1. Programming Embedded Systems in C++
Day 3: C++ & a Real-Time Operating System  
21 May 2014 Colin Walls, Mentor Graphics

2. [ # ]
This is what a question slide will look like. Please answer in the chat.
Also, questions to me start with ?

3. Agenda
Monday: C++ for Embedded Programming Tuesday: C to C++ Migration Strategy Wednesday: C++ and a Real Time Operating System Thursday: Case study #1 Friday: Case Study #2

4. [ 1 ]
Are you using an RTOS? If so, which one?

5. Today’s Topics Hiding the RTOS API Thread local data Paired operations
Using classes to encapsulate the API
Thread local data
Paired operations
Dynamic memory management

6. Today’s Topics Hiding the RTOS API Thread local data Paired operations
Using classes to encapsulate the API
Thread local data
Paired operations
Dynamic memory management

7. C++ and an RTOS Benefits: Wide range of possibilities
application programmers do not need detailed knowledge of RTOS
code portability
Wide range of possibilities

8. Task/Object Creation Encapsulate API using classes Task example:
have base class for generic task
constructor/destructor – task creation
empty virtual main()
application task defined by deriving class
instantiate to create tasks
note separation of definition from creation

9. Task/Object Creation
class task { unsigned taskID; public: virtual void main(); task() taskID = task_create(main); } ~task() task_delete(taskID); ... };

10. Task/Object Creation Same approach to other RTOS objects: Semaphores
Mailboxes
Queues
Event flags
Memory pools
Timers
...

11. Today’s Topics Hiding the RTOS API Thread local data Paired operations
Using classes to encapsulate the API
Thread local data
Paired operations
Dynamic memory management

12. Thread Local Data C++, like C, has no concept of thread
Using a class adds this facility
Benefit is ability to have variables (class members) which are local to a thread
new scope concept
Can share by declaring static
Maybe complex variables – e.g. RTOS objects
Same idea could be applied to other objects

13. Thread Local Data
class mytask : task { unsigned taskID; public: int tld[100]; static int state; void main(); ... }; mytask alpha, beta; alpha.tld and beta.tld are distinct [thread local] alpha.state and beta.state are the same variable

14. Today’s Topics Hiding the RTOS API Thread local data Paired operations
Using classes to encapsulate the API
Thread local data
Paired operations
Dynamic memory management

15. [ 2 ]
Is dynamic memory an issue/problem for a real time system?

16. Paired Operations
Common requirement for a pair of complementary operations
e.g. disable/enable interrupts
Problem can be ensuring that both are executed
Object oriented “trick” solves the problem
Constructor performs first, destructor performs second

17. Paired Operations
<normal code> { object declaration (constructor disables interrupts) <critical code> (destructor re-enables interrupts) }

18. Paired Operations
class Device_Locked { private: static Semaphore device; // NB static public: Device_Locked() { device.acquire(); }; ~ Device_Locked() { device.release(); }; };

19. Today’s Topics Hiding the RTOS API Thread local data Paired operations
Using classes to encapsulate the API
Thread local data
Paired operations
Dynamic memory management

20. Dynamic Memory Management
Dynamic behavior is troublesome in real time embedded systems
Issues:
non-determinism
key to real time is predictability
resource exhaustion
also fragmentation

21. Memory Fragmentation #define K (1024) ... char *p1, *p2;
Heap [10K]
Heap [10K]
#define K (1024)
...
char *p1, *p2;
p1 = malloc(3*K);
p2 = malloc(4*K);
free(p1);
p1 = malloc(4*K); //fails!
[3K free] p2 4K
[3K free]

22. Dynamic Memory Define series of partition pools
Geometric series of sizes
e.g. 16, 32, 64, 128, 256 bytes
Write malloc() to select best fit
can be deterministic

23. Dynamic Memory malloc(16); 16 byte pool malloc(9); 32 byte pool

24. Dynamic Memory In C++ new maps onto malloc()
new is also used when an object is created
Can create a pool for each class with blocks of exactly the right size
Overload new to use this pool
Or create a base class that does the work and derive all new classes from that

25. Agenda
Monday: C++ for Embedded Programming Tuesday: C to C++ Migration Strategy Wednesday: C++ and a Real Time Operating System Thursday: Case study #1 Friday: Case Study #2

26. [ 3 ]
What aspect of embedded systems programming is most challenging?

27. Question review ...

28. [ Review - 2 ]
Is dynamic memory an issue/problem for a real time system?

29. Colin Walls