1. Topics Covered What is Real Time Operating System (RTOS)
Standard for RTOS
VxWorks Features as RTOS
Layer Structure
Microkernal Functions:
Multitasking
Scheduling
Interrupt Service Code
Shared Code and Reentrancy
Intertask Communications
Applications for VxWorks

2. What is Real Time OS (RTOS)
RTOS Characteristics
Processing must be done within a time constraint or the system will fail.
All delays in the system will be bounded; from the retrieval of stored data to the time RTOS finishes the request.
Types of RTOS
Hard RTOS
Soft RTOS

3. Standard for RTOS Standard requirement for RTOS
POSIX b by ISO/IEEE chapter.
POSIX interfaces:
Queued signals
Scheduling
Clocks and timers
Asynchronous I/O
Semaphore
Message queues
Memory management

4. VxWorks OS as RTOS

5. Real-Time Embedded Application
Layer Structure
Real-Time Embedded Application
Internet
Graphics
Multiprocessing
Java Support
POSIX Library
File System
WindNet Networking
Core OS:
Wind Microkernal

6. Multitasking P R D S Ready – wait for CPU,
Suspended – used primarily for debugging
Ready – wait for CPU,
a single primitive=create+activate
Pended – blocked, wait t time for recourses. After t - Error.
Delay – asleep for t time, after t goes to ready state.
On a context switch, a task’s context is saved in TCB. All code executes in a single common space. Memory isn’t saved.

7. Scheduling, One CPU Priority-based preemptive scheduling
Each task has priority (from 0 - highest to lowest). Each priority has its own queue.
If two tasks are in the ready state, a lower priority task would be pre-empted by one of a higher priority.
Round-robin scheduling
After time slice for a task expires, another task with the same priority will execute during the given time slice.
Preemption locks
These locks prevent task context switching, but do not prevent interrupt handling.

8. Interrupt Service Code
Via interrupts, the system is informed of external events.
Interrupt Service Routines (ISRs) run outside any task context.
Interrupt handling involves no task context switch.
Limitations of ISR:
All ISRs share the same stack
ISR has no context that can be suspended
Cannot take the semaphore, but can give the semaphores, releasing any task waiting on them.
Cannot perform I/O through drivers except pipe.

9. Shared Code and Reentrancy
Shared Code - a single copy of code executed by multiple tasks.
Shared Code must be reentrant. A subroutine is reentrant if a single copy of the routine can be called simultaneously from several task contexts without conflict.
Reentrancy Techniques:
Dynamic Stack Variables
Global and Static Variables Guarded by Semaphores
Task Variables

10. Shared Code and Reentrancy, Continued
Dynamic Stack Variables
Task Stacks
Common Function
Tasks
Task1( ){ …..
comFun(myData1) }
myData1
comFun(yourData){ }
Task2( ){ …..
comFun(myData2) }
myData2

11. Shard Code and Reentrancy, Continued
Guarded Global and Static Variables
Mutual-exclusion mechanism - only one task at a time can be in a critical section of the code.
Task Variables
Four byte variables are added to the task context block (TCB).

12. Intertask Communications, One CPU
Types of Intertask Communications:
Shared Data
Message Queue
Pipes - Virtual I/0
Network Intertask Communication
Sockets
Remote Procedure Call

13. Intertask Communication, Shared Data
Tasks
Memory
Access SharData 1.
Access SharData
SharData 2.
Access SharData 3.

14. Semaphores
Semaphore - mechanism for mutual exclusion and intertask communication.
Binary Semaphore
Semaphore
available
TimeOut=
No_Wait NO
Task is pended for timeOut NO NO
YES
YES
Task cont.
Semaphore taken
Task cont.
Semaphore not taken

15. Special Semaphore Option (Not in POSIX)
Semaphores, Continued
Counting Semaphore
Every time is semaphore is given count ++
Every time is semaphore is taken count --
If count=0, task that tried to take semaphore is blocked
If semaphore is given and task is blocked, task is unblocked
Is semaphore is given and no tasks are blocked, count ++
Special Semaphore Option (Not in POSIX)
Timeout
Time the task is willing to wait (time in ticks) in pended state. If time > timeout, ERROR occurs.

16. Intertask Communication, Message Queue
Allows a variable number of messages to be queued
Any task or ISR can send a message to a queue.
Multiple tasks can send and receive from the same message queue.
Timeout Parameter:
Send message: how long to wait for buffer space to place message.
Receive message: how long to wait to obtain message.

17. Applications for VxWorks
Some usage of RTOS
Flight simulators
Radio and optical telescopes
Navigation systems
Deep sea instrumentation
Traffic control systems
Modems
Printers
Digital cameras
Hand-held computing devices
Routers, switches, and other network devices
… any systems where rigid time requirement have been placed on the operation of a processor or the flow of the data.

18. Thank You
Any Questions ?

19. Bibliography
VxWorks 5.4 Programmer Guide