Operating Systems Concept

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Operating Systems Concept

?What is an operating system?

?Operating system architecture

?Process concept

?CPU scheduling

?Memory management

?File and I/O systems

File System

?Provides a mechanism for on-line storage of and access to both data and programs

?Components

?Files: logical unit abstracted by the OS.

?A directory structure: special files to organize and provide information to files.

?What should be considered?

?File attributes such as name, file operations such as seek, file types such as executable, file structures such as those for Word.

* “Operating system concept”, Silberschatz and Galvin, Addison Wesley, pp. 337-342,346-348.

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File Systems

?File Methods

?Sequential Access

?Basic Operations

?READ, WRITE + file pointers

?Direct Access

?Basic Operations

?READ N or Write N, where N is the relative block number.

File: a sequence of bytes … ..

a sequence of (logical) blocks

File System – A UNIX Approach

?Process Control Block Struct PCB {

char p_pid;

int pc; /* program counter */

…

int files[NFILE];

} PCB[NPROC];

Read(4, … )

Tables of Opened Files (per process)

System Open File Table

In-core i-node list

i-node i-node

i-node

sync

data block data block

kernel space user space

open create an entry

(file current position, etc)

file-open-count++ Load the corresponding i-node if it is absent.

* The i-node structure of a file includes info regarding the disk location of the file.

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BSD UNIX i-node

* “Operating system concept”, Silberschatz and Galvin, Addison Wesley, pp. 380.

mode owner timestamp size block ref-count

triple indirect double indirect

single indirect

direct blocks

data data data

…

data data data

…

data data

… data…

• 4KB block size

• 12 direct pointers

• 48KB

• 1 single indirect

• 4-byte block ptr

• 1K * 4KB = 4MB

• >> 4GB for the largest file!

Memory-Mapped File

?Allow a part of the virtual address space to be logically associated with a section of a file.

1 2

Virtual Memory for Process A

1 2

Virtual Memory for Process B

Physical Memory

1 2 3

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File System – Directory Structure

?Partition (/Volume):

?a low level structure in which files and

directories reside.

?Directory:

?Records info for “all”

files on a partition.

directory files directory

files

… disks

partition

* “Operating system concept”, Silberschatz and Galvin, Addison Wesley, pp. 349,354-358.

File System – Tree-Based Directory

? Path name

?Specify a file by listing “node”names from the root to the corresponding node in the structure

?/users/userA/info-sys/test vmunix

/

dev

console lp0 …

bin

csh …

lib

libc.a …

usr

include …

etc

passwd …

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Sharing of Files

?Hard Link

?Each directory entry creates a hard link of a filename to the i- node that describes the file’s contents.

?Symbolic Link (Soft Link)

?It is implemented as a file that contains a pathname.

?Example: Shortcut on Windows

foo

bar

/usr/joe

/usr/sue File i-node:

Reference = 2

foo

bar

/usr/sue

File i-node:

Reference = 1

/usr/joe

File i-node:

Reference = 1 data file:

/usr/joe/foo

File Systems - Mounting

?(name of the device, mount point)

tmp

Use an appropriate device driver to read the device directory and verify the format => mount!

•Mount point: the location within the file structure at which to attach the file system.

UNIX: manual mounting

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Protection

?How to prevent improper access?

?Access control!

?Read, Write, Execute, Append, Delete, List, etc

?Approaches:

?Complete isolation

?No protection at all

?Controlled access by limiting the “types” of file access based on some factors:

* “Operating system concept”, Silberschatz and Galvin, Addison Wesley, pp. 360-362.

Protection

?Access user list for each file/directory

?{ < user-ID, allowed access types> … }

?Tedious in maintenance and variable directory sizes -> condense such info (UNIX)

?Read/write/execute over owner/grp/others

?Issues

?Control in group memberships

?Membership per user?

?A password for each file/directory

?A large number of passwords

?Different passwords for different levels of protection?

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Operating Systems Concept

?What is an operating system?

?Operating system architecture

?Process concept

?CPU scheduling

?Memory management

?File and I/O systems

I/O Subsystems

?Why?

?Provide the simplest interface to the rest of the system

?Optimize I/O for the maximum concurrency

?Variations:

?Block vs Character I/O

?Sequential/Random Access

?Synchronous/Asynchronous Transfer

?Dedicated/Share

?Read-Only/Read-Write

* “Operating system concept”, Silberschatz and Galvin, Addison Wesley, pp. 398, 408-410, 414-415.

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I/O Subsystems

?Conflicting trends

?Increasing standardization of software/ hardware interfaces

?Increasing broad variety of I/O devices

?Device drivers which provide a uniform device-access interface to the I/O subsystem.

?How the I/O system improves the efficiency of the computer?

?Schedule I/O operations, e.g., those on disks.

?Use techniques such as buffering, caching, or spooling.

Principles in Doing I/O

?Reduce the number of context switches.

?Reduce the number of data copyings.

?Reduce the frequency of interrupts

?Large transfer, smart controller, etc.

?Increase concurrency

?DMA controller

?Move processing primitives into hardware.

?Balance CPU, memory subsystems, bus, and I/O memory.

* “Operating system concept”, Silberschatz and Galvin, Addison Wesley, pp. 424-425.