Intro_to_OS_v02

8/14/2019 Intro_to_OS_v02

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Definition

operating system (OS) is the systemsoftware responsible for the directcontrol and management of hardwareand basic system operations.Additionally, it provides a foundation

upon which to run application software


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Few OSs

1973 Unix the first kernel written in a portableprogramming language

1975 VAX/VMS1981 MS-DOS.

1983 AmigaOS

1984 Macintosh1987 OS/2

1993 FreeBSD


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Unix variations

AIX

BSDi

Digital UNIX (or DUNIX)

FreeBSD

GNU Hurd

HP-UX

IRIX

LINUXMac OS X

Mac OS X Server

NetBSD

NeXT

NeXTSTEP

OpenBSD

OPENSTEP

Pyramid

Rhapsody

SCO Solaris

SunOS

ULTRIX


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Kernel driver

Kernel driver can be:

System driver implement a system feature

like file system or TCP/IP stack.

Hardware driver implement a specific

hardware interface to the OS kernel.


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Hardware drivers

Provide general interface for the OS to accessdifferent brands with similar functionality.

For example all NICs driver have the same interfacewhile each driver speak differently to its NIC

hardware.

Hardware driver are layered!

The NIC drivers call the PCI drivers and so on.


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File System

Used mainly on Hard disks.

Provide directory hierarchy

And files with names and attributes.

Can provide security.

Modern file system provide journal forpower fail recovery

Example: FAT, NTFS, ext2, ext3, raiserfs.


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TCP/IP stack

Network stack layer 3.

Most OS today comes with its own stack.

However it is not a most!

In Linux the TCP/IP stack is a kernel driver.


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Memory management

Allocating memory in the form pages.

Separate kernel pages from user space pages

Activate the MMU (next slide)

Handle mapping of virtual addressing to

physical memory.

Kernel uses physical memory addressing.


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Memory management unit

Hardware component that provides:

Memory Protection

Access per mode.

Access can be read/write, read only and noaccess.

There is one access for data and one forinstruction fetch (running code)

Also enable the use of swap!


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Memory management unit

Virtual Addressing mapping:

Map real memory to virtual addressing.

Enable the OS to manipulate application

memory addressing.

Enable fork!


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Swap file system

A Kernel driver that control the virtualmemory that is located on the hard disk. This

memory belongs to application that aresleeping. When this application arise thekernel driver will fetch their memory fromthe hard disk back to physical memory.

Work tightly with the MM component.


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Processors

Processors are object of the OS that have: stack,heap, code and data, all mapped by the OS. Each

Processors have its own CPU registers whichincludes program counter and stack pointer.

The process memory is unique and can access only

by the OS when needed.

Each Process has its user ID and all other securityIDs!


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Threads

Threads are OS objects that exist inside aprocess. Each Thread has its own registers

and stack. But share the same memory space! The process itself is then called the main

threads!


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Tasks

Exist in system that do not use memorymapping, so all process are like threads

running in one process, which is the OSitself!

In this case we call them tasks.


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Scheduler

The Scheduler is a machine inside of the kernel thatshare the CPU among all process and threads in the

system include the kernel itself! The Scheduler share the CPU by priority and byorder.

A good Scheduler can improve system

performance, and should fit the OS main purpose ofuse!

Can share two or more CPUs among processes.


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Loader

The OS has a code loader.

The loader can load kernel driver into the kernel

and application (process) into the user space. The Linux load recognize elf a.out and coff

formats.

The loader perform dynamic link to resolve all of

the new module unresolved links. In case of application to load all the libraries

required by the new application.


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Blocking and non-blocking

Blocking operation is an operation thatblocks the thread (or process) until the

condition is true. In this case the Schedulerwill move to other threads.

Blocking operation can have time-out.

non-blocking operation returns immediatelywithout waiting.


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Event

Usually Pure blocking call (could be withtimeout) for an event to happen.

An event could be for example key pressed.

In this case the thread will wait until use

press a key. (Just like this slide)


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Semaphores

When two threats or tasks use the same resource,they must use a sort of a lock mechanism. This is

the Semaphore. There are three basic kinds:

Binary semaphore lock or not.

Count semaphore can be obtain several times

before it is locked! Mutex thread depended semaphore. Can be obtain

endlessly by the same thread that lock it.

Obtain Semaphore can be blocking call or not.


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Inter process communication

Message queues.

Shared Memory

Sockets (network!)


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Message queue

Sending messages between processes,threads or tasks.

In case of threads we can pass only thepointer to a memory block. That is all needed

since they all live in the same memory space!

In case of process we should pass all themessage since there is memory sharing

between two processes.


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Shared Memory

A block of memory that can be shared byseveral processes.

This enable different processes which bydefault live each in its own memory space to

share a memory block together.

In this case a semaphore is needed to avoidaccident write by two processes to the same

memory block on the same time!.


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CPU modes

Most CPU provide at least two mode of operation(Usually there are more!)

System mode is provide for the OS for its normaloperation. This mode has full privilege and full

memory access.

User mode Application area mode. In this mode,

processes can access only their own memory, using

the virtual memory supplied by the OS.


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Kernel versa OS

Kernel contains only the area that works in systemmode (or other privilege modes as interrupt orexception modes)

The OS contains the Kernel and other component

that runs in user mode but are related to the OS

itself like the X-server, the init process.

Processors and threads can run inside the kernel,

outside as part of applications or part of the system.


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System calls (syscalls)

The bridge between kernel and outside thekernel.

Syscalls are the fastest way to invoke theunderling system

Most CPUs this day have system call for this

purpose.


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GlibC and the standard library

GNU standard library for C basedapplication.

Implement all of the C standard functions.

Use the system calls to call the underlying

OS as needed!


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POSIX standard

IEEE standard

Stand for Portable Operating System

Interface.

A standard interface to the underlying OS.

Linux today have libraries to support POSIX.


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Interrupt, and interrupt handler

Interrupt is issued by hardware on an event.

For example, the timer interrupt interrupt every few

CPU cycles or every few micro seconds. For each interrupt there is interrupt handler which is

a routine part of the OS that handles this event

process it as needed.

For example a NIC interrupt will be a network

packet that should be forward to the TCP/IP stack.


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Exceptions and exceptionshandlers

Exception are issued by the CPU when a wronginstruction is issued.

Exception could be for example, divide by zero andaccess inaccessible memory.

Each exception has its own handler, the exception

handler in case of application will create a core

dump and clear the application from the memory.

(or call the debugger directly in case of windows)


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Classification of system

multi-user : Allows two or more users to runprograms at the same time.

multiprocessing : Supports running a program onmore than one CPU.

multitasking : Allows more than one program to runconcurrently.

Embedded: special-purpose computer system,which is completely encapsulated by the device itcontrols.

real time: Responds to input instantly.

Intro_to_OS_v02

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