An embedded system combines hardware and software to perform a specific task within a device, utilizing elements like microcontrollers and sensors. These systems have unique characteristics such as limited resources, reliability, and time constraints, and they can be classified based on generation, complexity, and specific functionalities. Major applications include consumer electronics, healthcare, and industrial automation, showcasing their critical role in various technological domains.

1. Unit – I
Embedded System Concepts

2. Introduction Embedded
System
An embedded system refers to the combination of hardware
and software to carry out a specific task inside a device or
bigger computer system
The elements that are combined here correspond to
microprocessors or microcontrollers, I/O peripheral devices,
sensors, computer memories, among others.
The electronic system which integrates the hardware circuit
with the software programming techniques for providing
Specific task is called as embedded systems
The software usually gets embedded into memory modules as
ROM, and it does not need any secondary memory as in a
computer.
Embedded software is also called ‘firmware’

3. Examples
1. Calculators
2. Washing machine
3. Drones
4. Tracking systems
5. Electric vehicle charging stations
6. Macropads
7. Wearable devices

4. Calculator Washing machine
Drones Tracking systems

5. Electric vehicle charging stations Macropads
Smart wearable devices

8. Embedded System Characterized by
Some Special Features
• Embedded systems do a very specific task, they cannot
be programmed to do different things.
• Embedded system have very limited resources,
particularly memory. (They do no have secondary storage
device )
• Embedded system have to work against some deadlines. A
specific job has to be completed with in a specific time.
• Embedded Systems are constrained for power.
• Embedded systems need to be highly reliable.( ex ALT+CTRL+DEL is
OK on your desktop but you cannot afford to reset your embedded system)
• It has to operate in extreme environmental conditions such
as very high temperatures and humidity.

9. Classification of Embedded
Systems
• Based on Generation
• Based on complexity and performance

10. Based on Generation
• First Generation – 8 bit microprocessor like 8085 and 4
bit microcontroller. ( Ex. Digital telephone keypad,
stepper motor control unit etc..).
• Second Generation – 16bit microprocessor and 8bit and
16bit microcontrollers. Instruction set are more
complex. (Ex. Data acquisition, SCADA system…)
• Third Generation- 32bit microprocessor and 16bit
microcontrollers. A new concept like ( ASIC, DSP). Ex.
Robotics, media, networking…).
• Fourth Generation – The advent of System on
chips(SoC) reconfigurable processors and multicore
processors are bringing high performance. Ex. Smart
Phone, Mobile internet devices…).

11. Based on Complexity and
performance
• Small Scale Embedded System
• Medium Scale Embedded System
• Large Scale Embedded System.

12. Small Scale Embedded System
• Single 8 bit or 16 bit Microcontroller
• Little hardware and software complexity
• They may even be battery operated
• Usually "C" is used for developing these system.
• They need limited power dissipation when system
is running continuously.
Example : Digital Watch
Programming Tools: Assembler and Cross Assembler

14. Medium Scale Embedded
System
• Medium scale embedded systems are designed with a
single 16 or 32 bit microcontrollers, DSPs or RISCs.
• These systems have both hardware and software
complexities.
• When developing embedded software for these types of
systems, the following programming tools are available.
Ex. C, C++, Visual C++, Java, and RTOS, source code
engineering tool, debugger, simulator and integrated
development environment.

15. Large Scale Embedded System
• Large scale embedded systems have huge
hardware and software complexities and may
need PLAs, IPs, ASIPs, scalable processors or
configurable processors.
• They are used for cutting-edge applications that
need hardware and software co-design &
components which have to combine in the final
system.

