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Operating System

A collection of system programs which allow theuser to run application software.

manages processes and resources.

abstracts the real hardware of the system andpresents the users and applications with a virtual

machine.

has utilities for system administration tasks.

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OS Mechanisms

Two mechanisms for controlling accessto system resources are

different modes of execution

system calls

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Processor Modes

Modern processors typically can operate in 2 modes:"user mode" and "kernel mode " .

User mode

processor executes normal instructions in the user's program.

Kernel mode

processor executes both normal and privileged instructions

Processor can access additional registers and memory address

space that are accessible only in kernel mode

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Kernel

The kernel is the core of the operatingsystem.

It consists of code and data structures that

are protected and can be accessed only in the

kernel mode.

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System Calls

User programs are not allowed to accesssystem resources directly. They must ask theOS to do that for them.

OS provides a set of functions that can be

called by user programs to request for OS

services. These functions are called system

calls

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System Calls

System calls run in kernel mode.

They can be called by executing a special

instruction (trap or software interrupt) which

causes processor to switch to the kernel mode andjump to a previously defined location in the

kernel.

When the system call finishes, processor returnsto the user program and runs in user mode.

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Differences between library functions andsystem calls

System calls run in kernel-mode but libraryfunctions run in user-mode and may callsystem calls.

System calls are not linked to user

programs.

There are only a small number of system

calls. (about 250 in Linux, see /usr/include/asm/unistd.h )

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Solaris System Calls

Solaris has about 200 system calls.

Files and I/O

Process management

Interprocess communication

Others

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System Calls: Files & I/O

open, close open and close a file

create, unlink create and remove a file

read, write read and write a file

lseek move to a specified byte in a file

chmod change access permission mode

mkdir, rmdir make and remove a directory

stat get file status

ioctl control device

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System Calls: Process Management

fork create a new process exec execute a file

exit terminate proces

wait wait for a child process to terminate

sbrk change the size of the space allocated in theheap data segment of the process (used by memory

allocation functions, e.g. malloc)

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System Calls: Interprocess Communication

kill, sigsend send a signal to a process pause suspend process until signal

sigaction set signal handler

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Programming in UNIX

Programming in a UNIX system (and modernoperating systems) often relies on two concepts:

controlled access to system services

realized by having system calls running in privileged

mode

common interface, various implementations

realized by API

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Programming with System Calls

High level languages, including C, providefunctions for programmers to use systemcalls just like normal function.

/usr/include/unistd.h

man -s 2 intro (Solaris) or man 2 intro

(Linux)

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errno and perror

When a system call fails, it usually returns -1 and

sets the global variable errno to a value describingwhat went wrong.

The function perror() translates this error code

into human-readable form and prints a system error

message.

void perror(const char *s);

The argument string s is printed first, then the

system error message. Usually s is a user message

indicating where the error occurs.

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API (Application Programming Interface)

An API is a set of functions provided by an operatingsystem or other system software.

An application program calls the functions to request the

services.

An API clearly defines how to call functions and what

the results are. (API is specification, not implementation)

Examples: APIs for file system, graphics user interface,networking, etc.

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Advantages of Using API

Portability User programs that follow the APIs definition are portable.

An API can provide a common interface for different

implementations of a service.

For example, the UNIX file system API is the same for all

kinds of devices.

X windows API has many implementations on different

machine architectures

Using an API allows upgrading system software without

changing user programs

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Privileged Instructions

Two modes of execution --> two kinds ofinstructions :

normal instructions, e.g., add, sub, etc.

privileged instructions, e.g.,

read/write reserved registers

read/write protected memory

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Exception Mechanism

Switching from user mode to kernel mode to perform systemoperations and return to user mode

Exception is caused by hardware interupt (I/O) and software

interrupt

Software interrupt or trap Some special instructions cause processor to switch between

user and kernel modes: trap , return_from_trap

Exception Handler

When an exception occurs, an exception handler is called.

Exception handler code is in kernel's protected memory Exception hander code for software traps are system calls