RTOS Class Notes

1 Prepared By: Yogesh Misra, FET, Mody University of Science & Technology

Course Title: Embedded System Design

Topic Covered in notes

(1) Operating System Basics

(2) Kernel

(3) Kernel Space and User Space

(4) Monolithic Kernel and Microkernel

(5) Important Features of RTOS

(6) Mutual Exclusion

(7) Semaphore

(8) Binary Semaphore (Mutex)

(9) Counting Semaphore

(10) Multiprocessing and Multitasking


OPERATING SYSTEM BASICS

The operating system acts as a bridge between the user application/tasks and the

underlying system resources through a set of system services

Fig. 1 : The Operating System Architecture

KERNEL

SERVICES

User Applications

Underlying Hardware

Memory Management

Process Management

Time Management

File System Management

I/O System Management


Kernel: It communicates between system resources and user applications. Kernel is the

core of an operating system and contains a set of system services. Various services of

kernel are as follows:

Process Management: It deals with managing the processes/tasks. Process

management includes:

setting up the memory space for the process

loading the process’s code into the memory space

allocating system resources

scheduling and managing the execution of the process

Primary Memory Management: It deals with:

(i) Keeping track of which part of the memory area is currently used by which process

(ii) Allocating and De-allocating memory space on a need basis

I/O System Management: It is responsible for routing the I/O requests coming from

different user applications to the appropriate I/O devices of the system.

The kernel maintains a list of all the I/O devices of the system. This list may be available

in advance, at the time of building the kernel. Some kernels, dynamically updates the list

of available devices as and when a new device is installed (e.g. Windows XP kernel

keeps the list updated when a new plug ‘n’ play USB device is attached the system).

The service ‘Device Manager’ of the kernel is responsible for handling all I/O device

related operations.

Protection System Management: It deals with implementing security policies to restrict

the access to system resources by different applications or processes or users (e,g,

Window XP has restrictions like ‘Administrator’, ‘Standard’. ‘Resticted’, etc).

Kernel Space and User Space:

The application services are classified into two categories, namely: user applications and

kernel applications.


The program code corresponding to the kernel applications/services are kept in the

primary memory and protected from un-authorized access by user programs/applications.

The memory space at which the kernel code is placed is known as ‘kernel space’.

Similarly, all user applications are placed to a specific area of primary memory and this

memory area is known as ‘user space’.

Some OS implements this kind of partioning and protection whereas some OS does not

segregate the kernel and user application code into two separate areas.

Monolithic Kernel and Microkernel: Based on the kernel design it can be classified in

to monolithic and micro kernels.

Monolithic Kernel:

In this all kernel services run in the kernel space. The major drawback of monolithic

kernel is that any error or failure in any one of the kernel service leads to the crashing of

the entire kernel application. LINUX, SOLARIS, MS-DOS kernels are examples of

monolithic kernel.

Fig. 2 : The Monolithic Kernel Model

Monolithic Kernel with all OS services running in kernel space

Applications


Microkernel: The microkernel design incorporates only the essential set of operating

system services into the kernel. The rest of the operating system services are

implemented in programs known as ‘Servers’ which runs in user space. Memory

management, process management, timer systems and interrupt handlers are essential

services, which forms the part of the microkernel.

Benefit of microkernel: If problem arises in any of the services, which run as

‘Server’ application, the same can be reconfigurable and re-started without the need

for re-starting the entire OS.

Fig. 3 : The Microkernel Model

Microkernel with essential services like memory management, process management, time system, interrupt handler, etc. in kernel

space

ApplicationsServer (kernel services running

in user space)


REAL-TIME OPERATING SYSTEM (RTOS)

Important Features of RTOS: The RTOS consumes only known and expected amount

of time regardless the number of services it is offering. The RTOS decides which

applications should run in which order and for how much time. A RTOS designed for an

embedded system should have as low as possible memory requirement. The Real-Time

kernel is highly specialized and it contains minimal set of services required to run the

user application/task.