16. Major Application Area of
Embedded Systems
• Consumer electronics – camcorders, digital cameras,
digital diary etc…
• Household appliances – TV, DVD Player, washing
machine, fridge, microwave oven…
• Office Automation – Copying machine, fax machine, key
telephone, modem, printer, scanner…
• Industrial automation – pharmaceutical, cement, sugar,
oil exploration, nuclear energy, electricity generation and
transmission.
• Medical electronics – ECG, EEG, blood pressure
measuring device, X ray, scanner, endoscopy….
• Computer networking – bridge, routers, ISDN, switches,
hubs, firewalls…

17. • Telecommunications – Key telephones, ISDN phone,
terminal adapters, web cameras, IP phone, IP cameras,…..
• Wireless Technologies- base station controllers, mobile
switching centers….
• Instrumentation – weight, temperature, pressure,
humidity, voltage, current……
• Security – biometric, fingerprint, face recognition…
• Banking and Retail– Smart card and ATM, credit card,
electronic wallet…..
• Card reader – barcode, smart card reader, hand held
devices…..

18. Categories of Embedded
Systems
Based on the functionality and
performancerequirements embedded
system can be categorized as.

19. Stand-alone Embedded System
• It will work on stand alone mode.
• They take input process them and produce the desire output.
• The input can be electrical signal from transducers or
commands from a human being such as pressing of button.
• The output can be electrical signal to drive another system
LED display or LCD display for displaying of information to
the users.
Ex:Digital camera,Microwave oven, CD Player, Air conditioner, TV

20. Real time System
• Real time embedded system have to complete specific task
in a specific time period. Meeting the deadlines is the most
important requirement of real time systems.
• For example consider a system that has to open a valve
within 30 milliseconds when he humidity cross a particular
threshold.
• If valve not open 30 milliseconds a catastrophe may occur…
such system called hard real time system.’
• For example consider a DVD Player ,suppose you give a
command to the DVD player from a remote control and there
is a delay wont lead to a serious implication. Such system
called soft real time systems.
Ex:
Aircraft
Missile embedded with a tracking system.

22. Networked Information Appliances
• Embedded systems that are provided with network
interfaces and accessed by networks such as LAN or the
Internet are called network information appliances.
• For example TCP/IP such as internet company’s intranet.
These systems have emerged in recent years.
• These system run the protocol TCP/IP stack and get
connected through Ethernet.
• A web camera can be connected to the internet.
• The door lock of your home can be a small embedded
system with TCP/IP and HTTP server software running on
it.
Ex:
Desktop Computer- Internet - Weather monitoring system.

23. Mobile Devices
• Mobile devices such as mobile phone personal digital
assistants smart phones etc… are a special category of
embedded systems.
Ex:
word processors, games etc…

24. • Every Embedded System consist of custom built hardware built
around a CPU.
• This hardware also contains memory chips onto which the
software is loaded
• The software residing on the memory chip is also called firmware.
• The embedded system architecture can be represented as a
layered architecture.
Layered Architecture of Embedded System
Hardware
Operating System
Application Software

25. • The same architecture is applicable to any computer including desktop
computer.
• In such case you need to integrate the application software with the
operating system and then transfer the entire software on to the memory
chip.
• Once the software is transferred to the memory chip the software will
continue to run for a long time and you don’t need to reload new
software.

26. Various Building Blocks of
Embedded System
CPU
Input
Devices
output
Devices
Communication
Interfaces
Application Specific
Circuitry
Read Only
Memory
Random
Access
Memory

27. CPU : At the heart of the embedded system is the central
processing unit or processor. It is the hardware that
executes the software and brings life to the embedded
system. It also controls the activities of all the other circuits.
The main criteria for selection is to provide the processing
power needed to perform the tasks within the system.
can be any one of the following
Micro controller, Microprocessor or Digital Signal
Processor (DSP).
A Micro Controller is a low cost processor. Its main
attraction is that on the chip itself, there will be many other
components such as memory, serial communication
interface, ADC etc..
A Microprocessor are more powerful, but you need to use
many external components with them.
DSP is used mainly for applications in which signal
processing is involved such as audio and video processing.

