Real-Time Systems,
COSC-4301-01,
Lecture 1
Stefan Andrei
7/31/2017
COSC-4301-01, Lecture 1
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Real-time systems
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COSC-5340-01 is a 3 credit points module
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Assignments: 20%
Research paper presentation: 30%
Written final exam: 50% (open book)
Module homepage:
http://galaxy.lamar.edu/~sandrei/cosc-4301-01
COSC-4301-01 is a 3 credit points module
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Assignments: 40%
Written final exam: 60% (open book)
7/31/2017
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Course etiquette
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Code of conduct
no copying
 for details, see webpage
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You are encouraged to attend to all lectures,
tutorials, and so on.
You are encouraged to ask questions.
You are encouraged to offer answers.
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The team
Associate Professor:
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Dr. Stefan Andrei, [email protected]
Office hours:
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8:30am-9:30am and 11:30am-12:30pm
please send an email to make an appointment
Lectures based of the book:
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Albert M.K. Cheng: Real-time systems. Scheduling, Analysis, and
Verification, Wiley-Interscience, 2002 (first printing), 2005 (second
printing), ISBN: 0-471-18406-3
Recommended books:
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John A. Stankovic, Marco Spuri, Krithi Ramamritham, Giorgio Buttazzo:
Deadline Scheduling for Real-Time Systems: EDF and Related
Algorithms, Springer Verlag, 1998
Herman Kopetz: Real-Time Systems. Design Principles for Distributed
Embedded Applications. Kluwer Academic Publishers, 1997
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Pre-requisites:
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It is recommended to be already familiar
with:
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Symbolic logic;
Automata and languages.
What is this course all about?
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Real-time scheduling algorithms and
schedulability analysis.
Formal analysis and verification of real-time
systems.
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Overview of this course
1.
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5.
Introduction in Real-Time Systems
Real-time scheduling and schedulability
analysis
Real-time logic, graph-theoretic analysis, and
MODECHART
Model checking of finite-state systems
Visual formalism, statecharts, and
STATEMATE
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Lecture structure
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Reminder of last lecture
Overview
Content (new notions + examples)
Summary
Reading suggestions
Coming up next
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Overview of this lecture
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Introduction in Real-Time Systems
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What is time?
What is a real-time system?
Simulation
Testing
Verification
Run-time monitoring
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What is time?
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Time is an essential concept in real-time
systems.
Keeping time using accurate clocks is
required to ensure correct operations.
Master source: Paris’s International Atomic
Time (TAI - Temps Atomique International) is
an average of several laboratory atomic
clocks in the world.
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What is time?
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The earth’s rotational rate slows by a few
miliseconds each day.
Universal Coordinated Time (UTC) is another
master time source that performs leap
corrections to the time provided by TAI,
making the time length of every natural solar
day constant.
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Clocks synchronization
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UTC time signals are sent from National Institute
of Standards and Technology (NIST) radio
station, in Fort Collins, Colorado, and other UTC
radio stations to specialized receivers.
Selected radios, receiver-clocks, some phoneanswering machines, etc, have the ability to
receive these UTC signals to maintain accurate
clocks.
Depending on the location of the receiver, the
clocks synchronization should take into account
the delay in receiving the UTC signal.
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Wall clock or absolute time
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For computers whose time is kept by quartzbased computer clocks, the clocks should be
periodically synchronized such that they
maintain a bounded drift relative to UTC.
In this course, when we want refer to wall
clock or absolute time, we refer to the
standard time provided by a bounded-drift
computer clock on UTC.
So, there is a mapping
clock: RealTime  StandardClockTime
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What is a real-time system?
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A system that should produce the result in a
certain time.
Example: Anti-lock braking controller:
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“When the driver of a car applies the brake, the
anti-lock braking controller analyzes the
environment in which the controller is embedded
(car speed, road surface, direction of travel) and
activates the brake with the appropriate frequency
within fractions of a second.”
Both the result (brake activation) and the time at
which the result is produced are important in
ensuring the safety of the car and passengers.
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Real-time (computer) systems
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Many years ago, there were only non-realtime computer systems (e.g., old cars braking
controller);
Recently, computer hardware and software
are embedded in real-time systems to monitor
and control their operations (these systems
are called real-time computer systems or
embedded systems).
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Examples: the computerized version of the cars
braking controller, avionics, multimedia tools, etc.
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Distinguishing embedded and realtime systems
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Real-time system: meets timing constraints
and respects the functional requirements.
Real-time systems
Embedded systems
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Embedded system: part of another system.
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Correctness of real-time systems
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Satisfaction of logical/functional constraints:
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The result (e.g., brake activation) should be
correct.
Satisfaction of timing constraints:
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The time at which the result is produced is also
very important (e.g., pressing the brakes should
instruct the computer system to stop the car in a
certain amount of time).
