System Software
CS 1308 Computer Literacy and the Internet
Introduction
 Von Neumann computer
 “Naked machine”
 Hardware without any helpful user-oriented features
 Extremely difficult for a human to work with
 An interface between the user and the hardware is
needed to make a Von Neumann computer usable
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System Software: The Virtual Machine
 System software
 Acts as an intermediary between users and hardware
 Creates a virtual environment for the user that hides the actual
computer architecture
 Set of services and resources created by the system software and
seen by the user
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Figure 6.1
The Role of System Software
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BIOS
 Basic Input/Output System
 The first piece of software that starts when the computer is
turned on
 Stored in Flash Memory
 Power-on Self-test (POST) makes sure that all the system
components are running properly (e.g. RAM check)
 Gets the keyboard, mouse, and other peripherals started
 Finds the “Boot” device and starts the operating system
 For more information (Google “How does the BIOS work”)
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Types of System Software
 System software is a collection of many different
programs
 Operating system
 Controls the overall operation of the computer
 Communicates with the user
 Determines what the user wants
 Activates system programs, applications packages, or user
programs to carry out user requests
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Functions of an Operating System
 Five most important responsibilities of the operating
system
 User interface management
 Program scheduling and activation
 Control of access to system and files
 Efficient resource allocation
 Deadlock detection and error detection
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The User Interface
 Operating system
 Waits for a user command
 If command is legal, activates and schedules the appropriate
software package
 Types of User interfaces
 Text-oriented – DOS, Unix, Linux
 Graphical – Mac, Windows, Linux
 Hybrid – Cell phones, digital cameras, PDAs
 Embedded –Microwave ovens, cars, etc.
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System Security And Protection
 The operating system must prevent
 Unauthorized people from using the computer
 User names and passwords
 Legitimate users from accessing data or programs they
are not authorized to access
 Authorization lists
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An OS provides security, e.g. against viruses!
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Microsoft Word
The virus code overwrites
other programs in memory.
Microsoft Word
Doom
the virus code
Doom
DOS OS
DOS OS
Shareware Game
16M
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When an “infected program”
is run, the program counter
(PC)
might take the value of the
first address of the virus code.
Should this happen, the
virus code will be executed.
Memory How might the OS prevent this?
Shareware Game
Memory
Efficient Allocation Of Resources
 The operating system ensures that
 Multiple tasks of the computer may be underway at one time
 Processor is constantly busy
 Keeps a “queue” of programs that are ready to run
 Whenever processor is idle, picks a job from the queue and
assigns it to the processor
 Memory and secondary storage devices are used
efficiently.
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An OS Allocates Resources Efficiently:
 It’s easy for an OS that supports a single user running a single
program
bus
Doom
Word
Windows
Netscape
JAVA
SPSS
disk
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ALU
PC
registers
memory
CPU
Peripherals:
•monitors
•printers
•network connections
•etc.
1. User tells OS to execute program
2. OS finds the program on disk
3. OS copies the program into memory
4. OS initializes the PC to the beginning of the program
5. Program monopolizes all computer resources
This wastes computer resources
 Input and Output are very slow
 Disk and peripherals are 10,000x slower than the CPU
 The user (typing on the keyboard) is even slower!
 CPU time is wasted while waiting for them
 During this time, CPU could be executing other
programs!
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Multitasking
 Although it seems like your computer is running many
programs at once, including the OS, it is not.
 Your computer runs a bit of one program before switching to
another.
 And then switching again, often because it is waiting for I/O.
 Because your computer is very fast, it seems like they are all
running at the same time.
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The Future
 Operating systems will continue to evolve
 Possible characteristics of fifth-generation systems
 Multimedia user interfaces
 Parallel processing systems
 Completely distributed computing environments – possibly over the
internet (cloud computing)
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Summary
 System software acts as an intermediary between the
users and the hardware
 Your computer would be useless without an
operating system and system software.
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