28. Memory :The embedded system also has memory. The
memory is an important part of any embedded system
The memory is categorized as RAM & ROM. The
contents of RAM will be erased if power is switched
off to the chip, whereas ROM retain the contents
even if he power is switched off.
The memory is used to store the software that the processor
will run. It also provides storage for data such as program
variables, intermediate results, status information
Input Devices : Input devices to an embedded
system have very limited capability.
Output Devices : output devices to an embedded
system have very limited capability. Some embedded
output systems will have a few LED to indicate health
status and LCD used to display some important
parameters

29. Communication Interfaces: The embedded
systems may need to interact with other embedded
systems or they may have to transmit data to a
desktop. like USB, RS232,RS422, IEEE, Ethernet….
Application Specific Circuitry : Sensors,
transducers, special processing and control circuitry
may be required for an embedded system depends on
application. Power consumption is minimized.

30. Specialties of Embedded
Systems
While designing embedded systems, the following
aspects need to be considered by the developers
An embedded systems have many specialties.
1. Reliability
2. Performance
3. Power Consumption
4. Cost
5. Size
6. Limited user interface
7. Software Up gradation Capability

31. Reliability
When we use a desktop, sometimes the
system ‘hangs’ and we need to reset the
computer. Generally this does not cause
any problem. They must work with high
reliability.
Performance
Many Embedded system have time
constraints. The system must meet such
deadlines.

32. Power Consumption
Most of the embedded system operate through
battery. In such case reducing the component
count hardware designer have using PLD and
FPGA. Reducing the component count apart from
reducing the power consumption also increase the
reliability of the system.
Cost
For embedded systems used in safety applications
of a nuclear plant or in spacecraft, the cost may
not be a very important factor. If using consumer
electronics or office automation the cost is utmost
importance.

33. Size
Size is certainly a factor for many embedded
systems. We do not like mobile phone that has to
be carried on our backs. The size and weight are
important parameter in embedded systems used
in aircraft, spacecraft, missiles etc.. Because in
such case each and every inch and gram matters.
Limited user interface
Unlike desktop which have full fledged I/O
devices, embedded systems do not have
sophisticated interfaces for I/O. Some embedded
systems do not have any user interface at all.

34. Software Up gradation Capability
Embedded systems are meant for very
specific task. Once software is transferred
to embedded system, the same software is
run throughout the life. In such case we
need to upgrade the software.

35. Recent Trends in Embedded Systems
 Processor Power- 8 bit, 16 bit, 32 bit and 64 bit-
clock speed
 Memory-cost and size of the memory chip
Operating System
Communication interfaces and N/W capability
Programming Language-C, C++, Java, Python…
Development tools-Keil, Matlab, AVR studio…
Programmable hardware- PLDS, PLCs, FPGA,
ARDUNO, Rasparipi…

36. Detailed Hardware Architecture of Embedded System

37. CPU : can be any one of the following Micro controller, Microprocessor
or Digital Signal Processor (DSP).
A Micro Controller is a low cost processor. Its main attraction is that on
the chip itself, there will be many other components such as memory,
serial communication interface, ADC etc..
A Microprocessor are more powerful, but you need to use many external
components with them.
DSP is used mainly for applications in which signal processing is involved
such as audio and video processing.
CPU consist of
•ALU
•General purpose registers
•Control Unit
- Instruction Pointer
- Stack Pointer
- Instruction Decoder
- Memory address register and Memory Data register

38. Internal Architecture of a Processor

39. ALU- performs arithmetic and logical operations
General purpose Registers. Registers constitute the processors
internal memory
it contain the current data and operands that are being manipulated by the
processor
Control unit- fetches the instructions from memory ,decodes them and execute
them
Instruction Pointer – points to the next instruction to be executed
Stack pointer- points to the stack in memory – Decodes the instruction
In addition to manipulating data, a processors job is to read data and
instructions from memory and write data to output devices and read data to
input devices to do these functions the processor communicates with other
devices using three buses
1.Data Bus
2. Address Bus
3. Control and Status bus

40. Bus System and architectures of a
Processor
Address Bus
Which carries address information from the processor to
memory, hence this is unidirectional bus.
Data bus
Which carries data between the processor and other
devices. Bi directional bus.
Control and Status Bus
which will carries control/status information.