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A model of a real-time system
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X: space of sensor input
vectors
x(t) is the value of x at
time t
Y: space of decision
vectors
y(t) is the value of y at
X
time t
S: space of state vectors
T: set of timing
constraints
I: set of integrity
constraints
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A
Y
D
S
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D: decision map
SXSY
D(s,x)=(s’,y) means
that given s and x at
time t, then D
determines the next
decision y and system
state s’ at time t+1.
A: relation over X and
Y about the effect of a
control decision on the
external world.
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A model of a real-time system. Example
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Anti-lock braking controller:
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X = the vector consisting of (wheel sensors)
S = the vector consisting of (car speed, direction of
travel)
Y = the vector consisting of (frequency)
A = the environmental constraint as a relation over
Y and X
D = the decision map that based on the S  X
value, it gives a value from S  Y
T = the timing constraint that requests the braking
mechanism to pump the brakes at a higher
frequency within 100ms.
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Anti-lock braking controller. Comments
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The frequency (decision Y) depends on S
and X.
For instance, if the car is turning (direction
component) and the wheel sensor detects a
wet road, then the frequency is higher
(system pumps the brake more times/sec to
avoid skid).
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Analysis techniques
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Simulation
Testing
Verification
Run-time monitoring
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Simulation
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Means constructing a model of an existing system to
be studied or a system to be built and then executing
actions allowed in this model.
The model can be:
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a physical entity (e.g., scale clay model) or
a computer representation.
Comparison:
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A computer model is usually less costly than a physical
model.
A computer model can represent a non-computer entity (e.g.,
airplane, component) or a computer entity (computer system,
program).
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Simulation
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Both physical and computer models are called
simulators of the actual systems.
A simulator can carry out simulated executions
of the simulated system and can display the
outcomes of these executions.
Examples:
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A physical model of an airplane in a wind tunnel
shows the aerodynamics of the simulated plane;
A software model can simulate the behavior of an
automobile crashing into a concrete barrier, showing
the effects on the occupants.
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Simulation
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Is an inexpensive way to study the behavior
of the simulated system and to study different
ways to implement the actual system.
If we detect behavior or events that are
inconsistent with the specification and safety
assertion, we can revise the model and
actual system to be built.
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Simulation. Target system
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Different levels of details of the actual system
(called also target system) can be modeled
and its events simulated by a simulator.
Example: when designing and simulating the
cockpit of an aircraft, we can restrict attention
to that particular component by simulating
only the cockpit with inputs and outputs to the
other aircraft components, without simulating
the behavior of the entire aircraft.
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Simulation techniques
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Real-time-event simulation
Discrete-event simulation
Hybrid simulation
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Real-time-event simulation
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Example: a physical scale model of an
automobile performs its actions in real-time
and the observable events are recorded in
real-time (crash testing simulator needs
instruments of recording events in real-time).
When the physical model is an implemented
target, the simulation is called testing.
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Discrete-event simulation
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Uses a logical clock and is usually software-based.
The variety of systems that can be simulated by
software-based is not limited by the speed at which
the hardware executes the simulator since the
simulated actions and events do not occur in real-time.
Rather, they can take place according to the speed of
the simulator hardware and the instructions in the
simulator program.
Example: simulation of a network of computers in a
single-processor system.
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Hybrid simulation
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The simulator works with a partial
implementation of the target system by acting
as the non-implemented part.
The hybrid simulator makes it possible to
predict the performance and behavior of the
target system once it is completely
implemented.
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Simulation. Disadvantages
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Simulation is not able to model all possible
event-action sequences in the target system
where the domain of possible sequences of
observable events is infinite.
Even if the domain is finite, the number of
possible sequence of events is so large that
the most powerful computer resources or
physical instruments may not be able to trace
through all possible sequences of events in the
simulated target system.
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Testing
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Is a technique for detecting errors or problems
in implemented software, hardware, or noncomputer systems.
It consists of executing or operating the
system to be tested using a finite set of inputs
and then checking to see if the corresponding
outputs or behavior are correct with respect to
the specifications.
To test a real-time system, the values as well
as the timing of the inputs are important.
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Approaches for testing
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Exhaustive testing
Functional testing
Structural testing
Code reading
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Exhaustive testing
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Considering every possible input values to be
checked against the expected answers is not
practical for larger systems.
For systems with infinite number of possible
inputs, this approach is of course not viable.
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Functional testing
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Uses a “black-box” approach in which the
programmer creates test data from the program
specification using one or a combination of
techniques such as boundary value analysis and
equivalence partitioning.
The program is executed using these test data as
input, and the corresponding behavior and output
are compared with those described in the program
specification.
If the program behavior or output deviates from the
specification, the programmer attempts to identify
the erroneous part of the program and correct it.
