ESE-RTOS-2004

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3 Unit Course, Winter 2004

CS Department, Univ. of Salzburg

Chapter 1: RTOS Concepts

Embedded Software Engineering

www.cs.uni-salzburg.at/~ck/teaching/ESE-Winter-2004

Christoph Kirsch

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Environment

Software

Embedded Programming

The Art of Embedded Programming

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Environment

Software

Software Processes

Environment Processes

What Do We Really Need From an RTOS?

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Environment

Software

Environment Communication Services

Sensing

Actuating

Memory

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Environment

Software

Environment Trigger Services

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Software

Software Communication Services

Sending

Receiving

Software

Memory

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Software

Software Trigger Services

Software

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Software

Software Scheduling Services

Software

Real-Time Scheduler

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Summary: RTOS Services

Device DriversSensing/Actuating

SchedulerSoftware Scheduling

SignalsSoftware Triggering

Shared VariablesSoftware Communication

Interrupt HandlersEnvironment Triggering

ImplementationService

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Software

The Illusion of Concurrent Software

Software

t1 t2 t1 t3 t2 t1

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Abstractions for Multiprogramming

Process

Thread

Task

Subroutine/Coroutine

Protected Behavioral Function

Behavioral Function

Triggered Function Subroutine

Coroutine

Coroutine/MMU

Function Stack/List

Programming Abstraction Runtime Overhead

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Environment Ports

Task Ports

reads

reads actuates

updates

Driver PortsReal-Time

Operating System

reads

writes

Memory Model

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Definition: Task

A task is afunction from its input and state ports to

its output and state ports

A taskruns to completion (cannot be killed)

A task ispreemptable

A task does not usesignals (except at completion)

A task does not usesemaphores (as a consequence)

API (used by the RTOS): initialize {task: state ports}

release {task}

dispatch {task: function}

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So, whats the difference between a task and a function?

A task has an operational semantics:

A task is implemented by asubroutine and a trigger

A task is eitherenvironment- orsoftware-triggered

The completion of a task may trigger another task

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Task t2

Task t2 Preempts Task t1

Task t1

t1

t2

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Task t2

Who Triggers Task t2?

t1

t2

Environment

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Definition: Event and Signal

An event is a change of state in some environment ports

A signal is a change of state in some taskports

A synchronous signal is a change of state in some driverports

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Definition: Trigger

A trigger is apredicate on environment, task, driverports

A triggerawaits events and/or signals

A trigger is enabledif its predicate evaluates to true

Trigger evaluation is atomic (non-preemptable)

A trigger can be activatedby the RTOS

A trigger can be cancelledby the RTOS

A trigger can be enabledby an event or a signal

API (used by the RTOS): activate {trigger}

cancel {trigger}

evaluate {trigger: predicate}

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My First RTOS

react() {

tasks t: initialize(t);

triggers g: activate(g);

while (true) {

if trigger g: evaluate(g) == truethen released-tasks :=to-be-released-tasks t: release(t);

schedule();

}

}

schedule() {

released-tasks t: dispatch(t);

released-tasks := {};

}

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Environment

Software

Tasks

Events

schedule()

react()

RTOS Model: Reaction vs. Scheduling

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Reactorvs. Schedulervs. Processor(Kirsch in the Proceedings of EMSOFT 2002)

Tasks

Processor

SchedulerStrategy

ReactorEvents

Disabled

Code

Enabled

Code

Running

Code

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RTOS with Preemption

react() {



while (true) {

if trigger g: evaluate(g) == truethen released-tasks :=to-be-released-tasks t: release(t);

schedule_concurrently();

}

}

schedule_concurrently() {

released-tasks t: dispatch(t);

released-tasks := {};

}

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Corrected RTOS with Preemption

react() {



while (true) {

if trigger g: evaluate(g) == truethen released-tasks := released-tasks

to-be-released-tasks t: release(t);}}

schedule() {

while (true) { t := select(released-tasks);

dispatch(t);

released-tasks := released-tasks \ { t }; }}

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Environment

Software

Tasks

Events

schedule()

react()

Signals

RTOS Model with Signals

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Definition: Thread

A thread is a behavioral function (with a trace semantics)

A thread may be killed

A thread ispreemptable

A thread may usesignals

A thread may usesemaphores

API (used by the RTOS or threads):

initialize{thread: ports} release{thread}

dispatch{thread: function}

kill{thread}

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So, whats the difference between a thread and a task?

A thread is a collection of tasks:

A thread is implemented by a coroutine

A thread requires signals

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Task t2

Task t2 Kills Task t1

Task t1

t1

t2

Kill t1

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Signal API

A signal can be awaitedby a thread

A signal can be emittedby a thread

Signal emission is atomic (non-preemptable)

API (used by threads): wait{signal}

emit{signal}

Literature:

emit: send(signal)

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Definition: Semaphore

A semaphore consists of asignaland aport

A semaphore can be lockedby a thread

A semaphore can be releasedby a thread

Semaphore access is atomic (non-preemptable)

API (used by threads): lock{semaphore}

release{semaphore}

Literature:

lock: P(semaphore)

release: V(semaphore)

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Binary Semaphore (Signal)

lock(semaphore) {

if (semaphore.lock == true) then

wait(semaphore.signal);

semaphore.lock := true;}

release(semaphore) {

semaphore.lock := false;

emit(semaphore.signal);}

must be atomic

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Binary Semaphore (Busy Wait)

lock(semaphore) {

while (semaphore.lock == true) do {}

semaphore.lock := true;