Some of the important features and functions required for a RTOS are as follows:

1 Task/Process Management – It deals with setting up memory space for the tasks,

loading the task’s code into the memory space, allocating system resources,

task/process termination/deletion and setting up a Task Control Block (TCB). A

Task/Process management service of RTOS creates a TCB for a task on creating a

task and delete the TCB when the task is terminated.

A TCB contains following informations:

Task ID – Task identification number.

Task state – The current state of task (e.g. READY state, RUNNING state,

BLOCKED/WAITING state, etc.)

Task Type – Indicate the type of task (e.g. Hard Real Time Task, Soft Real Time

Task, etc)

Task Priority – It indicates about the priority of task.

2 Task/Process Scheduling – A kernel application called ‘Scheduler’ handles the

task scheduling. A scheduler is an algorithm which deals with efficient sharing of

CPU among various tasks/processes.

3 Task/Process Synchronisation – In a multitasking system, multiple processes run

concurrently (in pseudo parallelism) and share system resources. Task

synchronization provides a mechanism to access the resources which are to be

shared among various Tasks/Processes. Without task synchronization problems

like racing and deadlock arises.

4 Error/Exception Handling – It deals with handling the errors raised during the


execution of tasks. Exceptions can be in the form of insufficient memory,

timeouts,deadlocks, deadline missing, etc. Watchdog timer is a mechanism for

handling timeouts for tasks.

5 Clock and Timer Support - Clock and timer services with adequate resolution

are one of the most important part of a RTOS. Hard real-time application requires

timer services with the resolution of the order of a few microseconds.

On the other hand, traditional operating systems normally do not provide timer

services with sufficiently high resolution.

6 Real-Time Priority Levels - A RTOS must support static priority levels. A

priority level supported by an operating system is called static, when once the

programmer assigns a priority value to a task, the operating system does not

change it by itself. Static priority levels are also called as Real-Time Priority

Levels.

A traditional operating system dynamically changes the priority levels of tasks

from programmer assigned values to maximize system throughput.

7 Fast Task Preemption - For successful operation of a real-time application,

whenever a high priority critical task arrives, currently executing low priority task

must free the CPU to high priority task. The time duration for which a higher

priority task waits before it is allowed to execute is quantitatively expressed as

task preemption time. A RTOS should have task preemption times of the order of

a few microseconds. On the other hand task preemption times of the traditional

operating system is of the order of seconds.

8 Fast Interrupt Latency - Interrupt latency is defined as the time delay between

the occurrence of an interrupt and the running of the corresponding Interrupt

Service Routine (ISR). In RTOS, the interrupt latency should be less than a few

microseconds.

9 Preemptive Kernel - The kernel of a RTOS must be preemptive type. Preemptive


kernel of RTOS ensures that every task / process gets a chance to execute. When

and how much time a process gets is dependent on the implementation of the

preemptive scheduling. In this the currently running task is preempted to give a -

chance to other tasks/processes to execute.

On the other hand, in case of non-preemptive kernel, the task currently running

runs till it terminates or enters the ‘wait’ state, waiting for an I/O or system

resources.

Multiprocessing and Multitasking

1 Multiprocessing - In the operating system context multiprocessing describes

the ability to execute multiple processes simultaneously. Systems which are

capable of performing multiprocessing, are known as multiprocessor systems.

Multiprocessor systems possess multiple CPUs and execute multiple

processes simultaneously.

2 Multitasking - In a uniprocessor system, it is not possible to execute multiple

processes simultaneously. However, it is possible for a uniprocessor system to

achieve some degree of pseudo parallelism in the execution of multiple

processes by switching the execution among different processes. The ability

of an operating system to hold the multiple processes in memory and switch

the processor from executing one process to another process is known as

multitasking. Multitasking creates the illusion of multiple processes executing

in parallel.

Whenever a CPU switching happens, the current context of currently running

process should be saved so that at a later point of time it is retrieved when the

current process is to be executed by CPU. The context saving and retrieval is

essential for resuming the process exactly from the point where it was

interrupted due to CPU switching. The act of switching CPU among the

processes or changing the current execution context is known as ‘Context

Switching’.


(A)

(B)

(C)

Multitasking involves the switching of execution among multiple tasks.

