost()、等待信号量OSSemPend()。
1.事件控制塊（ECB）
①µC/OS-II将信号量、互斥信号量、消息郵箱、消息隊列等統稱為“事件”，然後通過一個稱為“事件控制塊（ECB）”的資料結構來管理事件，也就是說，任務和中斷服務程式可以通過ECB向另外的任務程發送信号，任務也可以等待另一個任務或者中斷服務程式給它發送信号。
②在使用事件控制塊之前，需要将所有事件控制塊連結成一個空閑事件控制塊連結清單。每當建立一個信号量、互斥信号量、消息郵箱、消息隊列時，就從該連結清單中取出一個空閑事件控制塊，并對它進行初始化。當信号量、互斥信号量、消息郵箱、消息隊列被删除時，對應的事件控制塊也需要放回到空閑事件控制塊連結清單中。
③初始化
建立一個信号量、互斥信号量、消息郵箱或消息隊列時，需要對事件控制塊的等待任務清單進行初始化，通過調用函數OS_EventWaitListInit()實作。
④任務等待事件
當某個任務要等待一個事件的發生時，需要将任務從就緒任務表中删除，并放到相應事件的事件控制塊的等待任務表中，這些操作是通過調用函數OS_EventTaskWait()來實作的。
void OS_EventTaskWait (OS_EVENT *pevent)
{
OSTCBCur->OSTCBEventPtr = pevent; /* Store pointer to event control block in TCB */
if ((OSRdyTbl[OSTCBCur->OSTCBY] &= ~OSTCBCur->OSTCBBitX) == 0x00) { /* Task no longer ready */
OSRdyGrp &= ~OSTCBCur->OSTCBBitY; /* Clear event grp bit if this was only task pending */
}
pevent->OSEventTbl[OSTCBCur->OSTCBY] |= OSTCBCur->OSTCBBitX; /* Put task in waiting list */
pevent->OSEventGrp |= OSTCBCur->OSTCBBitY;
}
⑤等待事件任務就緒
當發生某個事件時，需要将等待事件的任務清單中優先級最高的任務置于就緒态，通過調用函數OS_EventTaskRdy()實作。
INT8U OS_EventTaskRdy (OS_EVENT *pevent, void *msg, INT8U msk)
{
OS_TCB *ptcb;
INT8U x;
INT8U y;
INT8U bitx;
INT8U bity;
INT8U prio;
y = OSUnMapTbl[pevent->OSEventGrp]; /* Find highest prio. task waiting for message */
bity = OSMapTbl[y];
x = OSUnMapTbl[pevent->OSEventTbl[y]];
bitx = OSMapTbl[x];
prio = (INT8U)((y << 3) + x); /* Find priority of task getting the msg */
if ((pevent->OSEventTbl[y] &= ~bitx) == 0x00) { /* Remove this task from the waiting list */
pevent->OSEventGrp &= ~bity; /* Clr group bit if this was only task pending */
}
ptcb = OSTCBPrioTbl[prio]; /* Point to this task's OS_TCB */
ptcb->OSTCBDly = 0; /* Prevent OSTimeTick() from readying task */
ptcb->OSTCBEventPtr = (OS_EVENT *)0; /* Unlink ECB from this task */
#if ((OS_Q_EN > 0) && (OS_MAX_QS > 0)) || (OS_MBOX_EN > 0)
ptcb->OSTCBMsg = msg; /* Send message directly to waiting task */
#else
msg = msg; /* Prevent compiler warning if not used */
#endif
ptcb->OSTCBStat &= ~msk; /* Clear bit associated with event type */
if (ptcb->OSTCBStat == OS_STAT_RDY) { /* See if task is ready (could be susp'd) */
OSRdyGrp |= bity; /* Put task in the ready to run list */
OSRdyTbl[y] |= bitx;
}
return (prio);
}
⑥等待事件任務逾時就緒
在µC/OS-II中，任務等待事件可以設定逾時時間，如果指定的事件沒有發生而等待逾時，那麼任務也會被時鐘節拍處理函數OSTimeTick()置為就緒。此時，等待任務清單中仍然儲存這個任務，由函數OS_EventTO()實作任務退出等待任務清單。
2.信号量管理
①建立信号量
OS_EVENT *OSSemCreate (INT16U cnt)
{
#if OS_CRITICAL_METHOD == 3 /* Allocate storage for CPU status register */
OS_CPU_SR cpu_sr;
#endif
OS_EVENT *pevent;
if (OSIntNesting > 0) { /* See if called from ISR ... */
return ((OS_EVENT *)0); /* ... can't CREATE from an ISR */
}
OS_ENTER_CRITICAL();
pevent = OSEventFreeList; /* Get next free event control block */
if (OSEventFreeList != (OS_EVENT *)0) { /* See if pool of free ECB pool was empty */
OSEventFreeList = (OS_EVENT *)OSEventFreeList->OSEventPtr;
}
OS_EXIT_CRITICAL();
if (pevent != (OS_EVENT *)0) { /* Get an event control block */
pevent->OSEventType = OS_EVENT_TYPE_SEM;
