CIS 106a: Introduction to Operating System Concepts

Chapter 1: Introducing Operating Systems



This chapter discusses operating systems. The objectives important to this chapter are:

  1. Understanding operating systems
  2. Understanding user interfaces
  3. Understanding the operating system tools
  4. Recognizing some major operating systems


As with the previous chapter, I will concentrate here on the most important features of the chapter.

Operating System software is defined as software "related to controlling the operations of the computer hardware". To the beginner, that sounds like a definition of all software. You need to know that some programs, like applications, only deal with their own concerns (such as processing data into a report). Such programs must interface with the system software to save files, to print, to display data, to access memory, and to perform any other function involving hardware.

A brief discussion of several operating systems follows. You will want know something about most of them, since an operating system is choice you make when purchasing a computer:

  • DOS - stands for Disk Operating System. Several versions have existed, from various vendors, notably Microsoft and IBM. This is an older system that does not support a 32 bit graphic environment. DOS was used as the basic operating system for most of the early versions of Windows. (See below)
  • Windows 3.x - this refers to versions 3.0, 3.1 and 3.11 of Windows. These products were not really operating systems. They were actually Graphical User Interfaces to the DOS operating system.
  • Windows 9x and Me
    • Windows 95 - this version of Windows is more of an operating system, but it is still an interface because it requires some DOS components be loaded before it is loaded.
    • Windows 98 - this version of Windows is a bit less reliant on DOS than Windows 95, but again, it is not truly an operating system.
    • Windows Me (Millennium Edition) - Few people liked this version of Windows. For our purposes, know that it still ran on top of a DOS layer.
  • Windows CE - this is a true operating system, but it is meant for handheld devices, not desktop or laptop computers.
  • Windows NT - this was the first version of Windows that was truly an operating system for desktops, laptops, and servers. The workstation and server versions were marketed as separate software.
  • Windows 2000 - more stable than NT, plug-and-play support introduced, supported Active Directory (a network database system for managing users and computers) When users need a command line interface, they start a DOS simulator program.
  • Windows XP - introduced multiple logons to the same computer (user profiles), introduced Windows Firewall
  • Windows Vista - current version of Windows (2007-2008), higher hardware requirements, many interface changes
  • Windows Server 2003 - several versions available (to be replaced by Server 2008)
    • Windows Small Business Server 2003
    • Storage Server 2003
    • Server 2003 Web Edition
    • Server 2003 Standard Edition
    • Server 2003 Enterprise Edition
    • Server 2003 Datacenter Edition
  • Macintosh - a proprietary system, because it has only been used on Apple Macintosh computers for most of its life. Several versions have been released. OS-X (version 10 of the Mac OS) can run on some Intel computers.
  • OS/2 - an IBM product, developed with Microsoft. This system could be run on many kinds of machines. It was not a marketing success.
  • UNIX - various versions exist. The original was invented at Bell Labs. Many people are promoting one version or another as a replacement for Windows.
  • Linux - started as a free version of UNIX, now marketed by several vendors as well, notably:
  • NetWare - this is a network operating system created by Novell. The operating system runs on servers, a client interface runs on workstations in the network. It has been a widely used system, and it supports workstations that run any of the operating systems listed above. Windows is used on more corporate networks now.

The text reminds us that all operating systems must provide four common features:

  • a user interface
  • file management
  • application management
  • hardware management

Operating systems can be discussed in terms of major components:

  • the Shell - provides the user interface, and an interface for applications to access the operating system
  • the Kernel - provides the interface between the operating system and hardware
  • Configuration settings - in newer versions of Windows, configuration settings for applications, and Windows itself, are kept in the Registry database. Previously, they were kept in separate files.

Operating systems can be grouped in terms of the interfaces they offer to users:

  • command line interface - user must type commands
  • menu-driven interface - user may choose options from menus, usually with a keyboard
  • graphical user interface - user may use a mouse to click buttons and icons

The menu-driven interfaces often allow a user to enter typed commands. The graphical interfaces often allow the user to use menus and typed commands as well.

All operating systems include file systems. Without a file system, there would be no way to store information on a floppy disk, a hard drive, an optical drive, or a USB connected device. File systems provide a means to make folders (also called directories) and subfolders (also called subdirectories) to organized files. This organization is for the user's convenience, but it is also for programs which need to find their own files in specific folders.

In order for disks to store data, there has to be a logical system of organizing the data. Disks are divided into tracks, which are concentric circles. Floppy disks typically have 80 tracks per side. Floppy drives read and write to the tracks with magnetic read/write heads (like in a tape recorder or VCR). The heads take turns, writing to one side then the other, so the user does not have to be aware that both sides are used (and does not have to flip the disk over, as I had to with my first computer: an Apple IIc). Floppy drives rotate at about 360 rpm (revolutions per minute), and the read/write heads are meant to make light contact with the disk. This is part of what causes a floppy disk to wear out after about 40 hours of actual use.

