REAL TIME OPERATING SYSTEM PART 1

Operating System
• Operating system is the software which is used as interface between
user and hardware.
• It provide uniform access to hardware for the user
• Operating system goals:
– Execute user programs and
make solving user problems easier.
– Make the computer system
convenient to use.
– Use the computer hardware
in an. efficient manner
• Ex:- Windows, Linux, Solaris etc

Real time system
• Real-time systems are defined as those systems in which the correctness of
the system depends not only on the logical result of computation, but also on
the time at which the results are produced.
• In such a type of system result must be obtained within the limited time
constraints.
• If result is not obtained within limited time then result may be incorrect or no
meaning of that result.

Example of a Real Time System
Consider a real-time system comprised of three motors and three switches.
The switches have two positions, ON and OFF.
The switches must be scanned at about 10 times per second,
and the motors turned on or off as appropriate.
S1 S2 S3
M1 M2 M3

Example System
0 100 200 300 400 500 600 700 800 900 1000 msecs

void main(void)
{
int i;
while(1)
{
for (i= 0; i < 3; i++ )
{
if (switchChanged(i))
changeMotor(i);
}
}
} .
S1 S2 S3
M1 M2 M3

void main(void)
{
while(1)
{
if (OneTenthSecondIsUp)
{
for (i= 0; i < 3; i++ )
{
if (switchChanged(i))
changeMotor(i);
}
OneTenthSecondIsUp = 0;
}
} .
S1 S2 S3
M1 M2 M3

Adding one sensor
Let a pressure gage must be checked every 50 milliseconds
A valve opened if the pressure is greater than 100 psi.
Once opened, the valve must be closed after the pressure drops below 90 psi.
S1 S2 S3
M1 M2 M3

Example System
0 100 200 300 400 500 600 700 800 900 1000 msecs
50 150 250 350 450 550 650 750 850 950 1000 msecs

Example
if (FiftyMsIsUp)
{
switch (valveState)
{
case CLOSED:
if (pressure() > 100)
{
openValve();
valveState = OPEN;
}
break;
case OPEN:
if (pressure() < 90)
{
closeValve();
valveState = CLOSED;
}
}
FiftyMsIsUp = 0;
}
S1 S2 S3
M1 M2 M3

Let us add datagrams (data packets)
Assume that the system is connected to a network
and that incoming datagrams must be processed.
This could be handled by adding yet another function call to the loop.
checkDatagrams();
S1 S2 S3
M1 M2 M3
DATA

Let us handle datagrams (data packets)
if the function checkDataGrams is not called at a sufficient rate,
datagrams can be lost.
In order to avoid this,
a queue must be created so that when the interrupt service routine for incoming
datagrams is entered
, the datagram is placed into a queue for processing by the function
checkDatagrams. . S1 S2 S3
M1 M2 M3
DATA

Final code for our system…
Void main()
{
While(1)
{
checkMotorSwitches();
checkPressure();
checkDatagrams();
}
}
S1 S2 S3
M1 M2 M3
DATA

Final code for our system Drawback…
there are no priorities
Busy waiting should not there
and more responsibility (e.g. timer management) is put on the programmer.
User has to manage queue
Priority to task is not given

RTOS Solution…
Three tasks
First Task ------------
Second Task ---------------
Third Task
void checkDatagrams(void)
{
typeMsg * msg;
while (TRUE)
{
msg = waitMsg(DATA_GRAM);
processDataGram(msg);
freeMsg(msg);
}
}

RTOS Solution…
Main Code
----------------------
#include <RTOS.h>
Void main()
{
InitRTOS();
Createtask (checkmotorswitches (),1);
Createtask (checkPressure(),2 );
Createtask (checkDatagrams(),3 );
StartSchedular();
}

Advantages
1. Busy waiting is eliminated.
2. Timer management is no longer a concern of the programmer.
3. checkPressure does not have to retain a state variable.
4. Queue development and management is no longer a concern of the
programmer.
5. Kernel API
1. waitMSG()
2. Pause()
3. freeMSG()
4. Createtask()
5. StartSchedular()

