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Computer Systems 1Fundamentals of Computing
Operating Systems
Computer Systems 1 (2004-2005)
CS1 - Week 22
Introduction to the OSOS Fundamentals
ProcessesMemoryFilesI/O
Computer Systems 1 (2004-2005)
The Operating System (OS)
Manages the hardware and software resources of the system
Resources Processor Memory Disk space, etc.
Allows applications to deal with the hardware without having to know all the details of the hardware
Computer Systems 1 (2004-2005)
Operating Systems
Hardware
Operating System
User
Application
Application
Application
User
User
•The OS acts a a ‘shell’ around the hardware.
•The applications software need an OS to work, and users need the applications software to interact with the hardware via the OS.
Computer Systems 1 (2004-2005)
Types of OS
Real-Time Operating System (RTOS) Fast Response (‘Real Time’ / Instant) Used to control machinery and industrial systems Usually Poor HCI
Single-User, Single Task One user can do one thing at a time Useful for low spec systems
Single-User, Multi-Tasking One user can perform many tasks at a time Common for current PC systems
Multi-User Many users can use the computer's resources simultaneously OS balances resources allocation against users and tasks N.B- A machine on a network is not necessarily multi-user!!!
Computer Systems 1 (2004-2005)
OS Generic Structure
•Kernel
•Memory & Process Management
•Input / Output
•File System
•User Interface
Computer Systems 1 (2004-2005)
The Kernel
Heart of the OS Provides secure access to the computer hardware Low-level software / hardware transfer Manages resource availability / usage A kernel includes
Interrupt handler Handles all requests for kernel service
Scheduler that Decides how and which programs share the kernel
processing time Supervisor
Gives use of the computer to each process when required
Computer Systems 1 (2004-2005)
Types of kernels
Monolithic Kernel Runs most of its services in the kernel workspace Error prone due to the amount of tasks in the kernel itself Used in most Linux systems
Microkernel Runs most services - like networking, filesystem, etc. - in
user space More stable, but more complex designs
Hybrid (Modified Microkernel) Microkernels that have some "non-essential" code in
kernel space in order for that code to run more quickly. Windows NT/2000 and Macintosh OS X use hybrid kernels
Computer Systems 1 (2004-2005)
Process Management
Process A single sequence of operation(s) The basic unit of software the operating system deals with
in scheduling the work done by the processor Not the same as an application
An application can contain multiple processes Process has some key attributes
Process ID (PID) Priority Level Pointers to upper and lower bounds of memory
allocated Status of all I/O devices needed
Computer Systems 1 (2004-2005)
Process Management CPU can only do one thing at a time
If a process wants to use the CPU it must tell the CPU Interrupts or Polling
To give the appearance multi-tasking, the quickly OS switches between different processes This function is performed by a process scheduler
Schedulers decide what process should run, when, and for how long
Scheduling decision factors Priority Resources needed Resources used to date Waiting time to date
Computer Systems 1 (2004-2005)
Memory Management Provide memory space to enable multiple processes
to be executed at the same time Give satisfactory level of performance for users Protect each process
Don’t get data mixed-up between processes Enable sharing of memory between processes
More dynamic & efficient use of the memory resources Make addressing of memory transparent to
programmers Provides Virtual Memory
Extra memory, larger than the physical allocation Done by storing portions of data to backing storage
Computer Systems 1 (2004-2005)
Input / Output
Need more input….