  • Red sector
  • Blue sector
  • Track
The disks are further divided into pie-shaped wedges called sectors. (Why is that word red? You'll see in a few lines.) Again, note in the chart that the number of sectors on a disk varies from type to type. Our common example, the 1.44 MB 3.5 inch disk has 18 sectors on each side. (The number of sectors is always the same on each side of any disk.) Now for the confusing part: the word "sector" has another meaning. Sector can mean one of the wedges on a disk, but it can also mean the part of a track inside that wedge. If we consider that a 1.44 MB disk has 18 sectors on a side, and that it has 80 tracks on that side, then those tracks are divided by the sector lines into 1,440 track segments that are also called sectors. It will become tedious if I continue using colors for that word, and it will be unrealistic: the real world is not color coded. You have to understand which meaning of the word is intended from the context in which it is used, just like any other oddity in English.

(In the picture on the right, the number of wedges has been reduced to make the image clearer. The picture is a metaphor, not an exact representation.)

Something that does not vary, at present, is that a sector can only hold 512 bytes of data, no matter what kind of disk is being used. (Okay, one more time. I made it blue so you would know I meant "a segment of a specific track".) Since there are two sides to that disk, that means it can hold 512 bytes per sector, times 2 sides, times 1,440 sectors per side, making 1,440 Kilobytes, which is 1.44 Megabytes. (In this paragraph, every time I used the word "sector", I meant "a segment of a specific track".)

The next major concept is the cluster, sometimes also called a file allocation unit. Think of sectors and tracks as being physical aspects of a disk that are dealt with by the BIOS. Think of clusters as being logical aspects of a disk that are dealt with by the Operating System. A cluster is defined as the smallest unit on a disk that the operating system can read from or write to at one time. Again, the chart above is helpful. For the type of disk we are discussing (1.44 MB), the smallest amount of data that can be written to the disk (a cluster) is the size of one sector. For the other types, it varies between one sector and two sectors.

On computers that run Windows, a hard drive keeps track of the location of data with a directory listing and a FAT table, just like a floppy disk. A hard drive can have two (redundant copies) of several kinds of FAT tables (FAT stands for File Allocation Table. Redundant, isn't it?):

  • Floppy disks use 12 bit FAT tables (FAT12)
  • DOS and every version of Windows since 3.1 can use 16 bit FAT tables (FAT16)
  • Windows 95, version 2, and later can use 32 bit FAT tables (FAT32), which are larger and can use larger addresses. Windows 2000 and XP can use FAT32, but only if the drive is no larger than 32 GB.
  • Windows NT can use any of these, or NT File System (NTFS) tables which are larger still, using the most addresses. Windows 2000 and XP also prefer NTFS.
  • FAT 16 is usable for hard drives up to 2 or 4 GB (see below) in size, but there is a penalty for large drives. The larger the drive, the larger the cluster size for any FAT table.
  • FAT 32 is better organized, and is practical for drives up to 8 GB, where its cluster size is about 4 KB. From 8 GB to 16 GB, the cluster size goes up to about 8 KB per cluster.
  • NTFS allows the use of much larger hard drives.

This is a chart of cluster sizes for various logical drive sizes using FAT12, FAT16, FAT32, and NTFS. For another view of the same data, see this page at the Microsoft Help and Support site.

DEFAULT Cluster Sizes, from Microsoft Help and Support site
FAT Type Logical Drive Size Cluster Size
FAT12 360 KB 2 sectors = 1KB
  720 KB 2 sectors = 1KB
  1.2 MB 1 sector = 512 bytes
  1.44 MB 1 sector = 512 bytes
  < 1 MB to 15 MB 8 sectors = 4 KB
FAT16 16 MB to 127 MB 4 sectors = 2 KB
128 MB to 255 MB 8 sectors = 4 KB
256 MB to 511 MB 16 sectors = 8 KB
512 MB to 1023 MB 32 sectors = 16 KB
1 GB to 2 GB (limit for DOS and Windows 9x) 64 sectors = 32 KB
2 GB to 4 GB (must be using NT, 2000, or XP) 128 sectors = 64 KB
FAT32 257 MB to 8 GB 8 sectors = 4 KB
8 GB to 16 GB 16 sectors = 8 KB
16 GB to 32 GB (limit for Windows 2000 and XP) 32 sectors = 16 KB
NTFS Up to 512 MB 8 sectors = 4 KB
512 MB to 1 GB 8 sectors = 4 KB
2 GB to 2 TB 8 sectors = 4 KB
2 TB to 16 TB 8 sectors = 4 KB
16 TB to 32 TB 16 sectors = 8 KB
32 TB to 64 TB 32 sectors = 16 KB
64 TB to 128 TB 64 sectors = 32 KB
128 TB to 256 TB 128 sectors = 64 KB
More than 256 TB not supported

You may wonder why we don't just use NTFS on all large hard drives. It is because NTFS is not available unless you are running Windows NT, 2000, XP, or later versions of Windows. We can consider a file system to be inefficient if it uses more than 8 sectors for a cluster size. By this measurement, FAT16 is only efficient for hard drives up to 256 MB, and FAT32 is only efficient for hard drives up to 8 GB.

