1. Computer Science & Engineering Electrical Engineering
EE 472 – Embedded Systems
Dr. Shwetak Patel
Assistant Professor
Computer Science & Engineering
Electrical Engineering

2. Agenda Critical regions Semaphores Lab 3 questions Midterms results 2
Shwetak N. Patel - EE 472

3. Mars Pathfinder 1997

4. Multitasking
Need to share resources
Think about the data in Lab 2

5. Sharing
Global variables
Shared buffers
Producer / Consumer model

6. Buffers
Single buffer
Multiple buffers

7. Intertask Communication
Messages
Mailbox

8. Critical Sections Only one task can access a shared resource
A short segment of code that cannot be preempted

9. Critical Sections
What are ways to address the concurrency problem?

10. Critical Sections Ways to address this
Don’t use a preemptive scheduler
Mask interrupts
Use protection code
Flags
Semaphores

11. Semaphores A type of flagging scheme Used to protect critical sections
Atomic operation
Have two operations:
Wait(s) or Pend(s) or P(s)
Signal(s) or Post(s) or V(s)

12. Semaphores
Also used for synchronization

13. Semaphores Task can relinquish control until blocking is over
Task is put in a waiting queue

14. Semaphores – Pros/Cons
Easy to use and implement
One semaphore per resource (reduce blocking)
Drawback: Can cause priority inversion

15. Priority Inversion Hard to debug and detect Prevention techniques
Priority inheritance

16. Deadlock
Two or more processes wait indefinitely

17. Deadlock Prevention Conditions for deadlock (all 4 have to hold)
Mutual exclusion
No preemption
Hold and wait
Circular wait

18. Class Exercise Task1 1 X Task2 2 Task3 3 Task4 4 Task5 5 Task6 6 X
Priority
Resources
R R R3
Task1 1 X
Task2 2
Task3 3
Task4 4
Task5 5
Task6 6 X

19. Class Exercise How many semaphore are needed?2
Where do you see priority inversion problems?
Tasks 1,2,3 (R2)

20. Class Exercise Task1 1 X Task2 2 Task3 3 Task4 4 Task5 5 Task6 6 X
Priority
Resources
R R R3
Task1 1 X
Task2 2
Task3 3
Task4 4
Task5 5
Task6 6 X

21. Class Exercise How many semaphore are needed?3
Where do you see priority inversion problems?
Tasks 1,2,3 (R1) and 1,2,3 (R2) and 1,2,4 (R1)
Do you see any other problems?
Potential deadlock between Tasks 1 and 3

22. Class Exercise Task 1 1. Initialize digital I/O port for input
2. Read digital input
3. Count rising edges on each input
Task 2 1. Read Data from serial port 1
2. Check for errors in serial data
3. if data buffer is empty
{write data payload to buffer}
Task 3 1. Initialize digital I/O port to output
2. write data to digital outputs
3. toggle bit 0 of digital I/O port on and off (i.e. make a brief pulse on bit 0).
Task 4 1. if data buffer is full
{take data from buffer}
2. Write data to serial port 2
Task 5 1. Count number of bytes in data buffer
2. display byte count on LCD

23. Class Exercise What resources are being shared?
Do you see critical sections?
Which tasks?
Where in each task?

24. Class Exercise What resources are being shared?
IO and the buffer
Do you see a critical sections?
Yes
Which tasks and what resources?
1 - io,2 - buffer,3 - io, 4 – buffer, 5 - buffer
Where in each task?

25. Questions?
Shwetak N. Patel - EE 472