1. Real Time Operating Systems for Networked Embedded Systems

2. Real Time System
A system is said to be Real Time if it is required to complete it’s work & deliver it’s services on time.
It responds within stipulated time constraint
It provides a software abstraction layer for MC and CPU.

3. Hard and Soft Real Time Systems
Hard Real Time System
Critical task are completed on time.
Failure to meet deadlines is fatal
Secondary storage is limited
Air traffic control
Soft Real Time System
Critical real time task gets priority over other task.
System performance degrades as more & more jobs miss deadlines
Online Databases

4. Hard and Soft Real Time Systems (Operational Definition)
Hard Real Time System
Validation by provably correct procedures or extensive simulation that the system always meets the timings constraints
Soft Real Time System
Demonstration of jobs meeting some statistical constraints suffices.
Example – Multimedia System
25 frames per second on an average

5. Responsibilities of RTOS
Task management and scheduling
Interrupt servicing
IPC and synchronization
Memory management

6. Benefits of RTOS’s Simplify the development and improve productivity
Present an abstraction to processor.
Implements reliable scheduling
Integrates and manage resources.
Optimizes the resources.
Brings all elements together in a single platform.

7. Characteristics of RTOS
Deterministic
Operations are performed at fixed
Long os delay before ack an interrupt
Responsiveness
User control
Reliability
Fail soft operation

8. OS services Initial loading of programs Process management function
Memory management function
Device management function
File management function
Secondary storage management function
Network management function
Protection and security management function
Command interpretation.

9. Process management It is a program in a state of execution
It handles allocation of resources.
Various states during execution of the process
New: process is created
Ready: process is ready to be executed on processor.
Running: processor executes the process.
Waiting: processor waits for some event to happen
Terminated: process has finished the execution.

10. Process management (continued)
Multiple processes share the hardware at same time.
Process manager implements CPU sharing, process synchronization and deadlock strategy.
Process manager carries out several services such as creating, suspending, executing, terminating and destroying the processes.

11. Functions of Process manager
CPU sharing.
Resource allocation
Implements process synchronization and IPC.
Implements deadlock strategies and protection mechanism.

12. Creation of process
Process manager allocates the process control block when it creates the process and later manages it.
PCB is a process descriptors used by process manager
Identifier.
Process state
Priority.
Program counter
CPU registers
CPU scheduling information (e.g., priority, scheduling queue information)
Memory management information (e.g., base and limit registers)
Accounting information (e.g., time)
I/O status information

13. Process termination
A process terminates when it finishes executing its last statement.
Its resources are returned to the system, it is purged from any system lists or tables, and its process control block (PCB) is erased
i.e., the PCB's memory space is returned to a free memory pool

14. Process termination
The new process terminates the existing process, usually due to following reasons:
Normal Exist :Most processes terminates because they have done their job. This call is exist in UNIX.
Error Exist :When process discovers a fatal error. For example, a user tries to compile a program that does not exist.
Fatal Error: An error caused by process due to a bug in program for example, executing an illegal instruction.
Killed by another Process: A process executes a system call telling the Operating Systems to terminate some other process. In UNIX, this call is kill.
Cascading termination: if parent process is terminated, child processes are automatically terminated.

15. Timer functions
Timer function is a simple service that delays a task for a period of time.
Timer function: TaskDelay()- It creates a period of time of delay in its argument.
Timer functions must be accurate to the nearest system tick (period)

16. Timer functions in RTOS.
OS_TICK_PER_SEC─ to set the system clock ticks and hence SysClkIntr interrupts per second.
OSTickInit ( )─ to initiate system clock ticks.
OSTimeDelay ( )─ to delay the process making call by a fixed number of system clock ticks specified in argument.
OSTimeDelay-Resume ( )─ to resume a delayed process specified in the argument
OSTimeSet ( )─ to set the counts of system clock ticks
OSTimeGet ( ) ─ to read the counts of system clock ticks to find the time interval from the previous read or write of the counts.
OSSemPend (semVal, twait, *semErr)─ to wait for a semaphore release
OSMboxPend (semVal, twait, *mboxErr) ─ to wait for a message in mailbox (wait for message pointed not Null)

17. Event function Event is an boolean flag that tasks can set or reset
Features of event
More than on task can block waiting for an event and RTOS will unblock all of them.
RTOS woks on group of events and task can wait for any subset of event within group.
OS event register flag bits
Flag=1 occurrence of an event in an event register.
Flag=2 an event in a group has not occurred.

18. Event functions in RTOS.
OSEventQuery ( )─ to query an event register to find the event register existence and its contents.
OSEventDelete ( ) ─ delete the event register
SET (event_flag) to set one of the bits at the event register
CLEAR (eventFlag) clears the flag in the event register. An event flag can be cleared by an ISR or Task
WAIT_ALL function─ task waits for the occurrences of setting all the event flags in a group. [Wait till AND operation between all flags in the group equals true.]
WAIT_ANY function─ task waits for the occurrences of setting at least one event flag setting in a group. [Wait till OR operation between all flags in the group equals true.]

