computer system programming

8/12/2019 computer system programming

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Ch1:Intro to Unix

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Unix (officially trademarked as UNIX, sometimes

also written as Unix) is a multitasking, multi-

user computer operating system originallydeveloped in 1969 by a group of AT&T employees

at Bell Labs.

The Unix operating system was first developed

in assembly language, but by 1973 had been

almost entirely recoded in C.

Among all variants of Unix, Linux (1991) is themost widely used, powering everything from huge

data centers to desktop systems to mobile phones

to embedded devices such as routers.2


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Unix shell is a command-line interpreter or shellthat provides a traditional user interface for theUnix operating system and for Unix-like systems.

Users direct the operation of the computer byentering commands as text for a command lineinterpreter to execute or by creating text scriptsof one or more such commands.

The program is called a "shell" because it hidesthe details of the underlying operating systembehind the shell's interface.

The shell manages the technical details of the

operating system kernel interface, which is thelowest-level, or 'inner-most' component of anoperating system.

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System call is how a program requests a

service from an operating system's kernel. This

may include hardware related services (fore.g. accessing the Hard Disk), creating and

executing new processes, and communicating

with kernel services(like scheduling). System

calls provide the interface between a process

and the operating system.

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Systems Programming

Systems programming is an area of computer

engineering that deals with the developmentof complex program systems and their

components.

Unix Standards

ANSI C

a language standard

includes standard C library

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Basic modules of UNIX

User program

Library

Shell

User level

OS command

Library

System call interface

Kernel

level

Process management

&memory management

File

Subsystem

Character block

I/O I/O

Devicedrivers

Hardware control

Hardware

Hardware

level

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In computing, a device driver is a computer program allowinghigher-level computer programs to interact with a hardware device.

A driver typically communicates with the device throughthe computer bus to which the hardware connects Drivers are

hardware-dependent and operating-system-specific. A device driver simplifies programming by acting as translator

between a hardware device and the applications or operatingsystems that use it. Programmers can write the higher-levelapplication code independently of whatever specific hardwaredevice.

There are device drivers for printers, displays, CD-ROM readers,diskette drives, and so on. When you buy an operating system,many device drivers are built into the product. However, if you laterbuy a new type of device that the operating system didn'tanticipate, you'll have to install the new device driver. A devicedriver essentially converts the more general input/outputinstructions of the operating system to messages that the devicetype can understand.

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An abstract view of UNIX architecture

hardware

Text editor Command-line interpreter (shell)

VI sh pico

a.out

open( ), read( ), close( )

write()

fork(

)

UNIX kernel

who

cc

Application

programs

C compiler

System

calls

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UNIX and its basic ser ices


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UNIX and its basic services

User Processes System Processes

System callinterface

Interface todevices

U U U S S

Kernel

LAN

Devices

Modem

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B i S i


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Basic ServicesSystem initializationProcess management

Memory managementFile system managementInput/output managementCommunication facilities

Program interface (system calls)UNIX - overview:

UNIX is a Multitasking, Multi-user OS, with a Virtual

Memory, Programmable Shell and Networking capability.

UNIX has access controls and securityUNIX environment is organized into three layers kernel -

programs - shell

UNIX has a hierarchical, universal file system

UNIX - set of processes 13


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Multitasking Capability of UNIXProcess:

A single program in execution (e.g. running a Cprogram, reading I/O, sending an email are all kinds ofprocesses)Each process in UNIX has a unique Process ID (PID)

Created by fork() systems callsProcess termination (return from main() or callingexit())

Multiprocess application: a process wants to execute

another programProcess Synchronization: wait() system call is used

Process communication: local and remote IPCmechanisms

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Multitasking: is the ability to execute severalprocesses simultaneously.

Processes are not really executed at the sametime, but the operating system executes a pieceof one process and then switches to another (thisis called task switching).

This switching happens so fast (10x per secondsor more) that it looks like the computer is doingseveral things at a time.

At any given time, only one process is running. For a certain period of time, all the processes

have made progress.

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Process (task)

1

Process (task)

2

Process (task)

3

Context

switching

Time

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Multiuser capability of UNIX

UNIX

hostLocal Area Network (LAN)

UNIX

is here

No UNIX

here

(Windows

OS)Users workstations (PCs)

UNIX

Shell commands

Shell

Each host runs a number of independent processes that executes in different protection domains (WSs).

