1. Chapter 6: Event Control Block (ECB)

2. Synchronization & Communication
Semaphore (counting semaphore)
Mutual Exclusion Semaphore (binary semaphore + PCP)
Event Flag
Communication
Message Mailbox
Message Queue

3. General operations OSXxxCreate OSXxxDel OSXxxPend OSXxxAccept
Wait for Xxx
OSXxxAccept
A non-blocking version of OSXxxPend
OSXxxPost
Release Xxx
OSXxxQuery

4. Blocking System call
TaskA
TaskB
“Device Not Ready”

5. Nonblocking System call
TaskA
return -1
“Device Not Ready”

6. Asynchronous System call
TaskA
TaskA
return 0
“Device Not Ready”

7. Semantics of ECB ECB (Event Control Block)
An ISR or a task can signal an ECB
Only a task can wait for an ECB (error!!!)
An optional timeout can be specified in case the ECB is not signaled within the specified time period

8. Semantics of ECB Multiple tasks can wait for an ECB.
Only the highest priority task (HPT) is made ready to run when the ECB is signaled.
When a ECB is used as a semaphore, tasks can both wait and signal the ECB.

9. The use of ECB A building block to implement services such as
Semaphore
Mutual Exclusion Semaphore
Message Mailbox
Message Queue
All ECB functions only consider how to manipulate data structures.
Caller must consider synchronization issues

10. ECB data structure

11. Type of event control block Pointer to message or queue structure
ECB data structure
Type of event control block
.OSEventType
.OSEventGrp
.OSEventCnt
.OSEventPtr 7 6 5 4 3 2 1 15 14 13 12 11 10 9 8 . 55 54 53 52 51 50 49 48 63 62 61 60 59 58 57 56
~ OSRdyGrp
Semaphore Count…
Pointer to message or queue structure
~ OSRdyTbl
“red” fields are Initialized by OS_EventWaitListInit
~: is “similar to”

12. OSEventType OSEventGrp OSEventCnt OSEventPtr OSEventTbl[]
OS_EVENT_TYPE_SEM
OS_EVENT_TYPE_MUTEX
0x00
0x00
cnt
prio
OS_MUTEX_AVAILABLE
NULL
NULL
0x00 … 0x00
0x00 … 0x00
OS_EVENT_TYPE_QM
Point to msg
0x00
OS_EVENT_TYPE_MBOX
0x00 … 0x00
0x00
cnt
Point to Q
0x00 … 0x00

13. ECB Functions OS_EventWaitListInit() OS_EventTaskRdy()
Initialize an ECB
OS_EventTaskRdy()
Make a task ready
OS_EventTaskWait()
Put the task to sleep
OS_EventTO()

14. 5 Task A ECB Task B ready queue (waiting queue) 2. OS_XXX_Post1
ready queue
2. OS_XXX_Post
(OS_Event_TaskRdy)
1. OS_XXX_Pend
(OS_Event_TaskWait)
Task A
ECB
(waiting queue)
Task B 5

15. 5 Task A ECB Task B ready queue (waiting queue) OS_XXX_Post1
ready queue
2. OS_EventTO
1. OS_XXX_Pend
(OS_Event_TaskWait)
Task A
OS_XXX_Post
(OS_Event_TaskRdy)
ECB
(waiting queue)
Task B 5

16. OS_EventWaitListInit()
"loop unrolling"

17. OS_EventTaskRdy()
This function is called by the POST functions for a semaphore, a mutex, a message mailbox or a message queue when the ECB is signaled.
OS_EventTaskRdy() removes the highest priority task from the wait list of the ECB and makes this task ready to run.

18. OS_EventTaskRdy() - 1 EventGrp EventTbl Waiting Queue Ready Queue 1 1
OSRdyGrp
EventGrp 1 1 1
EventTbl
OSRdyTbl 1 1 1 1 1

19. OS_EventTaskRdy() - 2 EventGrp EventTbl Waiting Queue Ready Queue 1 1
OSRdyGrp
EventGrp 1
EventTbl
OSRdyTbl 1 1 1 1 1

20. OS_EventTaskRdy() - 3 EventGrp EventTbl Waiting Queue Ready Queue 1 1
OSRdyGrp
EventGrp 1 1
EventTbl
OSRdyTbl 1 1 1 1 1

21. OS_EventTaskRdy() - 4 Task Control Block (TCB) Event Control Block
OSTCBDly=? 1
OSTCBEventPtr
OSTCBStat 1 1
Task Control Block
(TCB)
Event Control Block
(ECB)

