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Real-Time Operating Systems

Suzanne Rivoire

November 20, 2002

http://www.stanford.edu/~skrufi/rtospres.ppt

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Motivating Example

void main() {

do forever{

check keypad;measure temperature;

control oven power;

decrement timer;

update display;wait for clock tick;

}

}

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Motivating Example - 2

void main() {

do forever{

check keypad;measure temperature;

check keypad;

control oven;

check keypad;}

}

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What isreal-time?

Correctness of output depends on timing as

well as result

Hard vs. soft real-time

Are Windows and Linux real-time?

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In a Hard RTOS

Thread priorities can be set by the client

Threads always run according to priority

Kernel must be preemptible or bounded

Interrupts must be bounded

No virtual memory

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In a Soft RTOS

Like a hard RTOS: Priority scheduling, with no degradation

Low dispatch latency Preemptible system calls

No virtual memory (or allow pages to be locked)

Linux: guarantees about relative timing of

tasks, no guarantees about syscalls

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Basic RTOS References

http://www.dedicated-

systems.com/encyc/publications/faq/rtfaq.htm

http://www.steroidmicros.com/mtkernel.html

http://www.qnx.com/developer/articles/dec1200b/

Silberschatz and Galvin, Operating System

Concepts.

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Example RTOSs

Micrium c-OS II

http://www.ucos-ii.com/

AvrXhttp://www.barello.net/avrx/

RTLinuxhttp://fsmlabs.com/developers/man_pages/

QNX Neutrino

http://www.qnx.com/

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c-OS II

Features

Sample main()function

Calling OSTaskCreate()

Creating a task

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C-OS II

Ports to the AVR, ATmega103

Comes with book:Micro-C OS: The Real-Time

Kernelby Jean Labrosse ($52)

Features

Semaphores and mutexes

Event flags

Message mailboxes and queues

Task management (priority settings)

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void main (void) {

/* Perform Initializations */

...OSInit();

...

/* Create at least one task by

calling OSTaskCreate() */

OSStart();

}

C-OS Sample Code

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INT8U OSTaskCreate (

void (*task)(void *pd),

void *pdata,

OS_STK *ptos,INT8U prio);

C-OS Sample Code

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void UserTask (void *pdata) {

pdata = pdata;

/* User task initialization */

while (1) {/* User code goes here */

/* You MUST invoke a serviceprovided by C/OS-II to: */

/* ... a) Delay the task for nticks */

/* ... b) Wait on a semaphore */

/* ... c) Wait for a message from atask or an ISR */

/* ... d) Suspend execution of this

C-OS Sample Code

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AvrX

Specs

Internal structures

Sample code: task creation

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AvrX Specs

Code size: 500-700 words (2x bigger with

debug monitor)

16 priority levels

Overhead: 20% of CPU

Version 2.3 for assembly code, 2.6 for C

interface

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AvrX Internals

Routine _Prologsaves state, _Epilog

restores it

Task info stored in a PID block and a task

control block

Modules for timers, semaphores, etc.

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An AvrX Task

AVRX_TASKDEF(myTask, 10, 3){TimerControlBlock MyTimer;while (1){

AvrXDelay(&MyTimer, 10);// 10ms delay

AvrXSetSemaphore(&Timeout)

;}}

Arguments: task name, additional stack bytes, priority

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RTLinux

Additional layer between Linux kernel and

hardware

Worst-case dispatch latency on x86: 15 s

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RTLinux: Basic Idea

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QNX Neutrino

Powerful hard RTOS

Includes GUI

Memory protection and fault tolerance

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Summary

Hard real-time, soft real-time

Characteristics of an RTOS

Example RTOSs

Specs

Internal structure/ideas

Sample code