After a drive is installed, you may have to partition it. The FDISK utility in DOS or Windows 9x is used for this. As you may know, FDISK is also used to assign drive letters to the disk. You should be aware that the system will boot from the primary partition, which is why it must be set as active. The primary partition cannot contain more than one drive letter, but Windows 2000 and XP will allow up to four primary partitions on a single hard drive. If more drive letters are desired, they must be assigned in an extended partition.

An operating system manages applications run by the user. The operating system provides memory and processor time to the applications, tracks their installation and removal, and provides access to system hardware. Five tasks performed when installing a program under Windows are listed:

  • file folders are created
  • files are copied or expanded to the folders
  • Registry data is saved about the program
  • Icons may be added to the desktop
  • menu items are added to the Start menu

Once software is installed in on a computer running Windows, you can run it several ways, including:

  • icons can be double-clicked (an icon is most often a shortcut to the program file itself)
  • a menu item under Start can be selected
  • you can click Start, Run, and enter the path to the main executable file
  • you can find the main executable file with Windows Explorer or My Computer, and double-click it

Software can run in several modes under Windows. The modes can be referred to by their names and by the number of bits each mode is capable of processing at the same time:

  • Real Mode (16-bit) - DOS used real mode, current Windows programs should not. Programs were given direct access to hardware, which led to crashes caused by programs trying to access the same resource at the same time
  • Protected Mode (32-bit) - the operating system works as a interface between the programs and the hardware
  • Long Mode (64-bit) - the operating system works as a interface between the programs and the hardware

Confusingly, programs can be written to process 16, 32, or 64 bits at a time, regardless of the mode they may be run in:

  • 16 bit programs were typically written for DOS or Windows 3.x
  • 32 bit programs were typically written for Windows NT, 2000, or XP (The text tells us that a program will typically run under only one operating system. Be aware, however, that sometimes programs are written to run on several similar operating systems.)
  • 64 bit programs are written to run under Windows XP Professional 64-bit Edition (or other 64 bit OSs)

This leads to a discussion of process management. A process is a single task a computer is trying to fulfill. Computer operating systems can approach tasks in a number of ways. Two broad categories are:

  • single tasking - the operating system can only do one thing at a time
  • multitasking - the operating system loads more than one task at a time. Often, this is an illusion. In cooperative multitasking, several tasks are loaded into memory and the processor give a few nanoseconds to each one in turn until they are done. In preemptive multitasking, the same thing happens, except that more important tasks are given more time than less important tasks.

Multiprocessing is mentioned. An operating system that supports multiple processors can do real multitasking, assigning different tasks to different processors.

As the operating system provides an interface between the user and the hardware, drivers are programs that provide interfaces between the operating system and hardware. When you install new hardware, it typically comes with driver programs to install for various operating systems. Drivers may be obtained from hardware manufacturer web sites, and from operating system publisher web sites. Like applications, drivers may be 16, 32, or 64 bit programs.

Another way to manage access to hardware is through the system BIOS (Basic Input Output System). A keyboard is an example of a device that is made available through the BIOS. The disadvantage to doing so is that devices managed through the BIOS run more slowly than devices managed by drivers, so keyboards are typically made available when you start a computer by the BIOS, but a driver is loaded in Windows to provide better, faster access.

You should be aware that Windows provides several tools to examine your system and its performance.

  • The Windows Desktop - be aware of the messages that appear on the Task Bar and what the author calls the System Tray. (Actually, most of the world calls it the System Tray. It is the area in your task bar where the time is shown, along with icons for programs and trouble messages about them. Microsoft calls this the Notification Area. No one else does.)
  • My Computer and Windows Explorer - folders and files can be managed through these interfaces
  • System Properties - access this through the Properties of My Computer, or through the System program in Control Panel. Several critical function are accessed from here. You can:
    • View processor and memory information
    • Change the name of the computer on the network
    • Access Device Manager on the Hardware tab
    • Control performance on the Advanced tab
    • Switch System Restore on or off
    • Use the Automatic Update tab to control updating
  • Control Panel - just as the name implies, it provides controls for many operating system features. Be aware of the two views of the Control Panel: Classic and Category View.
  • Device Manager - a graphical diagnostic aid that shows yellow icons for hardware that is malfunctioning and red icons for hardware that is disabled
  • System Information - gives you detailed information about installed components. Click Start, Run, then enter msinfo32.exe (You don't have to type the .exe part.)
  • Windows Help and the Microsoft Web Site - when wondering what to do with a problem in Windows, try both of these sources. The built-in help in Windows often checks online resources from Microsoft when you have access to the Internet.

For some reason, the chapter fades into notes about common keyboard commands in Microsoft compliant programs. You should be familiar with several of them:

  • Ctrl-x - cut the selected material to the clipboard
  • Ctrl-c - copy the selected material to the clipboard
  • Ctrl-v - insert whatever is on the clipboard at the current cursor position
  • Ctrl-z - a one-level undo (can also be activated by alt-backspace)