Your most important program:

  O p e r a t i n g   S y s t e m s  

CPT 105 · Franklin College · Erich Prisner · 2002-2007


Operating systems

operating systems may be single task or multi-tasking, and also single user(Windows, Mac OS) or multi-user (unix, and mainframe OSs). Between are single-user networking OSs like Windows 2000 or Novell.


BIOS (basic input/output system): It checks, displays some information, and loads the operating system from hard disk or Floppy/CD if you start your computer. Stored as Flash memory or ROM on the motherboard.

You may change entries in your BIOS, but be VERY CAREFUL there!

Different brands:

(late) TOS (for Atari)

This was my first computer. The Atari computers died when Windows 95 got popular.

released in the mid 80s, they had it all: Graphical interface, Icons, Multi-Windowing, Drop-down menus,... like the Macs, unlike the Windows system at that time.


This single-user OS has a command line UI. It was developed in 1979 by Tim Paterson, purchased by Microsoft, and released in 1981 for IBM computers.

You can still see it if you click on "Start-Programs-Accessories-Command Prompt". Try the following commands:


started in the mid-eighties, as an attempt to create a GUI. multi-tasking was possible

Windows 3.1

early nineties. The first really successfull Windows variant.

Windows 95 and 98

The breakthrough for Windows was Windows 95 (sold more than 1 Million copies within 4 days!), at the same time when the internet got important over night. Windows 3.1 was just a "shell" between MS-DOS and the user, only Windows 95 got rid of MS-DOS.

Windows Me

More multimedia and home networking support.

Windows NT and 2000

business version

Windows XP

combines the 95-98-ME and the NT-2000 lines.

Windows CE

operating system for pocket systems.


This is the IBM successor to MS-DOS.

Mac Operating System

introduced in 1984 for the Apple/Macintosh platform. First and, for a long time, best GUI.


created in 1971 at Bell Labs. Used at many universities. Command-line user interface. Processor type independent. Unix is written in the language C


created by a student, Linus Thorvals", from 1991 - 1994. Very reliable, very secure. Open source software: It's free, and you can make changes (if you can). Originally command-line interface, but GUIs are also available. But there are not as many programs running under Linux than under Windows.

Resource Allocation of OSs

Sharing the CPU

Multiprocessing means handling several processors. Only large computers have several.

Otherwise we are talking about multitasking/ multiprogramming. The OS schedules the different processes/threads to run on the CPU concurrently. Each application may have several processes active at a time, like if you are downloading a file but still browsing on. The CPU deals with only one process at a time, but since the processes treated alternate so quickly, it looks like simultaneous processing.

Interrupts are special signals send to the CPU, calling for attention (and quick access to the CPU). Otherwise, processes take (very short) turns on the CPU, but the rules for that depend on the OS. There may be priorities. Each time the process changes, the registers have also to be copied and cleared.

Time sharing is a case of multiprogramming where fixed time slices are assigned to the different processes.

the response time is the time between your request and the computerīs response. It depends on the number of "parallel" processes.

Thrashing means that most of the time the OS only puts information from and to CPU and doesnīt have time to "compute".

Sharing Memory

Each application running needs a separate part of the memory. If there is not enough memory, part of the data/programs is stored on hard disk. This is called virtual memory management.

Sharing Devices

for printer, spooling is used: When programs send their output to the printer, the OS catches it and stores it to a disk file. Only when the program has finished printing, the file is sent to the printer. Advantage: No scrambled lines for different users (or programs). Spooling takes also care of the slow connection speed to printer.

A driver is a special program used for communication between OS and all hardware not on the motherboard. It translates.

Sharing Storage Resources

Keeps track of files. More complicated if several users (or programs) require the same file or at least the same storage.