OS Used in Embedded System
Non Real Time
Embedded OS
Real Time OS Handheld/ Mobile OS
Embedded Linux
( kernel 2.4.x)
(www.embedded-linux.org)
www.ghs.com
QNX Neutrino (QNX Software Ltd.)
(www.qnx.com)
VxWorks (Wind River)
(www.windriver.com)
MicroC/OS-II (Jean J. Labross)
(http://www.micrium.com/)
Linux (Kernel 2.6.x)
(www.linyx.org)
Symbian OS
(www.symbian.com)
Windows CE
http://windowsce.com/

Open Source RTOS
1. ucLinux www.uclinux.org
2. Symbian www.symbian.org
3. Linux Kernel 2.6.x www.kernel.org
4. Ecos http://ecos.sourceware.org
5. NemuetOS http://www.menuetos.net/
6. FreeRTOS www.freertos.org
7. RTEM http://www.rtems.com/
8. Solaris www.sun.com
9. etc

Real time Operating system
- Real time operating systems are used as OS in real time system.
- In RTOS tasks are completed in given time constraints.
- RTOS is a multitasking system where multiple tasks run concurrently
- system shifts from task to task
- must remember key registers of each task
(this is called context of task)

Characteristics of Real time Operating system
• Single purpose
• Small size
• Inexpensively mass-produced
• Specific timing requirements

Example of RTOS
.
- INTEGRITY, VelOSity and µvelOSity From Green hills co
www.ghs.com
-RTLinux and VxWorks From Windriver
-www.windriver.com
-µC/OS-II from Micrium
- PowerPac from IAR
-www.iar.com

Real time Operating system categories
Two types
Soft RTOS
SOFT real-time system, tasks are
performed by the system as fast as
possible, but the tasks don't have to finish
by specific times
Hard RTOS
In HARD real-time systems, tasks have
to be performed not only correctly but
on time

Real time System Pitfalls -1: Patriot Missile
When the system turned on then it measure the time in 100 ms. Intervals.
This value is multiplied by 10 to obtain second.
The calculation was performed in 24 bit floating register.
So, value of 1/10 th second is truncated in 24 bits
where as exact value is 0.00011 00110 01100 11001 1001100…
So, if value was truncated then en error occurred. (3.4 ms per hour)
The patriot missile was switched on for about 100 hours, so the accumulated error
was 0.34 second.
Scud travels at speed of 1,676 mps, i.e. more then half a kilometer in 0.34 second.
So, patriot could not intercept the scud.
During 1991 Gulf war, an Iraqi Scud Missile hit an American
army barrack killing many soldiers.
Actually an American patriot missile could not track and
intercept the scud missile.
Reason:
A software bug in patriotic missile

Real time System Pitfalls -2: Lockheed Martin
In 1998 Lockheed Martin Titan 4 booster carrying a $1 billion LockMart Vortex
Class Spy satellite pitched sideways and exploded 40 seconds after liftoff cape
Canaveral, Fla.
Reason:
Fried wiring that had not been inspected. The guidance system remain without
power for fraction of second.

Real time System Pitfalls -3: Mars Orbiter
One of the mars orbiter probe crashed into the planet in 1999.
It did turn out that engineers who build the Mars Climate orbiter had provided a
data table in “proud force” rather then Newton's, the metric measure of force.
$125 Million Dollar lost
Reason:
NASA flight controllers at jet propulsion laboratory in pasadena calif., had used the
faulty table for their navigation calculations during the long trip from Earth to Mars.

Real time System Pitfalls - 4: The Ariane 5 satelite launch rocket
Rocket self destructed in 4 June -1996.
Exactly after 40 second of lift off at an attitude of 3700 meters, the
launcher exploded and became a ball of fire.
Cost: $500 Million USD
Reason:
Bad floating-point exception handling.
A 64 bit floating no is converted into 16 bit signed value.
During one calculation the converted value was more then of 16
bit.
This error is know as “500 million dollar software error”

REAL TIME OPERATING SYSTEM PART 1

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REAL TIME OPERATING SYSTEM PART 1