pevent->OSEventCnt = cnt; /* Set semaphore value */
pevent->OSEventPtr = (void *)0; /* Unlink from ECB free list */
OS_EventWaitListInit(pevent); /* Initialize to 'nobody waiting' on sem. */
}
return (pevent);
}
②等待信号量
void OSSemPend (OS_EVENT *pevent, INT16U timeout, INT8U *err)
{
#if OS_CRITICAL_METHOD == 3 /* Allocate storage for CPU status register */
OS_CPU_SR cpu_sr;
#endif
if (OSIntNesting > 0) { /* See if called from ISR ... */
*err = OS_ERR_PEND_ISR; /* ... can't PEND from an ISR */
return;
}
#if OS_ARG_CHK_EN > 0
if (pevent == (OS_EVENT *)0) { /* Validate 'pevent' */
*err = OS_ERR_PEVENT_NULL;
return;
}
if (pevent->OSEventType != OS_EVENT_TYPE_SEM) { /* Validate event block type */
*err = OS_ERR_EVENT_TYPE;
return;
}
#endif
OS_ENTER_CRITICAL();
if (pevent->OSEventCnt > 0) { /* If sem. is positive, resource available ... */
pevent->OSEventCnt--; /* ... decrement semaphore only if positive. */
OS_EXIT_CRITICAL();
*err = OS_NO_ERR;
return;
}
/* Otherwise, must wait until event occurs */
OSTCBCur->OSTCBStat |= OS_STAT_SEM; /* Resource not available, pend on semaphore */
OSTCBCur->OSTCBDly = timeout; /* Store pend timeout in TCB */
OS_EventTaskWait(pevent); /* Suspend task until event or timeout occurs */
OS_EXIT_CRITICAL();
OS_Sched(); /* Find next highest priority task ready */
OS_ENTER_CRITICAL();
if (OSTCBCur->OSTCBStat & OS_STAT_SEM) { /* Must have timed out if still waiting for event*/
OS_EventTO(pevent);
OS_EXIT_CRITICAL();
*err = OS_TIMEOUT; /* Indicate that didn't get event within TO */
return;
}
OSTCBCur->OSTCBEventPtr = (OS_EVENT *)0;
OS_EXIT_CRITICAL();
*err = OS_NO_ERR;
}
③發送信号量
void OSSemPend (OS_EVENT *pevent, INT16U timeout, INT8U *err)
{
#if OS_CRITICAL_METHOD == 3 /* Allocate storage for CPU status register */
OS_CPU_SR cpu_sr;
#endif
if (OSIntNesting > 0) { /* See if called from ISR ... */
*err = OS_ERR_PEND_ISR; /* ... can't PEND from an ISR */
return;
}
#if OS_ARG_CHK_EN > 0
if (pevent == (OS_EVENT *)0) { /* Validate 'pevent' */
*err = OS_ERR_PEVENT_NULL;
return;
}
if (pevent->OSEventType != OS_EVENT_TYPE_SEM) { /* Validate event block type */
*err = OS_ERR_EVENT_TYPE;
return;
}
#endif
OS_ENTER_CRITICAL();
if (pevent->OSEventCnt > 0) { /* If sem. is positive, resource available ... */
pevent->OSEventCnt--; /* ... decrement semaphore only if positive. */
OS_EXIT_CRITICAL();
*err = OS_NO_ERR;
return;
}
/* Otherwise, must wait until event occurs */
OSTCBCur->OSTCBStat |= OS_STAT_SEM; /* Resource not available, pend on semaphore */
OSTCBCur->OSTCBDly = timeout; /* Store pend timeout in TCB */
OS_EventTaskWait(pevent); /* Suspend task until event or timeout occurs */
OS_EXIT_CRITICAL();
OS_Sched(); /* Find next highest priority task ready */
OS_ENTER_CRITICAL();
if (OSTCBCur->OSTCBStat & OS_STAT_SEM) { /* Must have timed out if still waiting for event*/
OS_EventTO(pevent);
OS_EXIT_CRITICAL();
*err = OS_TIMEOUT; /* Indicate that didn't get event within TO */
return;
}
OSTCBCur->OSTCBEventPtr = (OS_EVENT *)0;
OS_EXIT_CRITICAL();
*err = OS_NO_ERR;
}