41. Based on number of memory and data buses
used there are 3 types.
– Von Neumann Architecture
– Harvard Architecture
– Super Harvard Architecture

43. Von Neumann architecture is most widely used architecture
It has one memory chip which stores both instructions and
data
Harvard Architecture
two separate blocks-
program memory and data memory
program memory – stores only instructions
Data memory –stores only data
Program memory address bus and Program memory data
bus used to access the program memory
Data memory address bus and Data memory data bus used
to access the data memory
SHARC ( Super Harvard Architecture)
Is a slight but significant modification of the Harvard
architecture.
The data memory is accessed more frequently than the
program memory

44. Interrupts
Interrupt is a signal to the processor that some important event has
occurred.
CISC (COMPLEX INSTRUCTION SET COMPUTER )
RISC( REDUCED INSTRUCTION SET COMPUTER)

45. Memory : The memory is divided into 2 categories: Data memory and
Program memory. The program memory stores the information permanently
whereas the contents of data memory are erased when the power is switched
off.
Memory chips are classified as:
RAM- perform both read-wrīte operation
RAM is of two types :
RAM: SRAM and DRAM
ROM- store firmware in embedded sysstems
PROM, EPROM,EEPROM(hybrid memory)
Programmable ROM-PROM devices can be programmed only once
Erasable Programmable ROM-can be programmed many times
Hybrid memory devices
Electrically Erasable PROM(EEPROM)similar to EPROM but its content can be
erased by applying electrical signal to one of te pins of the device
Non –Volatile RAM-if power is switched off, the battery will ensure that contents
are not erased
Flash memory-is a type of EEPROM
It’s a low cost chips are characterized by their fast read quality
The contents of RAM will be erased if power is switched off to the chip, whereas
ROM retain the contents even if the power is switched off.
ROM:

46. While evaluating the processor, the following specifications to be
considered:

47. I2
C (Inter-Integrated Circuit) Bus

48. Input Devices : Sensors and Transducers, Keypad
switches, fingerprint sensors, Voice command..
Output Devices : LEDs, LCD, OLED, 7segment
display, speaker, motor..
Communication Interfaces: USB, RS232,RS422,
IR,IEEE, Ethernet….
Application Specific Circuitry : Sensors,
transducers, special processing and control circuitry
may be required for an embedded system depends on
application. Power consumption is minimized.
Power Supply: The embedded system can be
operated through 230V AC supply or by Batteries.
Since the components in the system needs different
DC voltages such as +3v, +5v, +12v, -5v, -12v. So,
the DC convertor is required.
Linear regulator: 7805, 7812, and
switching regulator: SMPS

49. Clock Circuitry
The processor has to be given the clock input to one of the pins. To
generate the clock signal a crystal oscillator are required.
Watchdog Timer/Reset Circuit
Watchdog timer is used to reset the processor automatically if it
does not receive a signal periodically from the processor indicating its
healthy status. This mechanism is used for reset button on the embedded
system.
Chip Select
I/O Devices
Sensors and Transducers
ADC and DAC
Debug Port
Debugging a processor based board is very difficult. In earlier many
processor manufacturer used to provide proprietary interfaces to debugging.
JTAG and IEEE… are used.

50. Software Architecture
• The software is an embedded system consists of
an OS and the application software.
• OS is optional it is not present we need to write
our own software routines to access the
hardware.
• As embedded systems are constrained for
memory, we cannot use an OS such as windows
or Unix. But still we need the services provided
by an OS.

51. Service provided by an OS
• Every computing device or embedded system needs a piece
of software using which the user interacts with the hardware.
• The software is the OS, when the computer is switched on
first the OS is loaded into the main memory(RAM).
• We can use the computer for doing different things
simultaneously such as word processing, downloading the
email etc….
• Each job is done by an application software package.
• To write an application software package we divide the job
into different smaller jobs, write the code for each smaller job
and then combine the entire code.

