Embedded Systems. Chapter 6. Real-Time Operating System

Transcription

1 Embedded Systems Chapter 6 Real-Time Operating System

2 6. Real-Time Operating System [8 Hrs.] 6.1 Operating System Basics 6.2 Task, Process, and Threads 6.3 Multiprocessing and Multitasking 6.4 Task Scheduling 6.5 Task Synchronization 6.6 Device Drivers

3 How the increasing need for time critical response for task/events is addressed in embedded applications? -Assign priority to task & execute the high priority task when the task is ready to execute. -Dynamically change the priorities of tasks if required on a need basis. - Schedule the execution of tasks based on the priorities. -Switch the execution of task when a task is waiting for an external event or a system resource including I/O device operation.

4 Operating System Basics - acts as a bridge between the user application/ tasks & the underlying system resources through a set of system functionalities and services. - Manages the system resources and makes them available to the user application/task on a need basis. - Primary functions are: - Make the system convenient to use - Organize & manage the system resources efficiently and correctly.

5 Operating System Basics contd The Kernel is: - core of operating system - responsible for managing the system resources and the communication among the hardware and other system services. - act as the abstraction layer between system resources and user applications. - contains a set of system libraries and services.

6 Operating System Basics contd

7 Operating System Basics contd Process Management: deals with managing the processes/tasks. Includes setting up the memory space for the process Loading the process s code into the memory space Allocating system resources Scheduling and managing the execution of the process Setting up and managing the process control Block (PCB) Inter process communication and synchronization Process termination/deletion

8 Operating System Basics contd Primary Memory Management: Refers to the volatile memory (RAM) where processes are loaded and variables and shared data associated with each process are stored. Memory Management Unit (MMU) of the kernel is responsible for Keeping track of which part of the memory area is currently used by which process Allocating and De-allocating memory space on a need basis (DMA)

9 Operating System Basics contd File System Management: responsible for The creation, deletion and alteration of files. Creation, deletion and alteration of directories Saving of files in the secondary storage memory Providing automatic allocation of file space based on the amount of free space available Providing a flexible naming convention for the files.

10 Operating System Basic contd I/O System (Device)Management loading and unloading of device drivers exchanging information and the system specific control signals to and from the device Secondary storage management Disk storage allocation Disk scheduling (time interval at which the disk is activated to backup data) Free disk space management

11 Operating System Basic contd Protection systems (deals with implementing the security policies to restrict the access to both user and system resources by different application or processes or users) Interrupt Handler (Kernel provides handler mechanism for all external/internal interrupts generated by the system)

12 Operating System Types contd General Purpose Operating System (GPOS) Real - Time Operating System (RTOS) Implies deterministic timing behavior Means the OS services consumes only known and expected amounts of time regardless the number of services. Implements policies and rules concerning time critical allocation of a system s resources Applications should run in which order and how much time need to be allocated for each application.

13 Operating System Types contd The Real-Time Kernel: is highly specialized and it contains only the minimal set of services required for a running the user application/tasks. Basic functions are Task/Process management Task/Process scheduling Task/Process synchronization Error/Exception handling Memory management Interrupt handling time management

14 Tasks Task are very simple to write: under most RTOSs a task is simply a subroutine. 1. Running the microprocessor is executing the instructions that make up this task. one microprocessor, and hence only one task that is in the running state at any given time. 2. Ready some other task is in the running state but that this task has things that it could do if the microprocessor becomes available. Any number of tasks can be in this state. 3. Blocked this task hasn't got anything to do right now, even if the microprocessor becomes available. Tasks get into this state because they are waiting for some external event. For example, a task that handles data coming in from a network will have nothing to do when there is no data. A task that responds to the user when he presses a button has nothing to do until the user presses the button. Any number of tasks can be in this state as well.

15 Tasks Blocked Ready Task States Running

16 Tasks Here are answers to some common questions about the scheduler and task states'. How does the scheduler know when a task has become blocked or unblocked? What happens if all the tasks are blocked? What if two tasks with the same priority are ready?