19. Memory management Allocation of portions of memory to programs
Main parts of main memory
Resident monitor: area of memory accessed by user process and OS resides
User area: main memory space which is allocated for user process.

20. Memory allocation Contiguous Memory Allocation.
Single partition allocation
Multiple partition allocation
Static or fixed partition
Dynamic or variable partition.
Non contiguous Memory Allocation.
Paging
Basic paging
Demand paging
segmentation

21. Device management
Device management: the OS component that manages hardware devices
Provides a uniform interface to access devices with different physical characteristics
Optimize the performance of individual devices
I/O device hardware includes buses, port and device controllers
Device controller
Conversion a block of bytes and I/O operations
Performs error correction if necessary
Expose hardware registers to control the device
Typically have four registers:
Data-in register to be read to get input
Data-out register to be written to send output
Status register (allows the status of the device to be checked)
Control register (controls the command the device performs)

22. Device manager
It provides the appropriate level abstraction to handle system devices.
Functions of device manager
Achieve devices independent.
Handle error
Speed up transfer of data
Handle deadlock
Enable multi user system to use dedicated devices.
Techniques for device management
Dedicated devices
Shared devices
Virtual devices

23. File management Requirement of file system Store the info in long term
Data sharable among processes.
Huge size data accommodated in appropriate storage.
Files are structured, named, accessed and protected by OS
File management system objectives
Data management need and requirement by user
I/O support by storage devices
Standardized set of I/O interface routine
Minimize the potential for lost or destroyed data

24. File manager Keeps track of every file in the system Need for file
Long term existence
Shareable between processes
structure
Functions of File manager
Enforces the restriction on who access to which file
Controls the user are allowed to access the file
Allocates resources by OS
Controls the naming of files
Responsible for achieving and backups

25. I/O subsystem
Combination of I/O device, device driver and I/O subsystem
Purpose of I/O system
Hide the device specific info from kernal
Provide uniform access

26. Basic design of RTOS
An embedded system with a single CPU can run only one process at an instance.
The process at any instance may either be an ISR, or kernel function or task

27. Other RTOS’s (contd) VxWorks pSOS Monolithic Architecture
Real Time Posix compliant
Cross development System
pSOS
Object Oriented OS

28. RTOS facilities
Provides running the user threads in kernel space so that they execute fast.
Provides effective handling of the ISRs, device drivers, ISTs, tasks or threads
Disabling and enabling of interrupts in user mode critical section code
Provides memory allocation and deallocation functions in fixed time and blocks of memory.
Provides for effectively scheduling and running and blocking of the tasks in cases of number of many tasks
I/O Management with devices, files, mailboxes, pipes and sockets becomes simple using an RTOS
Effective management of the multiple states of the CPU and, internal and external physical or virtual devices

29. Design principles of RTOS
Design with the ISRs and Tasks.
Provides of nesting of ISRs, while tasks run concurrently
A task─ wait and take the messages (IPCs) and post (send) the messages using the system calls
Each ISR design consisting of shorter code
A design principle is that the ISR code should be optimally short and the detailed computations be given to an IST or task by posting a message or parameters for that.
Design with using Interrupt Service Threads or Interrupt Service tasks
In certain RTOSes, for servicing the interrupts, there are two levels, fast level ISRs and slow level ISTs, the priorities are first for the ISRs, then for the ISTs and then the task
Design with using Interrupt Service Threads.
Each task has a while loop which never terminates.
A task waits for an IPC or signal to start.
The task, which gets the signal or takes the IPC for which it is waiting, runs from the point where it was blocked or preempted.

30. Design principles of RTOS
Design Each Task with an infinite loop.
Each task has a while loop which never terminates.
A task waits for an IPC or signal to start.
Design in the form of tasks for the Better and Predictable Response Time Control.
provide the control over the response time of the different tasks.
The different tasks are assigned different priorities and those tasks which system needs to execute with faster response are separated out.
Design in the form of tasks Modular Design.
System of multiple tasks makes the design modular.
The tasks provide modular design
Design in the form of tasks for Data Encapsulation.
System of multiple tasks encapsulates the code and data of one task from the other by use of global variables in critical sections getting exclusive access by mutex

31. Design principles of RTOS
Design with taking care of the time spent in the system calls.
expected time in general depends on the specific target processor of the embedded system and the memory access times.
Some IPCs takes longer than the other
Design with Limited number of tasks
Limit the number of tasks and select the appropriate number of tasks to increase the response time to the tasks, better control over shared resource and reduced memory requirement for stacks
Use appropriate precedence assignment strategy and Use Preemption.
Use appropriate precedence assignment strategy and Use Preemption in place of Time Slicing
Avoid Task Deletion
Create tasks at start-up only and avoid creating and then deleting tasks later at the later times.