Devices use an I/O sub-system of a computer use to communicate Communication carried out via registers
I/O devices use interrupts to signal to the CPU that a task has been completed IRQ – Interrupt ReQuests. Each device has a unique IRQ
On ISA buses, data can be sent from the I/O controller to memory, bypassing the the CPU This is DMA (Direct Memory Access)
Computer Systems 1 (2004-2005)
Input / Output Input / Output Control System (IOCS)
The part of the O/S that deals with I/O activity Deals with requests and routes to and from devices
Device Driver A software module which manages the communication
with, and control of, a specific I/O device Interprets requests from the application to specific
commands to the I/O device
Application Program
Input-output Control System
(IOCS)
Device Driver
Device Controller
I/O Device
System Calls
I/O Bus
}Operating System
Computer Systems 1 (2004-2005)
File System
File can be stored data, although programs and folders are also considered files at a low level
OS knows nothing about file structure Applications can interpret file data
File System tasks Creating & deleting files Storing and organising files on storage medium
A new file is allocated one cluster As the file grows the cluster becomes filled and
more clusters are assigned to the file
Computer Systems 1 (2004-2005)
File System Ideally, successive file clusters should be in
sequence on the disk Faster access as less disk movement in required
Usually a disk contains many files Clusters get mixed out of sequence This is known as fragmentation
Fragmentation can reach a critical level where system performance becomes greatly reduced
A A B A B C C BB
End of file A
Free cluster
End of file B
Computer Systems 1 (2004-2005)
File System Windows file systems are FAT
File Allocation Table FAT16
Uses 16-bit value to record file properties FAT32
Very similar to FAT16, except it uses 32-bit values
Linux / UNIX Use iNodes Similar procedure to Windows Usually 32-bit values Every file has an iNode OS tracks iNodes and file names
Computer Systems 1 (2004-2005)
User Interface
The user interface (UI) is a layer above the operating system What users “see” or interact with
UI deals with getting user commands and providing feedback / representation
Two common types of user interfaces: Command-Line Interface (CLI) Graphical User Interface (GUI)
Computer Systems 1 (2004-2005)
Command-Line Interface
Textual commands Either from the keyboard or from a script
User types a command after the computer displays a prompt character For example, MS-DOS, UNIX, etc.
Advantages Faster for skilled users All options and commands available in a standard format Can perform operations in a batch processing mode without user
interaction Disadvantages
Perplexes new users / Not user-friendly Multiple operations made complex Have to remember commands / not intuitive
Computer Systems 1 (2004-2005)
Graphical User Interface
Graphical representations & metaphors GUIs have a number of common features:
Windows, Icons, Menus, Pointers (WIMP) As well as buttons, toolbars, etc.
Advantages Consistent look and feel, even across platforms Commands can be executed using either mouse or keyboard High levels of operation and dialog
Disadvantages Often restrictive Use more system resources / more intense
Computer Systems 1 (2004-2005)
UNIX Designed by Ken Thomson of AT&T Bell Laboratories
improved by Dennis Ritchie Multi-user
More than one person suing the OS at a time Multitasking
Each user can run more than one process Platform independent
Runs on almost any hardware platform Complex Large distribution User control and administration Used as multiple types of server
Computer Systems 1 (2004-2005)
LINUX Created by Linus Torvalds Particularly aimed at PC users Two parts:
Kernel The Linux operations section The UNIX part of things
Distribution Particular flavour of Linux
Open source Free! (more or less)
Many now have a windows style interface Easy to get to grips with Some expertise is required to become a master
Low hardware requirements 100 MHz CPU, 12Mb RAM, 500Mb HDD
Computer Systems 1 (2004-2005)
Microsoft Windows Founded by Bill Gates Developed from MS-DOS
Microsoft Disk Operating System ‘Borrowed’ from XEROX Q-DOS
Windows 1.0 began life as an interface running on MS-DOS
GUI improved and networking capabilities added in future versions
Is now independent of MS-DOS Many varieties available
Majority are multi-user and multi-task Windows XP Windows Server Windows 2000
Computer Systems 1 (2004-2005)
Macintosh OS X
Rooted in a UNIX system Providing all the good bits of a powerful UNIX OS
Super-nice GUI as front-end Ease of use and interaction
Macintosh OS have always been pioneers in user-friendly interfaces and functionality High level of accessibility Scalable from first-time users to advanced Seamless integration of OS and system applications
Computer Systems 1 (2004-2005)
Week 22
Operating System Structure & ComponentsKernelMemory managementProcess management Input / outputFile systemUser interface