EMBEDDED SYSTEMS.pptx

7/28/2019 EMBEDDED SYSTEMS.pptx

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EMBEDDED SYSTEMS

UNIT IV

by

Venkatesan NLect. | CSE | KITE


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Introduction

What is RTOS?

A Real-Time Operating System (RTOS) is a

computing environment that reacts to input

within a specific time period

Purpose?

To handle the Deadline Time Constraints


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Process, Tasks, Thread

A process is an instance of a computer

program that is being executed

Ex:

-This can be anything from a small background

task, such as a spell-checker or system events

handler to a full-blown application like

Internet Explorer or Microsoft Word.


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TASK

a task is a basic unit of programming that an

operating system control

Depending on how the operating system defines a

task in its design, this unit of programming may be

an entire program or each successive invocation of

a program

THREADS

A thread is a single sequence stream within in a

process

they are sometimes called lightweight processes


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Processes Vs Threads Similarities

Like processes threads share CPU and only one thread

active (running) at a time. Like processes, threads within a processes, threads within

a processes execute sequentially.

Like processes, thread can create children.

And like process, if one thread is blocked, another thread

can run Differences

Unlike processes, threads are not independent of oneanother.

Unlike processes, all threads can access every address in

the task . Unlike processes, thread are design to assist one other.

Note that processes might or might not assist one anotherbecause processes may originate from different users.


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Process

Thread

Threads and Processes

Process

Code

DataChild Processes

Resource List

Critical

region

int main(){

int input;

cout input;

cout


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ISRs in RTOS ISRs have the higher priorities over the RTOS

functions and the tasks.

An ISR should not wait for a semaphore,

mailbox, message or queue message

An ISR should not also wait for mutex else ithas to wait for other critical section code to

finish before the critical codes in the ISR can

run.


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Three alternative ways systems to respond

to hardware source calls from the interrupts

- Direct Call to an ISR by interrupting

- Event Message (IPC) from the ISEvent ISR

Direct Call to an ISR and ISR Enter Direct Enter

message


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Operating System Services Goals

Facilitating easy sharing of resources as perschedule and allocations

Facilitating easy implementation

Optimally scheduling the process Maximizing the system performance to let

difference processes

Providing management functions

Management and organization functions for files

and files like devices


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Providing easy interfacing and management

functions for the network protocols

Providing portability for hardwareconfigurations

Providing interoperability of applications on

different networks Providing a common set of interfaces that

integrates various applications and open

systems


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Structure of RTOS

Top-down Layers Application software

Application Program Interface

System software other than the one providedat the OS

Hardware-OS Interface Hardware


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Generally OS runs in two modes

USER Mode(The user process is permitted to run

and use only a subset of functions)

Supervisory Mode(The OS runs the privileged

functions and instructions in a protected mode)


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Kernel

The OS includes certain structural units.

Kernel is also one of the structural unit

It is the secured unit of an OS

It operates under the supervisory mode

But the application software operates in the user

mode


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Kernel Services in OS

Process Management Process Creation

Management of the created process(creation,activation, running, blocking, resumption,

deactivation, deletion) Memory Management

Memory Allocation

Memory Management after initial allocation

Managing Strategy(Fixed blocks, Dynamic Blocks,Dynamic page, Data Memory Allocation, Addressrelocation, Multiprocessor Memory Allocation)


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Device Management

It manages physical as well as virtual devices likepipes and sockets

Device mgmt. has three approaches to 3 types ofdrivers Programmed I/Os by polling

Interrupts from the device drivers ISR

DMA operation

It has certain functions like Device detection,addtion, deletion, allocation, registration,detaching, deregistration, queue,etc


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OS command functions for a device

Create and open

Write Read

Ioctl(Configuring the device to operate at

particular baud rate) Close and delete


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File System organisation and

implementation

A file is a named entity on a magnetic disc, opticaldisk or system memory

It may be a named entity or a structured recordor an unstructured record of bits

It uses set of command functions in POSIX filesystem Open

Write

Read

Lseek

close


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I/O Subsystems

They are the subsystems of OS device

management systems

It is used for the drivers to communicate with

many devices which is present in the system


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Interrupt Routines Handling in RTOS

There are three ways to call the services on

interrupting

Direct call to ISR By an Interrupting Source

Direct call to RTOS by an Interrupting Source and

Temporary Suspension of a Scheduled Task

Direct call to RTOS by an Interrupting Source and

Scheduling of Tasks as well as ISRs by the RTOS


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RTOS Task Scheduling Models

Control Flow Strategy

Data Flow Strategy

Control -Data Flow Strategy


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Scheduling Models used by RTOS

Schedulers

Cyclic cooperative scheduling of ready tasks in acircular queue.

Cooperative scheduling with precedence constraints

Cyclic cooperative scheduling with time slicing

Preemptive scheduling Fixed times scheduling

Scheduling of Periodic, Sporadic and aperiodic tasks

Dynamic Real Time Scheduling using Earliest Deadline

First(EDF) precedence Advanced scheduling algorithms using the probabilistic

timed petri nets or multi thread graphs.

Th C ti R d R bi


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The Cooperative Round Robin

Scheduling Using a circular queue of

Ready Tasks RTOS inserts a list of ready tasks for sequentialexecution

Program counter changes whenever the CPU

starts executing another process

Worst case latency is same for each task

It is calculated by summing up detection time,switching time, execution time

Cooperative scheduling with


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Cooperative scheduling with

precedence constraints

In the first layer ISR has set of short codes thathave to be executed immediately

Its sends a flag or token and its priority

parameter for the task to be initiated


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Cyclic cooperative scheduling with time slicing(Rate

Monotonic Cooperative Scheduling)

The RTOS defines a time slice for each task to

finish

If the process is not executed within the time

then it is executed in the next cycle


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Preemptive scheduling

In the cooperative scheduling the low priority

job is executed for long time and thus making

a high priority job to wait for execution

Preemptive scheduling lets a pending high-

priority job take resources away from a

running job of lower priority.


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Critical section service by a preemptive

scheduler

The blocking of a lower priority process for

executing high priority process should not be

done in critical situations

It creates tasks and


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EMBEDDED SYSTEMS.pptx