32. Design principles of RTOS
Use CPU idle CPU time for internal functions
Read the internal queue.
Manage the memory.
Search for a free block of memory.
Design with Memory Allocation and DeAllocation By the Task.
If memory allocation and de-allocation are done by the task then the number of functions required as the RTOS functions is reduced.
This reduces the interrupt-latency periods. As execution of these functions takes significant time by the RTOS whenever the RTOS preempts a task.
Design with taking care of the Shared Resource or Data among the Tasks.
The ISR coding should be as like a reentrant function or should take care of problem from the shared resources or data such as buffer or global variables
Design with limited RTOS functions
Use an RTOS, which can be configured. RTOS provides during execution of the codes enabling the limited RTOS functions. For example, if queue and pipe functions are not used during execution, then disable these during run

33. RTOS programming

34. Types of RTOS In-House developed RTOS Broad based commercial RTOS
General purpose OS with RTOS
Special focus RTOS.

35. RTOS µCOS-II
MicroC/OS-II (commonly termed as µC/OS-II or uC/OS-II), is the acronym for Micro-Controller Operating Systems Version 2
It is a priority-based real-time multitasking operating system kernel for microprocessors, written mainly in the C programming language.
It is a very small real-time kernel.
Memory footprint is about 20KB for a fully functional kernel.
Source code is written mostly in ANSI C.
Highly portable, ROMable, very scalable, preemptive real-time, deterministic, multitasking kernel.
It can manage up to 250 application tasks (56 user tasks available).

36. Features of µCOS-II Source code Multitasking Portability Scalability
Services
Automotive add-on

37. system level functions
Initiating the os before starting the use of the RTOS functions.
Starting the use of RTOS multi tasking functions and running the task.
Starting the use of RTOS system clock.
Sending msg to RTOS Kernel for take control At the start of an ISR.
Sending msg to RTOS Kernel for quitting the control at the return from an ISR.
Sending msg to RTOS Kernel for take control At the start of a critical section.
Sending msg to RTOS Kernel for quitting the control at the return from a critical section.
Locking & Unlocking OS scheduler

38. Task Service & Time Functions
Service functions means the functions to create task, Suspend and resume and time setting and time retrieving functions.
Time functions set Time & Get Time in terms of the no.of system clock ticks.
1. The functions OS task create () returns the following
OS_NO_ERR
OS_PRIO_EXIST
OS_PRIO_INVALID
OS_NO_MORE_TCB
2. suspending a task
OS_TASK_SUSPEND_PRIO
OS_TASK_SUSPEND_IDLE
3. resuming a task
4. setting time in system clock
5. getting time of system clock

39. Time delay functions
Delaying by defining a time delay by no.of clock ticks function. Function void Os time dly delays tasks by tick of system clock it returns no parameter.
Resuming a delay task by Os time dly. function unsigned byte Os time dly resume a previously delayed task whether delay parameters was interms of the delay counticks or hours, minutes, seconds.
Delaying by defining a time delay in units of hours, minutes, seconds, milli seconds function void Os time dly HMSM delays up to ticks.

40. Memory Allocation –Related Functions
Mem0ry functions are required to allocate fixed size memory Blocks from the memory partition having integer no.of blocks the allocation takes Place with out fragmentation the allocations & deallocations takes places in fixed & Deterministic.
Creating a memory blocks at memory address. Function OSMem *OSMem create is an Os function which partitions the memory from an address with partitions in the blocks.
Getting a memory blocks at memory address.Function void *OSMem get.
Querying a memory block.Function unsigned byte OSMemquery.
Putting a memory block into a partition.function unsigned byte OSMemput.

41. Semaphore – Related functions
Creating a semaphore for the IPCs. Function Os_event Os sem create.
Waiting for an IPC for semaphore release. Function void Os sempend.
Check for availability of an IPC after a semaphore release.
Sending an IPC after a semaphore release function unsigned byte Os sempost.
Retrieve the error information for a semaphore.

42. Queue – related Functions
By using a queue we can communicate an array of msg pointers From the task. The msg pointers can be posted into queue by the task either at the
Back as in a queue or at the front as in a stack
Creating a queue for An IPC.
Waiting for an IPC msg at a queue.
Emptying the queue & eliminating all the msg pointers.
Sending a msg pointer to the queue.
Sending a msg pointer & inserting it at the queue front.
Querying to find the msg & error information for the queue ECB

43. Mail Box – Related Functions
Creating a mail box for an IPC.
Check for availability of an IPC after a msg at mail box.
Wating for availability of an IPC for a msg at mail box.
Send a msg for an IPC through mail box.
Finding mail box data and retrieving the error information for mail box.

46. Peripheral devices and protocols
Interfacing
Serial/parallel ports, USB, I2C, PCMCIA, IDE
Communication
Serial, Ethernet, Low bandwidth radio, IrDA,
802.11b based devices
User Interface
LCD, Keyboard, Touch sensors, Sound, Digital
pads, Webcams
Sensors
A variety of sensors using fire, temperature,
pressure, water level, seismic, sound, vision