Depending upon how the switching process is implemented the multitasking

can be classified as follows:

Co-operative Multitasking – In this process a task/process gets the chance of

execution only when the currently executing task/process voluntarily releases

the CPU.

In this method, any task can hold the CPU as much time it wants. In this

method the co-operation of each task/process is required for utilization of

CPU.

Preemptive Multitasking – In this method every tasks/processes get a

chance to execute. The currently running task/process is preempted to give

chance to other task/process. When and how much time a task/process gets for

execution is dependent on the implementation algorithm of preemptive

scheduling.

Non-preemptive Multitasking - In this method the task/process, which is

currently in execution, is allowed to execute until it terminates (enters the

‘Complete’ state) or enters the ‘Bloked/Wait’ state, waiting for an I/O or

system resources.

The Co-operative and Non-preemptive Multitasking behave differently in

‘Bloked/Wait’ state. In Co-operative Multitasking, the currently executing

task/process need not to release the CPU when it enters the ‘Bloked/Wait’

state, waiting for an I/O, or a shared resource access or an event to occur

whereas in Non-preemptive Multitasking the currently executing task/process

releases the CPU when it enters the ‘Bloked/Wait’ state, waiting for an I/O, or

a shared resource access or an event to occur.


Fig. 4: Process States and State Transitions

Process – A process is a program or part of it, in execution. Process is also

known as an instance of a program in execution. Multiple instances of the

same program can ececute simultaneously. A process requires various system

resources like CPU for executing the process, a stack for holding the values

of local variables associated with the process, I/O devices for information

exchange, etc.

Process States and State Transition – The creation of a process and it’s

termination is not a single step operation. The transition of a task/process in

various state from newly created task state to completed task state is called as

‘Process Life Cycle’.


Created State – The state at which a task/process is created is called as

‘Created State’. The operating system recognizes the task but no resources

are allotted to the task in ‘Created State’.

Ready State – During the ‘Ready State’ task is allocated the memory and

task waits for processor so that the execution of task begins. During this

stage, the task is inlisted in the queue of Ready State tasks. This queue is

maintained by the operating system.

Running State – In this state the execution of the source code of the task

starts.

Blocked/Wait State – In this state the task is temporarily suspended from

execution. This state generates due to given reasons:

Task is waiting for getting access to a shared resource.

Task is waiting for user inputs such as keyboard input.

Completed State – A state where the process completes its execution is

known as ‘Completed State’.

Task Scheduling

Determining which task/process is to be executed at a given point of time is

known as task/process scheduling. Task scheduling is an algorithm which is

run by kernel as a service.

Selection of scheduling algorithm should consider following factors:

Throughput – This gives an indication of the number of tasks/processes

executed per unit of time. It is always high for good scheduling algorithm.

Turnaround Time – It is the amount of time taken by a process for

complete execution. It includes the time spent by process in ready queue,

time spent for waiting for the main memory, time spent in completing I/O

operation and time spent in execution. Turnaround Time should be low for

good scheduling algorithm.

Waiting Time – It is the time spent by process in ready queue waiting to get


the CPU time for execution. Waiting Time should be low for good

scheduling algorithm.

Various queues maintained by OS in association with CPU scheduling are:

Job Queue – It contains all the processes in the system.

Ready Queue – It contains all the processes which are ready for execution

and waiting for CPU to get their turn for execution.

Device Queue – It contains the set of processes waiting for an I/O device.

Non-Preemptive Scheduling - Non-Preemptive Scheduling is employed in

systems which implement Non-Preemptive multitasking model. In this

scheduling the current process is allowed to execute until it terminates or

enters in Blocked/Wait State waiting for an I/O or system resource. The

various types of non-preemptive scheduling are as follows;

First-Come-First-Served Scheduling

Shortest Job First Scheduling

Priority Based Scheduling

First-Come-First-Served (FCFS) Scheduling – This scheduling algorithm

allocates CPU time to the processes based on the order they enter the

‘Ready’ queue.

Example 1: Three processes P1, P2 and P3 with estimated completion time

10ms, 5ms and 7ms respectively enters the ready queue together in the order

P1, P2, P3. Calculate the waiting time and turn around time (TAT) for each

process and the average waiting time and average TAT (assuming there is no

I/O waiting for the process).