17 Tasks

18 Tasks Microprocessor Responds to a Button under an RTOS;

19 Tasks RTOS Initialization Code

20 Process: - is a program, or part of it execution. - an instance of a program in execution; multiple instances of the same program can execute simultaneously. - Requires various system resources like CPU for executing the process, memory for storing the code corresponding to the process and associated variables, I/O devices for information exchange. - is sequential in execution.

21 Process Structure: Process Stack Stack Pointer Working registers Status registers Program Counter (PC) Code memory corresponding to the Process

22 Process Life Cycle process changes its state from newly created to execution completed Created state a process is being created is referred. OS recognizes a process but no resources are allocated to the process. Ready State the state, where a process is incepted into the memory and awaiting the processor time for execution. Process states and state transition representation

23 Ready List queue maintained by the OS. Running State the state where in the source code instructions corresponding to the process is being executed. Blocked State/Wait state refers to a state where a running process is temporarily suspended from execution and does not have immediate access to resources.. Process states and state transition representation

24 Completed State a state where the process completes its execution State transition the transition of a process from one state to another Process Management deals with the creation of a process, setting up the memory space for the process, loading the process s code into the memory space, allocating system resources, setting up a Process Control Block (PCB) for the process and process termination / deletion. Process states and state transition representation

25 Process Management Deals with the creation of a process Setting up the memory space for the process Loading the process s code into the memory space Allocating system resources Setting up a Process Control Block (PCB) for the process termination / deletion

26 Stack Memory for process Threads: Is the primitive that can execute code Is a single sequential flow of control within a process Stack memory for thread 1 Stack memory for thread 2 Data memory for process Code memory for process Also known as light weight process Memory organization of a process and its associated Threads A process can have many threads of execution

27 Threads: contd. Different threads, which are part of a process, share the same address space; meaning they share the data memory, code memory and the heap memory area. Threads maintain their own thread status (CPU register values), Program Counter (PC) and stack.

28 Multithreading Application may complex and lengthy Various sub - operations like getting input from I/O devices connected to the processor Performing some internal calculations / operations Updating some I/O devices

29 Multithreading all the sub-functions of a task are executed in sequence (?) the CPU utilization may not be efficient Advantages of multiple threads to execute: Better memory utilization (same process share the address space of the same memory & reduces complexity of inter threads comm.) Speed up execution of the process (splitting into different threads, when one thread enters a wait state, the CPU can be utilized by the other threads of the process that do not require the event, which other thread is waiting, for processing) Efficient CPU utilization. CPU engaged all time.

30 Multithreading Thread Standards: deals with different standards available for thread creation and management; utilized by OS Thread Class libraries are: POSIX Threads (Portable Operating System Interface) Win 32 Threads Java Threads

31 Multithreading POSIX Threads (Portable Operating System Interface) POSIX.4 standard deals with the Real-Time extensions POSIX.4a standard deals with thread extensions Pthreads library defines the set of POSIX thread creation and management functions in C language

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39 Thread is a single unit of execution and is part of process Does not have its own data memory and heap memory. Shares these memory with other threads of the same process Cannot live independently; it lives within the process Can be multiple threads in a process; the first thread (main thread) calls the main function and occupies the start of stack memory of the process Are very inexpensive to create Context switching is inexpensive and fast If a thread expires, its stack is reclaimed by the process Process Is a program in execution & contains 1 or more threads Has its own code memory, data memory & stack memory Contains at least one thread Threads within a process share the code, data & heap memory. Each thread holds separate memory area for stack (shares the total stack memory of the process) Are very expensive to create. Involves many OS overhead Context switching is complex and involves lot of OS overhead & is comparatively slower If process dies, the resources allocated to it are reclaimed by OS & all the associated threads of the process also dies

40 Multiprocessing & Multitasking

41 Multiprocessing & Multitasking Context switching

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48 Data in an RTOS-Based Real-Time System

49 Semaphores

50 Priority Inversion

51 Telegraph Operation