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Computer Systems 1
Fundamentals of Computing
Operating Systems
CS1 - Week 22
Introduction to the OS
 OS Fundamentals
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Processes
 Memory
 Files
 I/O
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Computer Systems 1 (2004-2005)
The Operating System (OS)
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Manages the hardware and software
resources of the system
Resources
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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
•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.
Application
Operating System
Application
Application
User
User
Hardwar
e
User
Computer Systems 1 (2004-2005)
Types of OS
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Real-Time Operating System (RTOS)
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Single-User, Single Task
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One user can do one thing at a time
Useful for low spec systems
Single-User, Multi-Tasking
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Fast Response (‘Real Time’ / Instant)
Used to control machinery and industrial systems
Usually Poor HCI
One user can perform many tasks at a time
Common for current PC systems
Multi-User
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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
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Heart of the OS
Provides secure access to the computer hardware
Low-level software / hardware transfer
Manages resource availability / usage
A kernel includes
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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
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Monolithic Kernel
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Microkernel
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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
Runs most services - like networking, filesystem, etc. - in
user space
More stable, but more complex designs
Hybrid (Modified Microkernel)
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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
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Process
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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
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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
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CPU can only do one thing at a time
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If a process wants to use the CPU it must tell the CPU
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To give the appearance multi-tasking, the quickly OS
switches between different processes
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Interrupts or Polling
This function is performed by a process scheduler
Schedulers decide what process should run, when,
and for how long
Scheduling decision factors
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Priority
Resources needed
Resources used to date
Waiting time to date
Computer Systems 1 (2004-2005)
Memory Management
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Provide memory space to enable multiple processes
to be executed at the same time
Give satisfactory level of performance for users
Protect each process
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Enable sharing of memory between processes
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Don’t get data mixed-up between processes
More dynamic & efficient use of the memory resources
Make addressing of memory transparent to
programmers
Provides Virtual Memory
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Extra memory, larger than the physical allocation
Done by storing portions of data to backing storage
Computer Systems 1 (2004-2005)
Input / Output
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Devices use an I/O sub-system of a computer
use to communicate
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I/O devices use interrupts to signal to the CPU
that a task has been completed
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Communication carried out via registers
IRQ – Interrupt ReQuests.
Each device has a unique IRQ
Need more input….
On ISA buses, data can be sent from the I/O
controller to memory, bypassing the the CPU
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This is DMA (Direct Memory Access)
Computer Systems 1 (2004-2005)
Input / Output
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Input / Output Control System (IOCS)
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The part of the O/S that deals with I/O activity
Deals with requests and routes to and from devices
Device Driver
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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
Input-output
Control System
(IOCS)
Application
Program
Device
Driver
I/O Device
}
System
Calls
Operating
System
I/O Bus
Computer Systems 1 (2004-2005)
Device
Controller
File System
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File can be stored data, although programs and
folders are also considered files at a low level
OS knows nothing about file structure
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File System tasks
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Applications can interpret file data
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
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Ideally, successive file clusters should be in
sequence on the disk
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Usually a disk contains many files
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Faster access as less disk movement in required
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
End of
file A
Computer Systems 1 (2004-2005)
B
Free
cluster
B
End of
file B
File System
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Windows file systems are FAT
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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
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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
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The user interface (UI) is a layer above the
operating system
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What users “see” or interact with
UI deals with getting user commands and
providing feedback / representation
Two common types of user interfaces:
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Command-Line Interface (CLI)
Graphical User Interface (GUI)
Computer Systems 1 (2004-2005)
Command-Line Interface
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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
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Graphical representations & metaphors
GUIs have a number of common features:
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Advantages
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Windows, Icons, Menus, Pointers (WIMP)
As well as buttons, toolbars, etc.
Consistent look and feel, even across platforms
Commands can be executed using either mouse or keyboard
High levels of operation and dialog
Disadvantages
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Often restrictive
Use more system resources / more intense
Computer Systems 1 (2004-2005)
UNIX
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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
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Created by Linus Torvalds
Particularly aimed at PC users
Two parts:
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Kernel
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Distribution
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Free! (more or less)
Many now have a windows style interface
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Particular flavour of Linux
Open source
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The Linux operations section
The UNIX part of things
Easy to get to grips with
Some expertise is required to become a master
Low hardware requirements
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100 MHz CPU, 12Mb RAM, 500Mb HDD
Computer Systems 1 (2004-2005)
Microsoft Windows
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Founded by Bill Gates
Developed from MS-DOS
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Microsoft Disk Operating System
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‘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
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Majority are multi-user and multi-task
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Windows XP
Windows Server
Windows 2000
Computer Systems 1 (2004-2005)
Macintosh OS X
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Rooted in a UNIX system
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Providing all the good bits of a powerful UNIX OS
Super-nice GUI as front-end
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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
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Computer Systems 1 (2004-2005)
Week 22
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Operating System Structure & Components
Kernel
 Memory management
 Process management
 Input / output
 File system
 User interface
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Computer Systems 1 (2004-2005)