}

release(semaphore) {

semaphore.lock := false;

}

each round

must be atomic

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The Embedded Machine

Environment

Software

Tasks

Events

schedule(): The Scheduler and Dispatcher

react(): The Embedded Machine

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Environment

Software

Human: Programming in terms of environment time

Compiler: Implementation in terms of platform time

Proposal

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Environment

Software

Programming in terms of environment time

yields platform-independent code

Portability

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Environment

Software

Programming in terms of environment time

yields deterministic code

Predictability

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Environment

Software

sense actuate

endstart

The Task Model

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Environment

Software

sense actuate

endstart

Preemptable

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Environment

Software

1 2

f( ) =1

but Atomic

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Environment

Software

1 2

The Driver Model

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Environment

Software

1 2

f( ) = 1

f( ) = 2

Non-preemptable, Synchronous

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Environment

Software

1 2

s

a

t1

Syntax

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Program

A Triggerg

g

g: c c

b:

c

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Environment

Software

1 2

s

a

t1call(s)

release(t)

future(g,b)

call(a)b:

2

a

1

s

An Embedded Machine Program

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Environment

Software

1 2

s

a

t1call(s)

release(t)

future(g,b)

call(a)b:

2

a

1

s

Synchronous vs. Scheduled Computation

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INACTIVE

Synchronous vs. Scheduled Computation

releases

Scheduled computation

User context

Synchronous computation

Kernel context

Trigger related interrupts disabled

gc:

READY RUN

gb:

c

e

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Environment

Software

1 2

Environment-triggered Code

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Environment

Software

1 2

Software-triggered Code

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Triggerg: Input-, Environment-Triggered

gcall(s)

release(t)

future(g,b)

call(a)b:

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Environment

Software

1 2

Time Safety

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Environment

Software

1

s

11 t 2a

ss

t 2a2

Input-determined If Time Safe

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Environment

Software

1

s

1

s

t 2

a

1

s

2

a

2

a

t

Environment-determined If Environment-triggered

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The Zrich Helicopter

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Navigation

Control

Sensor

Actuator10

5

Helicopter Control Software

a

s

i

g

cClock

g: c = c + 5

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sensor gps_typeGPS uses c_gps_device ;

actuator servo_typeServo := c_servo_inituses c_servo_device ;

output

ctr_typeCtrOutput := c_ctr_init ;nav_typeNavOutput := c_nav_init ;

driver sensing (GPS) output (gps_type gps){ c_gps_pre_processing ( GPS, gps ) }

task Navigation (gps_typegps) output (NavOutput){ c_matlab_navigation_code ( gps, NavOutput ) }

Giotto Syntax (Functionality)

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mode Flight ( )period10ms

{

actfreq 1 do Servo( actuating ) ;

taskfreq 1 doControl( input );

taskfreq 2 doNavigation ( sensing) ;

}

Giotto Syntax (Timing)

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a ia

s

Navigation Navigation

Control

i

s s

Block of synchronous

code(nonpreemptable)

Scheduled tasks

(preemptable)

Environment Timeline0ms 5ms 10ms

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a ia

s


Control

i

s s

E Code0ms 5ms 10ms

b1: call(a_ctuating)

call(s_ensing)

call(i_nput)release(Control[10])

release(Navigation[5])

future(g,b2)

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a ia

s


Control

i

s s

E Code0ms 5ms 10ms

b2: call(s_ensing)

release(Navigation[5])

future(g,

b1)

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a ia

s

i

s s

Platform Timeline: EDF0ms 5ms 10ms

2ms 2ms

5ms

Navigation

Control3ms

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Environment

Software

1 2

s

a

t1

Time Safety

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Environment

Software

1 2

s

a call(a_ctuating)

Runtime Exceptions I

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Environment

Software

1 2

s

a

call(s_ensing)

Runtime Exceptions II

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Environment

Software

1 2

s

a

release(t)

Runtime Exceptions III

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Environment

Software

1 2

s

a

release(t,e)

call(a)e:call(s)

release(t)

future(g,b)

call(a)b:

a

An Exception Handlere

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a ia

s

i

s s

0ms 5ms 10ms

2ms 4ms

2msNavigation

Control2ms

How to Loose Determinism: Task Synchronization

3ms

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Environment

Software

1 2

s

a

terminate(t)

How to Loose Determinism: Termination

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Environment

Software

1 2

s

a

s

1

Time Liveness: Infinite Traces

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Dynamic Linking

t

Functionality

CodeE Code

E Machine

d

call(s)

release(t)

future(g,b)

call(a)b:

g

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The Berkeley Helicopter

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t+10mst+10mst t t+5ms t+5ms t+7ms

HeliCtr

HeliNav

TDMA Slot

HeliNet

Platform Timeline: Time-triggered Communication

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t+10mst+10mst t t+5ms t+5ms t+7ms

HeliCtr

HeliNav

b2: call(s_ensing)release(Navigation[2])

release(Connection[(7,10)])

future(g,b1)

Code Generation for HeliNav

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Instructions

Triggering: future(g,b)b:

Scheduled

Task:

release(t)

tSynchronous

Driver:

call(d)

d

g: c c

g

Documents

ESE-RTOS-2004