52. • In this each job is called “Process” and in an
embedded system each job is called a “Task”.
• Each task needs memory and needs to access I/O
devices.
The OS has following functions
• Process/Task Management
• Memory management
• I/O management including file system.
• Provide services to applications
• Providing User interfaces

53. Based on the capabilities OS are divided
into different categories
• Single tasking OS versus Multi tasking OS
• Single user OS versus Multi user OS
• Command driven OS versus GUI based OS
• Reliability
• Multi tasking with time constraints.
• Small footprint
• Support diskless systems
• Portability
• Scalability
• Support for standard API

54. Developing Embedded software
• Identifying the tasks and assigning priorities to each task.
• Identifying the time critical tasks.
• Scheduling the tasks in such a way that all the tasks are
completed and the tasks which are time critical meet the
deadlines.
• Working out when to send interrupts to the processor.
• Identifying the shared resources and working out
mechanisms for sharing the resource by multiple tasks.
• Working out the strategies for inter task communication
• Keeping track of the time.
Architecture of Embedded Operating
System

55. Tasks
The work to be done by an embedded
system is divided into a number of tasks. Each
task competes for the CPU time independently.
Each task will have a separate stack. Each
task is implemented as an infinite loop.
Ex
for ( )
{
Statements;
}
Alternatively we use while loop
While ()
{
Statements;
}

56. Task Scheduling
Since only one CPU has to handle multiple task, the task
have to share the CPU time in a disciplined way so that one task
does not get lot of time while others wait for unduly long time.
Important time critical task have to be given high priority
and a mechanism for deciding which task will get the CPU time
next has to be worked out. This is known as task scheduling.
Context Switching
A low priority task is presently being executed by the
processor but a high priority task has to run. In this case CPU
will be interrupted an interrupt signal. CPU will save the current
task information in stack and execute the high priority task. The
mechanism of storing the current CPU registers in a stack
to run the other task is known as context switching.

57. stack is defined as a linear data structure that is open at one end and the
operations follow the Last-In-First-Out (LIFO) order.
Characteristics of Stack:
The stack follows the LIFO order, which means that the last element added to the stack
will be the first element to be removed.
A register that points to the top of the stack is known as the stack pointer. It is used to
keep track of the current position of the top of the stack.
A stack is also characterized by its capacity, which is the maximum number of elements
it can hold at any given time. If an attempt is made to push an element onto a full stack,
a stack overflow error will occur.

58. Mutual Exclusion
Embedded system provide disciplined access to shared
resources through special objects such as semaphore and Mutexes.
Mutes and semaphore are like keys. To access resources a task will
obtain the key use the resource and release the key.
Inter Task communication
Task may need to exchange data amongst themselves. For
instance a task may write some data to a file and another task has to
read the data.
Memory Management
It is used to shared number of tasks. So memory management
is another service provided by OS.
Timer Services
The OS also provide time service

59. Software Architecture of ES

60. The operating system consists of
• Kernel
• Device Manager
• Networking protocol software
• Libraries
• File System (Optional)
Kernel
It is the heart of the OS. Its main function is to
manage the tasks. A task scheduling algorithm decides
which task has to run next.
Device Manager
The I/O devices are used to send/receive data
from the embedded system.

61. Communication protocol software
Communication interfaces such as Ethernet,
USB etc…
Libraries
The OS may have some C/C++ library files in
object code.
File System
Most of ES do not have secondary storage. In
such cases the ROM is used to store the program.

