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Operating System Concepts Three User Interfaces Command-line 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 folder • 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 task • 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 required 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 dynamic •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 Runnable Time-slice exceeded Event completed Running Suspended I/O or event request