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Networking capability of UNIX

Electronic mail (e-mail)

File transfer (FTP) service

World wide web (WWW) service Remote login (through Telnet)

Archie: is a tool for indexing FTP archives, allowingpeople to find specific files.

Gopher: is a TCP/IP application layer protocol designedfor distributing, searching, and retrieving documentsover the Internet. Gopher protocol was a predecessorof (and later, an alternative to) the World Wide Web.The protocol imposes a much stronger hierarchy on

information stored on it. Its text menu interface is well-suited to computing environments that rely heavily onremote text-oriented computer terminals, which werestill common at the time of its creation in 1991.

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A typical session in UNIX

Login: your identificator

Password: your password

%command 1

%

%logout

A few UNIX commands

ls or ls -l to list the current directory

cd to change a directory

rm to delete a file

mkdir to create a directory rmdir to remove a directory

more to show a text file

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Standard Files on a UNIX System:

/bin - (binary files) contains executable

programs, UNIX commands

/dev - special files that represent the physical

devices

/lib - libraries of programs used byprogrammers

/sys - system source files (system managers)

/tmp - temporary files

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Commands:

The basic form of any Unix command is:

command_name options argument(s) Most descriptions for commands such as

those given in the On-line Manualuse a muchmore precise syntax. For example:

cp [-iprR] filename ... directory UNIX on-line help manual

Section

Contents 1 User commands (Shell commands)

2 OS services (system calls)

3 Library functions

4 Devices networks interfaces21


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5 System file formats

6 Demo programs

7 Miscellaneous (ASCII, etc )

8 System maintenance commands

Getting Specific Information

Once you know the names of the man page or pages youneed, you can display them on the screen by giving the mancommand:

man [section] If you omit the section number, the man program searches

through each section in turn until it finds the named manpage.

Most manual sections have an ``intro'' man page, describing

that section in general. So to find out more about libraryfunctions (section 3 of the man pages), type

%man 2 intro

%man 3 intro

%man 2 fork

%man 3 sin22


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Interrupts are a commonly used technique

for computer multitasking, especially in real-

time computing. Such a system is said to beinterrupt-driven. An act of interrupting is

referred to as an interrupt request (IRQ). The

part of a program that deals with the interrupt

is referred to as an interrupt service routine

(ISR) or interrupt handler.

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Interruption handling in UNIX (simplified)

Interruptsignal

Save theinterruption

NoNew priority > currentpriority?

Yes

Create a new context,set current priority


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UNIX kernel

Basic services:

System initialization (bootstrap facility).

Process management (creation, control, andtermination of process).

Memory management (using a virtualmemory).

File system management.

I/O management

Communication facilities (inter-processcommunication, networks).

Program interface (system calls).25


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Kernel software

Machine dependent Machine independent

System initialization

Interrupt handling

Memory management

I/O device drivers

Etc.

System call interface (!)

Management of processes

Scheduling system

Etc.

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The UNIX Operating SystemBasic structure of the UNIX operating system is divided

into three parts:

kernelschedules programsmanages data/file access and storageenforces security mechanismsperforms all hardware access

shellpresents each user with a promptinterprets commands types by a userexecutes user commandssupports a custom environment for each user

utilitiesfile management (rm, cat, ls, rmdir, mkdir)user management (passwd, chmod, chgrp)process management (kill, ps)printing (lp, troff, pr)

program development tools 27


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Ch2: Preparing a C program and using

command line parameters in UNIX

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How to prepare a C program in UNIX

Case A: Your program consists of one source file

After starting a text editor (for example PICO), create and save a

source text file (for example myprog.c).

Compile and link the program. The result is executable file (for

example, myprog).

Start your program.

%pico

%cc -o myprog myprog.c

%myprog or

%myprog param1,param2.

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Case B: Your program consists of a few separate sourcefiles (for example, module1.c, module2.c, module3.c)

1. Create in a text editor, and save each source file.

2. Compile separately each source file. The result isobject files (module1.o, module2.o, module3.o)

3. Link the object files. The result is executable file.

%cc -c module1.c

%cc -c module2.c%cc -c module3.c

%cc -o myprog module1.o module2.o module3.o

%cc -o myprog module1.c module2.c module3.c

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If you execute as dayit will print usage: day .

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l f


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To clarify array pointers,

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computer system programming

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