22. OS_EventTaskRdy() - 5 Task Control Block (TCB) Event Control Block
OSTCBDly=? 1
OSTCBEventPtr
OSTCBStat 1 1
Task Control Block
(TCB)
Event Control Block
(ECB)

23. OS_EventTaskRdy() - 6 Task Control Block (TCB) Event Control Block
OSTCBDly=0 1
OSTCBEventPtr
OSTCBStat ≒ Rdy 1 1
Task Control Block
(TCB)
Event Control Block
(ECB)

24. OS_EventTaskRdy()
INT8U OS_EventTaskRdy (OS_EVENT *pevent, void *msg, INT8U msk) {
OS_TCB *ptcb;
INT8U x, y, bitx, bity, prio;
y = OSUnMapTbl[pevent->OSEventGrp];
bity = OSMapTbl[y];
x = OSUnMapTbl[pevent->OSEventTbl[y]];
bitx = OSMapTbl[x];
prio = (INT8U)((y << 3) + x);
if ((pevent->OSEventTbl[y] &= ~bitx) == 0x00)
pevent->OSEventGrp &= ~bity;
ptcb = OSTCBPrioTbl[prio];
ptcb->OSTCBDly = 0;
ptcb->OSTCBEventPtr = (OS_EVENT *)0;
ptcb->OSTCBMsg = msg;
ptcb->OSTCBStat &= ~msk;
if (ptcb->OSTCBStat == OS_STAT_RDY) {
OSRdyGrp |= bity; OSRdyTbl[y] |= bitx; }
return (prio);
Find highest priority task waiting for message
Remove this task from the waiting list
Prevent OSTimeTick() from readying task
Unlink ECB from this task
Clear bit associated with event type
Set the task ready to run if the task is not suspended

25. OS_EventTaskWait()
This function is called by the PEND functions for a semaphore, a mutex, a message mailbox or a message queue when a task must wait on an ECB.
It removes the current task from the ready list and places it in the wait list of the ECB.

26. OS_EventTaskWait() - 1 EventGrp EventTbl Waiting Queue Ready Queue 1 1
OSRdyGrp
EventGrp 1 1
EventTbl
OSRdyTbl 1 1 1 1 1

27. OS_EventTaskWait() - 2 EventGrp EventTbl Waiting Queue Ready Queue 1 1
OSRdyGrp
EventGrp 1
EventTbl
OSRdyTbl 1 1 1 1 1

28. OS_EventTaskWait() - 3 EventGrp EventTbl Waiting Queue Ready Queue 1 1
OSRdyGrp
EventGrp 1 1 1
EventTbl
OSRdyTbl 1 1 1 1 1

29. Put task in waiting list
OS_EventTaskWait()
void OS_EventTaskWait (OS_EVENT *pevent) {
OSTCBCur->OSTCBEventPtr = pevent;
if ((OSRdyTbl[OSTCBCur->OSTCBY] &= ~OSTCBCur->OSTCBBitX) == 0x00) {
OSRdyGrp &= ~OSTCBCur->OSTCBBitY; }
pevent->OSEventTbl[OSTCBCur->OSTCBY] |= OSTCBCur->OSTCBBitX;
pevent->OSEventGrp |= OSTCBCur->OSTCBBitY;
Task no longer ready
Put task in waiting list

30. OS_EventTO() OS_EventTO = OS-Event-Time-Out
This function is called by the PEND functions for a semaphore, a mutex, a message mailbox or a message queue when the ECB is not signaled within the specified timeout period.

31. Usage: OS_EventTaskRdy()
Taski
Taskj
xxx_post()
xxx_pend( 0 )
xxx_post()
xxx_pend( 0 )

32. Usage: OS_EventTO() 1 3 2 Taski Taskj Taskj xxx_post() xxx_pend( 3 )
Waiting queue
Ready queue
xxx_post()
xxx_pend( 3 )
xxx_post()
xxx_pend( 0 ) 1 3 2

33. Remove the task from the waiting list of the ECB
OS_EventTO()
void OS_EventTO (OS_EVENT *pevent) {
if ((pevent->OSEventTbl[OSTCBCur->OSTCBY] &= ~OSTCBCur->OSTCBBitX) == 0x00) {
pevent->OSEventGrp &= ~OSTCBCur->OSTCBBitY; }
OSTCBCur->OSTCBStat = OS_STAT_RDY;
OSTCBCur->OSTCBEventPtr = (OS_EVENT *)0;
Remove the task from the waiting list of the ECB