P1 P2 P3

10ms 5ms 7ms

Solution – Assuming the CPU is available at the time of arrival of P1. P1

starts executing without any waiting in the ‘Ready’ queue. Hence waiting

time for P1 is 0. The waiting time for each processes are as follows:


Waiting Time for P1 = 0ms

Waiting Time for P2 =1 0ms (P2 starts executing after completing P1)

Waiting Time for P3 =1 5ms (P3 starts executing after completing P1 and

P2)

Average waiting Time = (waiting time of all processes) / (No. of processes)

= (0ms + 10ms + 15ms) / 3 = 8.33ms

TAT for P1 = 10ms (Time spent in ‘Ready’ queue + Execution Time)



Average TAT = (TAT of all processes) / (No. of processes)

= (10ms + 15ms + 22ms) / 3 = 15.66ms

Shortest Job First (SJF) Scheduling – This scheduling algorithm allocates

CPU time to the processes which has shortest estimated completion time,

followed by the next shortest process, so on.

Example 2: Three processes P1, P2 and P3 with estimated completion time

10ms, 5ms and 7ms respectively enters the ready queue together in the order

P1, P2, P3. Calculate the waiting time and turn around time (TAT) for each

process and the average waiting time and average TAT for SJF scheduling

(assuming there is no I/O waiting for the process).

P2 P3 P1

5ms 7ms 10ms

Solution – The waiting time for each processes are as follows:

Waiting Time for P2 = 0ms

Waiting Time for P3 =5ms (P3 starts executing after completing P2)

Waiting Time for P1 =1 2ms (P1 starts executing after completing P2 and

P3)

Average waiting Time = (waiting time of all processes) / (No. of processes)

= (0ms + 5ms + 12ms) / 3 = 5.66ms





Average TAT = (TAT of all processes) / (No. of processes)

= (5ms + 12ms + 22ms) / 3 = 13ms

Task Synchronization

In a multitasking system, multiple processes run concurrently (in pseudo

parallelism) and share system resources. Task synchronization provides a

mechanism to access the resources which are to be shared among various

Tasks/Processes. Without task synchronization problems like racing and

deadlock arises.

Racing – It is the situation in which multiple processes try to access and

manipulate shared variable/data. In a racing condition the final value of the

shared data depends on the process which finally accessed it. Racing

condition produces incorrect results.

Deadlock – Deadlock creates a situation where none of the processes are

able to make any progress in their execution.

Fig. 5: Deadlock Condition


Mutual Exclusion: Mutual Exclusion is used to prevent dead lock condition.

The two mechanisms used for mutual exclusion are as follows:

(i) Mutual Exclusion through ‘Busy Waiting’ Mechanism – In

this mechanism the shared resource which is currently being used

by a process is locked. If a new process requires that resource

then it makes CPU always busy by checking the lock. This leads

in wastage of CPU time and results in higher power consumption.

(ii) Mutual Exclusion through ‘Sleep & Wakeup’ Mechanism –

In this mechanism when a new process is not allowed to access

shared resource, which is currently being locked by another

process, undergoes ‘Sleep’ mode and enters the ‘Blocked’ state.

The process which is blocked on waiting to access shared

resource is awakened by the process which currently using shared

resource before it leaves shared resource.

Semaphore: Semaphore is the implementation of a ‘Sleep & Wake’ based

mutual exclusion for shared resources.

A shared resource can be used exclusively by one process at a particular

instant (e.g. display device of a embedded system) or it can be shared by

multiple process at a particular instant (e.g. different sectors of secondary

memory can be used by different processes at a time). Based on the

limitation of sharing the shared resources semaphore can be classified as

‘Binary Semaphore’ and ‘Counting Semaphore’.

Binary Semaphore or Mutex: The binary semaphore provides exclusive

access to shared resource by allocating it to a single process at a time and not

allowing the other processes to access it when it is being used by a process.

In some RTOS kernel binary semaphore also referred as Mutex.

Counting Semaphore: The counting semaphore limits the access to shared

resource by a fixed number.

Documents

RTOS Class Notes