62. Categories of Embedded Operating Systems
1.Non real time embedded operating systems
This OS are not suitable for hard real –time embedded
applications Ex : Embedded Linux, Embedded NT and
Windows XP Embedded
2.Real time Operating system
This OS are suitable for hard real –time embedded
applications Ex :RT LinuX,OS/9
3.Mobile/Handheld Operating Systems
The OS used in mobile devices such as
palmtops,PDAs,smart phones are known as
3.Mobile/Handheld Operating Systems

63. Application Software
In embedded systems the application software is
integrated with the operating system software and networking
software, and then transferred to the memory of the embedded
system.
The various function calls provided by an OS are
•To create, suspend and delete tasks
•To do task scheduling for meeting real time requirements.
•To facilitate inter task communication and synchronization
between tasks.
•To initialize increment and reset counters to keep track of time
•To allocate and free memory
•To access the I/O devices.

64. Communication Software
Most of the embedded system need a communication interface
to interact with external world
To network enable an embedded system, the embedded
systems should have the complete TCP/IP protocol stack
integrated along with the operating system.

65. TCP/IP Protocol Suite

66. Physical layer
This layer defines the characteristics of transmission such as data rate
and signals encoding scheme.
Data Link Layer
This layer defines the protocols to manage the links.
The data link layer establishes and terminates a connection between
two physically-connected nodes on a network. It breaks up packets into frames
and sends them from source to destination.
Internet Protocol layer
Two important function of this layer are addressing and routing.
IP layer software runs on every end system and router connected to the
internet. Each system connected to internet is given a unique address known a
IP address.
Transport Layer
This layer provides end to end data transfer service between two
systems connected to the internet.
The transport layer carries out flow control, sending data at a rate that
matches the connection speed of the receiving device, and error control,
checking if data was received in correctly and if not, requesting it again

67. Transmission Control Protocol(TCP)
The job of TCP layer to ensure that the data is delivered to
the application layers without any errors
User Datagram Protocol(UDP)
It provides a connectionless service
It sends the packet to the destination one after other without
bothering whether they are received correct or not
Application Layer
The application layer is used by end-user software such as
web browsers and email clients. It provides protocols that
allow software to send and receive information and present
meaningful data to users.
Example
HTTP, FTP, SMTP, DNS….

68. Process of Generating Executable Image
• Creating the source file
• Create the executable file
• Give the command for execution
• The process is loaded into the RAM by the
loader.
• Loader transfer the control to the process
and the process execution.

69. The OS ,communication software and
application software have to be converted into a
single executable image and
transferred to the memory . This is the process
of creating an executable image

71. Source files written in C++ are converted into object file using compiler
Source files written in assembly language are converted into object
file using compiler
Each object file contain the binary code & program data
each object file created in the above process having following
information
1. Name of the source file
2.size of the file
3.process-specific binary instruction and data
4.symbol table
5. debugging information
linker combines the various object files including the library files
The linker command file contains a no of instructions .
These command tell the linker how to combine the object files and
extract locations in the memory
output of linker is an executable image that can be transferred to
memory chip

72. Development/Testing Tools
• Hardware Development Tools
– Digital MultiMate- this is the most instrument to measure voltages,
current
– Logic Analyzer- is used to check the timings of the signals
– Oscilloscope-used to analyze the time domain waveforms
– Spectrum analyzer –used to analyze the signals in frequency domain
• Software Development Tools
– OS Development Suite
– Cross Platform Tools
– ROM Emulator
– EPROM Programmer
– In circuit Emulator

73. In circuit Emulator is a device that emulates the CPU.
This device fits into the CPU socket of the target board
on one side , and on the other side it is connected to the
host through RS232 or USB port
ICE is processor –specific and costly

79. Introduction in RTOS
A Real Time Operating System is
multitasking operation system for applications,
which require that system tasks and functions
execute with real-time constraints.
The system controls the execution of all
functions such that time deadlines are met.

80. • It responds to inputs immediately(Real Time)
• Here the task is completed within a specified
time delay.
• In real life situations like controlling traffic
signal or a nuclear reactor or an aircraft.
• The operating system has to respond quickly.
There are two types
1.Hard Real time system
2.Soft Real time system

81. Hard Real Time System

82. Soft Real Time System

83. Characteristics of RTOS

84. Functions of RTOS
• Task Management
• Scheduling
• Resource Allocation
• Interrupt Handling
Task Management
* Real Time Applications the process is called as Task
which takes execution time and occupies
memory.
* Task management is the process of managing task
through its life cycle.

