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Operating System Concepts
Three User Interfaces
Job-Control Language (JCL)
Graphical User Interface (GUI)
Command Line Interface (CLI)
• An interactive terminal allows the system
to prompt
• What does the end user have to do?
– Then the end user types a command via a
keyboard. A well known example is MS-DOS
Command Line Interface (CLI)
• House-keeping command like…
• C:\>Copy Project. *a:
–This will cause all files named
Project to be copied to the current
folder on the A-drive
• > is the command prompt
• C:\ is the pathname for the current
• The command-line interpreter (CLI)
performs the task of identifying and
executing the command
Command Line Interface (CLI)
• Why could this type of interface be
considered the least user friendly?
– Large range of text-based commands
– Understand the syntax that goes with them
• An advantage of this type of interface is?
– Direct control of the O.S.
– There are more commands available than can
be displayed on the screen at any one time
– The user does not have to follow a sequence
of menus or windows and this saves time
Job Control Language
• Some sophisticated Op.Systems allow the user to
control the way the O.S. copes with tasks or jobs
– The user has no direct interaction with the computer
system but uses a series of instructions to alter the
priority of the tasks
• These instructions are prepared off-line using a
JCL to describe the system requirements of the
• A user’s job is executed, the execution is guided
by the JCL-prepared description and the results
are made at a later time via an off-line medium
(line printer paper)
– See chapter page 271 for an example
Graphical User Interface
• A GUI is also sometimes known as WIMP:
which means? Windows, Icons, Menu (or Mouse) and Pointer
• A GUI allows the end user to interact with
the system using windows, icons, menus
and a pointer to control the O.S.
• What can icons represent?
– Programs
– Groups of programs
– Folders
– Devices
– Files
Graphical User Interface
• How is selection achieved?
– Moving a pointer with a mouse and clicking on
a button
• What type of computer user is the GUI
more suitable for? Novice user
• Why
– The interface is more intuitive through the use
of metaphors and graphical symbols
– Only valid options are available and the layout
is consistent
– On-line help
– Command representation in applications
Graphical User Interface
• What are the disadvantages with a GUI?
– Use more immediate access store and
secondary store
– Require a more powerful processor and better
graphics display
– Slower when executing a command because
a lot more interpretation takes place
– Can be irritating to use for simple tasks
because a greater number of operations are
The Process Concept
• This is important in multi-programming
operating systems
• What type of an operating system is this?
– This is an O.S. that allows a number of
programs to appear to be running
simultaneously on one computer
• Definition: a process is a program
actually running on the CPU, even
though it might be waiting for I/O at a
particular moment
The Process Concept
• Fine! What does that mean?
– a Pascal Compiler may be executed by several
people simultaneously on a network
– There will only be one copy in memory
– Different parts of it will be used by several
different people to compile their programs
– They will share the same code
– Each instance of the program running will be a
process, with separate data areas maintained for
each execution
– A program working this way is said to be reentrant
A process is
•Process can contain one or more threads. Each thread has a
scheduling state
•OS maintains info about a process within a data structure called
a process control block
•Each thread has its own thread control block
I/O or event