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Structural testing
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Is a “white-box” technique in which the
programmer examines the source code of the
program and then creates test data for
program execution based on the percentage
of the program’s statements executed.
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Code reading
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Code reading by stepwise abstraction
requires the programmer to identify major
modules in the program, determine their
functions, and compose these functions to
determine a function for the whole program.
The programmer then compares this derived
function with the intended function as
described by the program specification.
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Verification
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Simulation and testing can reveal errors in
the simulated or actual system, but usually
cannot guarantee that the system satisfies a
set of requirements.
To apply formal verification techniques to a
real-time system, we must first specify the
system requirements and then the system to
be implemented using an unambiguous
specification language.
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Verification. Personnel
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Applications expert:
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He/she could be a programmer or a system designer;
Formal methods expert:
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A formal methods expert collaborates with the
applications expert to write the requirements and
system specification.
Once these specifications are written, the formal
methods expert can verify whether the system
specification satisfies the specified requirements
using his/her favorite formal verification methods and
tools.
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Specification of real-time systems
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Structurally and functionally specification (how
the real-time system components work as well
as their functions and operations):
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Mechanical components
Electrical components
Electronic components
Behavioral specification
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The result of each component in response to an
internal or external sequence of events.
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Example: real-time anti-lock braking
system in an automobile
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Structural-functional specification refers to:
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Braking system components and sensors
How they are interconnected, and
How the actions of each component affect the other
Example: how to connect the wheel sensors to the central
decision-making computer that controls the brake mechanism.
Behavioral specification refers to:
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Events and effects
Example: when the wheel sensors detect wet road conditions,
the decision-making computer will instruct the brake
mechanism to pump the brakes at a higher frequency within
100ms.
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Timing Constraints
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Behavioral specification without the
complexity of the structural specification often
suffices.
We restrict the specification language to
handle only timing relations.
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Specification and safety assertion
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An implementation of a real-time system is
built from the structural-functional
specification.
An implementation is correct (faithful) if
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The behavioral specification (denoted as SP)
implies safety assertion (denoted as SA).
In other words, we have to check whether SP 
SA is a theorem or not.
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Verification of timing properties
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In checking SP → SA, we may have the cases:
 (safe) SA is a theorem derivable from SP;
 (inherently unsafe) SA is unsatisfiable with
respect to SP;
 (safe if additional constraints are added) the
negation of SA is satisfiable under certain
conditions.
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Run-time monitoring
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Motivation:
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Despite simulation, testing, and verification, there
may be a system behavior that was not
anticipated.
This unexpected behavior may be because events
and actions were not modeled by the static
analysis tools or may be the result of making
simplified assumptions about the real-time
systems.
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Run-time monitoring (cont)
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So, there is a need to monitor the execution
of the real-time system and to make the
appropriate adjustments in response to a
monitored behavior that violates the specified
safety and progress constraints.
Even if the real-time system meets the
specified safety and progress constraints at
run-time, monitoring may provide information
that can improve the performance and
reliability of the monitored system.
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The monitoring system
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The monitored real-time system is the target
system and its components, such as
programs, are called target programs.
The monitoring system is the system used to
monitor and record the behavior of the target
system.
It consists of instrumentation programs,
instrumentation hardware, and other
monitoring modules.
It records the behavior of interest of the target
system and produces event traces.
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Monitoring techniques
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Intrusive monitoring
Non-intrusive monitoring
The availability of monitoring systems does not
mean that we can relax on the task of stringent
pre-run-time analysis and verification of the
target system.
Rather, monitoring should serve as an additional
guarantee on the safety and reliability of the
safety-critical target real-time systems.
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Intrusive monitoring
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Uses the resources of the target system to record its
behavior and thus may alter the actual behavior of
the target system.
Examples:
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(computer example) Insertion of print() statements in a
target program to display the values of the program
variables.
(computer example) Extra statements inserted in the
programs of computing nodes to record their states and the
exchanges of these state variables.
(non-computer example) Addition of speed and impactforce sensing instruments in an automobile to record its
performance.
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Non-intrusive monitoring
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Does not affect the timing and ordering
events of the monitored target system.
This is important in monitoring of real-time
systems where both timing and ordering of
events are critical to the safety of the realtime systems.
Example: use of additional processor(s) to
run monitoring programs used to record the
target system’s behavior in a real-time
environment.
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Summary
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Introduction in Real-Time Systems
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What is a real-time system?
What is time?
Simulation
Testing
Verification
Run-time monitoring
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Reading suggestions
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Chapter 1 of [Cheng; 2002]
Chapter 1 of [Kopetz; 1997]
Chapter 1 of [Stankovic, Spuri, Ramamritham,
Buttazzo; 1998]
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Coming up next
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Real-time scheduling and schedulability
analysis (Chapter 3 of [Cheng; 2002])
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Thank you for your attention!
Questions?
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