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2008 Chapter-8 L1: "Embedded Systems - Architecture, Programming

and Design" , Raj Kamal, Publs.: McGraw-Hill, Inc.1

REAL TIME OPERATINGREAL TIME OPERATINGSYSTEMSSYSTEMS

LessonLesson--1:1:OPERATING SYSTEM SERVICESOPERATING SYSTEM SERVICES

GOAL, MODES AND STRUCTUREGOAL, MODES AND STRUCTURE


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1. OS Services Goal1. OS Services Goal


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The OS Service GoalThe OS Service Goal Perfection andPerfection and

correctness during a servicecorrectness during a service

GoalGoal


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(i) Facilitating easy sharing of resources as

per schedule and allocations

No processing task or thread uses any

resource until it has been allocated by theOS at a given instance

OS Services GoalOS Services Goal


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Meaning of Resource of a systemMeaning of Resource of a system

processor (s), memory, I/O

devices, virtual devices,

system timer, software timers

keypad, displays, printer and other suchresources, which processes (tasks or

threads) request from the OS.


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(ii) Facilitating easy implementation of the

application program with the given system-

hardware through the system software



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(iii) Optimally scheduling the processes onone (or more CPUs if available) byproviding an appropriate context switchingmechanism



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and Design" , Raj Kamal, Publs.: McGraw-Hill, Inc.

8

(iv) Maximizing the system performance tolet different processes (or tasks or threads)share the resources most efficiently withprotection and without any security breach.



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(v) Providing management functions for the

processes (tasks or threads), memory, file,

physical and virtual devices and I/Os and

for other functions for which it is designed (vi) Providing management and

organisation functions for the I/Os, devices

and files and file -like devices



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(vii) Providing interoperability of the

application on different networks

(viii) Providing a common set of

interfaces that integrates various

devices and applications through

standard and open systems



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(ix) Providing easy interfacing and

management functions for the network

protocols and network

(x) Providing portability of the applicationon different hardware configurations



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2. User and Supervisory mode2. User and Supervisory mode

structurestructure


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Processor in the system running in twoProcessor in the system running in two

modesmodes

Clock, called system clock.

At every clock tick of the system-clock,there is system interrupt.

On each system interrupt, the system timeupdates, the system context switches to thesupervisory mode from user mode.

After completing the supervisory functionsin the OS, the system context switches backto user mode


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User modeUser mode

User processpermitted to run and use

only a subset of functions and instructions

in OS.

Use of OS functions in user modeeither

by sending a message to the waiting process

associated in the OS kernel space or by

system call (calling an OS function).


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User modeUser mode

The use of hardware resources including

memory is not permitted without OS

making the call to the OS functions, called

system call.


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User modeUser mode

User function call, which is not a systemcall, is not permitted to read and write intothe protected memory allotted to the OS

functions, data, stack and heap That protected memory space is also called

kernel space. Hence execution of userfunctions calls is slower than the execution

of the OS functions (on system call) due toneed to spend time in first checking theaccess permission to the protected space


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Supervisory modeSupervisory mode

Also called kernel mode

OS runs in protected mode the privilegedfunctions and instructions in protected mode

that are the privileged ones and the OS(more specifically, the kernel) is only onepermitted to access the hardware resourcesand protected area memory

Kernel space functions and processesexecute faster than the user space functionsand processes.


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Supervisory modeSupervisory mode

Only a system call is permitted to read and

write into the protected memory allotted to

the OS functions, data, stack and heap


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3. Example3. Example Improving system Improving system

performance by running applicationperformance by running application

program threads in supervisory (kernel)program threads in supervisory (kernel)

modemode


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RTOS Windows CE and several RTOSesRTOS Windows CE and several RTOSes

Run all the threads run in the supervisory

mode (kernel mode). Therefore, the threads

executes fast.

Improves the system performance.

If the threads are to execute in user mode,

as in Unix or in non-real time OS, then the

execution slows down due to time spent inchecks on the code access to the protected

kernel space


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3. Structure3. Structure


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Top to down Structural LayersTop to down Structural Layers

Software Application Programming

Interface (API)

System software other than the one

provided at the OS

OS Interface

OS

HardwareOS Interface

Hardware


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OS is the middle in-between layer

between the application software and

system hardware

Top to down Structural LayersTop to down Structural Layers


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4.4. Kernel Services in the OSKernel Services in the OS


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KernelKernel

Process, memory and IO managers areessential components of a kernel

Kernel may include file and devicemanagement functions as part of the

kernel in a given OS Kernel in certain OSes is without File and

Device management functions as part ofthe kernel in the given OS and any otherneeded functions other than process,memory and IO device managers, whichare essential components of a kernel arenot provided for at the kernel


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Process Management Kernel Services in anProcess Management Kernel Services in an

OSOS

Creation to deletion of Processes

Process structure maintenance

Processing resource requests

Scheduling Processes

Inter process Communication (IPC)

(communication between Tasks, ISRs, OS

functions)


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Memory Management Kernel Services in anMemory Management Kernel Services in an

OSOS

Allocation and de-allocation between

Tasks, ISRs, OS functions


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Kernel Services in an OSKernel Services in an OS

File Management

Device Management,

Device Drivers

I/O Management

Interrupts Control (by handling ISR)

Mechanism


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SummarySummary


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We learntWe learnt

OS goal is perfection, correctness,portability, interoperability, and providing a

common set of interfaces for the system,

resources and orderly access and control

when managing the processes


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We learntWe learnt

OS structure consists of kernel and otherservice functions outside the kernel. System

software includes the kernel. The OS lets

the application run on the hardware

OS is the middle in-between layer between

the application software and system

hardware

Process, memory and device managers,

which are essential components of a kernel


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Kernel - A basic unit of any OS that

includes the functions for processes,

memory, task scheduling, inter process

communication, management of

devices, IOs, and interrupts and may

include the file systems and network

subsystems in certain OSes

We learntWe learnt


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We learnt OS lets a process execute on the CPU

some process at OS for a resource-

management mechanism lets it use thesystem-memory and other system-

resources such as network, file, display

or printer


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We learntWe learnt

Kernel functions are management of

processes, process and resources scheduling

memory, file, devices, IOs, network and

handling the interrupts


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End of Lesson 1 of Chapter 8End of Lesson 1 of Chapter 8

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1 Os Services