The Operating System

What is an Operating System? The program that is loaded first and manages the hardware resources like main memory, backing storage and peripherals and communicates with the user. or or All of the other programs on your computer use the services that the Operating System supplies.

Starting Up Your Computer Power on Bootstrap loader program (BIOS) is run from ROM on the motherboard BIOS (Basic Input Output System) uses the MBR (Master Boot Record) to locate and load the rest of the Operating System Operating System is now loaded into main memory

What are the Functions of An Operating System? interpreting users commands file management memory management input/output management resource allocation managing processes Operating System Hardware Programs

Interpreting Users Commands Operating System Hardware Programs 1.The user moves the pointer and double clicks the Microsoft Word Icon using the mouse. The OS uses a mouse driver to understand the signals from this input device. 2. The Graphical User Interface then sends a command to open the program Microsoft Word 3. The File Management System then locates the program and loads it from the Hard Disk 4. The program is located and loaded into main memory by the memory manager 5. The program is then allocated processing time on the CPU by the process manager 6. The GUI draws the programs interface and outputs it to the monitor using the graphics card driver

The Evolution of Microsofts OS Command Line Interface Menu Driven InterfaceGraphical User Interface

File Management System

Indexed Allocation Table , 20, 17, 13 Index File CS Test.doc

Why Files Can Be Recovered Despite Being Deleted , 22, 17, 14 Index File CS Test.doc 1 Index File 1 1

Memory Management The operating system has to decide how much main memory to allocate to a program. The OS also needs to make sure that one program doesn’t accidentally use the memory allocated to another program. When a program does try to use memory set aside for the operating system, or another program, it usually crashes.

The Case of Hard Disk Thrashing and the Mysterious Swap File In a 32-bit PC the Operating System only has access to 3Gb of physical memory If the Operating System needs more memory it has to cheat and store some of the instructions and data on part of the hard disk instead When the user swaps to a program stored in virtual memory the hard disk has to load the program back into physical memory and swap out another program to the hard disk

Input/Output Management I/O Manager Keyboard Device Driver Graphics Card Device Driver Graphics Tablet Device Driver Rest of the Operating System Provides a way of controlling hardware devices connected to the computer Adding a new device means a new driver has to be installed before it will work

Who’ll Decide? The Kernel Will! The kernel schedules processor time and allocates computer resources to a process running on the machine If you press Ctr+Alt+delete at home you will see the work the kernel does in the Task Manager program.

Managing Processes and Resource Allocation