85. Task
• No of Tasks
• Resource Requirements
• Release Time
• Execution Time
• Deadlines
Resource Allocation
• Weighted Round Robin
• Priority Based
Interrupt
An interrupt is a signal from a device attached to
a computer or from a program with in a computer that
causes the main program that is operating system to
stop and figure out what to do next.

86. Examples of Real Time System
• Plant control
• Control of production processes / industrial automation
• Railway switching systems
• Automotive applications
• Flight control systems
• Environmental acquisition and monitoring
• Telecommunication systems
• Robotics
• Military systems
• Space missions
• Household appliances.

87. Architecture of the Kernel
• The embedded software consists of the
Operating System and the application
software.
• The services provided by the operating
system are accessed through Application
Programming Interface(API) to develop
application software.
• The API is a set of function calls using which
you can access the various kernel objects and
the service provided by the kernel.

88. The Various Kernel Objects are
• Task
• Task Scheduling
• Interrupt Service Routines
• Semaphores
• Mutexes
• Mailboxes
• Message queues
• Pipes
• Event Registers
• Signals
• Timers

89. The Kernel provides various services through
operations on the kernel objects.
These services are
•Memory Management
•Device Management
•Interrupt Handling
•Time Management

90. Tasks and Task Scheduler
• The Embedded software consists of a number of
tasks.
• These tasks include the OS tasks and
Application specific task.
• Each task in ES is implemented as an infinite
loop.
• The task objects consist of Name, unique ID,
priority, stack and task control block.
• In addition to the tasks required for the
application software, the kernel has its own
system tasks with priorities.

91. • Startup task: Which is executed when the
operating system starts.
• Exception handling task to handle the
exceptions.
• Logging task: log the various system messages.
• Idle task : Which will have lowest priority and will
run when there is no other task to run.
One CPU has to handle multiple tasks, the tasks
have to share the CPU time in a disciplined
manner so that one task does not get lot of
time while others are waiting forever.

92. Task Scheduling
• Each task has to be assigned a priority
mechanism for deciding which task will get CPU
time next has to be worked out. This is known
as task scheduling.
• The object that does task scheduling is the task
scheduler.
• The task have to share the system resources
such as CPU registers, external memory and I/O
devices.
• The important requirement is that one task
should not corrupt the data of another task.

93. While scheduling Task no of Issues are
Many tasks may make calls to a function.
•A function that can be used by more than one task
without data corruption is called reentrant
function.
•If data is corrupted when more than one task
calls the function such a function is called non
reentrant function.
•Every task requires resources such as serial port,
keyboard, display or memory locations.
•The resources shared by two or more tasks are
called as shared resources.

94. Task States

95. For Instance consider an embedded system that
obtains data from a serial port and converts the
data into Ethernet packets.
Task 1: reads the data from serial port another
task (task2) keeps converting the serial data
(Character) into packets and put them into buffer.
Another Task (Task3) read data from buffer and
sends it over an ethernet interface.
Task2 has to wait for Task1 to complete its work
and then inform task3 can do its work.

96. Task can be in one of the following 3 states
• Running
• Waiting
• Ready to Run
Running State:
A task is said to be in running state If it is being
executed by the CPU.
Waiting State:
If it is waiting for another event to occur. Waiting state
cannot be executed till the external event occurs.
Ready to Run State:
If it is waiting in a queue for the CPU time.

97. Task Stack:
Every task will have a stack that stores the local
variables, function parameters, return address
and CPU registers during an interrupt.
Context Switching:
The state of the CPU registers when a task has to
be preempted is called the context. Saving the
contents of the CPU registers and loading the new
task parameters is called context switching (High
Priority to low priority).