Lec6 OS Structure

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Silberschatz, Galvin and Gagne 2002Modified for CSCI 399, Royden, 2005

3.1Operating System Concepts

Operating Systems

Lecture 6System Calls

OS System Structure


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System Calls

System calls provide the interface between a running program andthe operating system.

Generally available as assembly-language instructions.

Languages defined to replace assembly language for systems

programming allow system calls to be made directly (e.g., C,C++)


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System Calls are Used Frequently

A single program may make numerous system calls. For example, aprogram to read from one file and write to another would needsystem calls for the following:

Prompt the user to enter file names

Read in filenames

Open input file

Read from input file

Open/create output file

Write output to file

Close input and output files

The system must be able to signal and handle errors that occur.


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Passing Parameters

Three general methods are used to pass parametersbetween a running program and the operating system.

Pass parameters in registers.

Store the parameters in a table in memory, and the

table address is passed as a parameter in a register. Push (store) the parameters onto the stackby the

program, and pop off the stack by operating system.


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Passing ofParameters As A Table


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Types of System Calls

Process control

create (fork), execute (exec), wait, end (exit), abort (kill), etc.

File management

creat, delete, open, close, read, write, cp, rm, mkdir, rmdir, ls, cat, more, grep,

etc. get/set file attributes

Device management read, write, attach (mount), detach (unmount), get/set device attributes

Information maintenance

get/set time or date, get/set file attributes (chmod, chown, chgrp), get/set

process/device attributes (du, ps, etc) Communications

send, receive, connect, accept, get/set status information, gethostid/sethostid,

gethostbyname, etc.


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Process Control

A process executing one program may want to load and execute another program(e.g. the shell loads and executes programs). The following are important

considerations:

Where does control return when the new process is finished?

If return control to existing program, must save memory image of existing

program before loading new process. If both programs are to run concurrently, the new process is added to the

multiprogramming set of processes.

Controlling execution of the process:

get/set process attributes, terminate process

Waiting for the process to finish wait event, signal event

Terminating the process

Normal (exit) or abnormal (abort) termination.

There are multiple ways of implementing process control.


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MS-DOS Execution

At System Start-up Running a Program

MS-DOS runs the commandinterpreter on startup.

It loads a new program into

memory, writing over part of

the interpreter.

When the program terminates,

the part of the interpreter not

overwritten begins execution.

It loads the rest of the

interpreter from disk.


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UNIX Running Multiple Programs

UNIX runs the shell on startup.

To start a new process, it uses the fork command to

create the process and exec to execute it.

If the process is in the foreground, the shell waits for the

process to finish.

If the process is in the background, the user can

continue to issue commands while the process is

running.

When the process is finished, it executes an exit system

call.


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Message Passing

Processes use message passing to send messages to one another. First, the connection must be opened.

The name of the communicator must be known (hos tname or host id,

process name or process id). Use get process id or get host id.

open connection, close connection

recipient uses accept connection

The initiator is the client. The recipient of the request is the server.

Exchange of information made with write message system calls.


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Shared Memory

In memory sharing, processes communicate by writing and readingto the same memory addresses.

Processes use map memory system calls to access memory

owned by other processes.

Both processes must agree to remove O.S. memory restriction sothat they can access the same region of memory.

The processes are responsible for the form and location of the

data.

The processes are responsible for making sure that they are notwriting to the same memory area simultaneously.


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Communication Models

Msg Passing Shared Memory

Communication may take place using either message passing or

shared memory.


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System Programs

System programs provide a convenient environment for program

development and execution. The can be divided into:

File manipulation

Status information

File modification Programming language support

Program loading and execution

Communications

Application programs

Most users view of the operation system is defined by system

programs, not the actual system calls.


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MS-DOS System Structure

MS-DOS written to provide the most functionality in the least

space

not divided into modules

Although MS-DOS has some structure, its interfaces andlevels of functionality are not well separated


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MS-DOS Layer Structure


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UNIX System Structure

UNIX limited by hardware functionality, the original UNIXoperating system had limited structuring. The UNIX OS

consists of two separable parts.

Systems programs

The kernel Consists of everything below the system-call interface and

above the physical hardware

Provides the file system, CPU scheduling, memory

management, and other operating-system functions; a large

number of functions for one level.


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UNIX System Structure


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Layered Approach

The operating system is divided into a number of layers (levels),each built on top of lower layers. The bottom layer (layer 0), is

the hardware; the highest (layer N) is the user interface.

With modularity, layers are selected such that each uses

functions (operations) and services of only lower-level layers. Advantage: Modularity makes it easy to modify and extend.

Disadvantage: Some functions may depend on one another,

making a strict hierarchy difficult to implement.


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An Operating System Layer


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OS/2 Layer Structure


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Microkernel System Structure

Moves as much from the kernel into user space.

Communication takes place between user modules using

message passing.

Benefits:

- easier to extend a microkernel- easier to port the operating system to new architectures

- more reliable (less code is running in kernel mode)

- more secure

Example: Mac OS X is based on the Mach micro kernel.

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