Computer Hardware PC Operating Systems

What is an operating system? An OS is the interface between the user and the computer hardware It provides the user and programmer with standard routines to access the hardware without resorting to machine language. This is true for a command driven O/S or GUI

What does an O/S provide? Command line operating systems allow the user to type in a command to initiate a task e.g.  The ability to type in a program name to run it  The ability to write “batch files” to automate a sequence of tasks  Dir command to view the contents of disk drives  Copy command to copy files from one location to another  Type command to view the contents of text files  Ren command to rename files  Ability to use “switches” to modify the behaviour of commands  Some commands are internal (in memory) and some are external programs (stored on the boot disk) e.g. “DIR” is an internal command stored in memory (RAM) “FDISK” is an external command used to configure hard disks for use

Graphical Operating Systems The most familiar graphical systems are Microsoft Windows and the Macintosh OS  They perform the same tasks as the command line system but via a different user interface e.g.  Point and double click an “icon” to run an application  Open a “window” to view the contents of a disk drive Essentially they both do the same job… The major benefit with graphical systems is that they can run multiple programs concurrently

Common Operating Systems MSDOS – command driven MS Windows - GUI UNIX - command driven (inc Solaris GUI) Linux – GUI/cmd, a “variant” of Unix IBM OS/2 - GUI

MSDOS Developed by Microsoft on license from IBM for their IBM “PC” in 1981 Acronym for Micro Soft Disk Operating System

Multi Tasking OS Multi tasking is the term for running multiple applications concurrently We now take for granted that under Windows a user can run many applications concurrently and this was a major selling point when Windows was initially released MSDOS was not a good multi tasking environment but some third party developers such as Quarterdeck allowed a crude form of multitasking under DOS which needed additional “expanded” memory which was very expensive Unix allows multitasking and was the mainstay of mainframe computers in the 1980’s

The Graphical User Interface In the late 1980’s the IBM PC relied on the MSDOS O/S and a command prompt The Apple Macintosh on the other hand had a graphical user interface (GUI) which was much more user friendly With the advent of and based PC’s the Windows environment became a reality as this processor could now address far more memory than it’s predecessors (the could only address 16Mb!!)

Device Drivers for the PC Device drivers are an interface between the physical hardware and the O/S. The driver software is normally written and supplied by the hardware vendors DOS and WINDOWS rely on these drivers to communicate with the many devices now available (printers, modems, cameras etc) 16 bit DOS device drivers (real mode) 32 bit native windows drivers

Editable MSDOS Config files Autoexec.bat - exampleexample Config.sys - exampleexample MSDOS.SYS (hidden) - exampleexample

Windows configuration files Early Windows such as 3.0 and 3.11 relied on MSDOS drivers and configuration files Windows 95 and later does not rely on the DOS config files so heavily and uses native 32bit Windows drivers when available O/S and application settings are stored in the Windows Registry (introduced in 3.11) which is accessed using regedit.exe  This is made up of two physical files for Windows 95-ME SYSTEM.DAT USER.DAT  NTUSER.DAT for Windows NT/2000/XP Windows ME copies any config settings to the registry but still relies on MSDOS In Windows NT, 2000 and XP the MSDOS config files are ignored altogether.

Regedit.exe

Windows NT/2000 Is not based on MSDOS like Windows 95/98 and even ME… Does not use autoexec.bat and config.sys and ignores them at bootup Does use BOOT.INI to find out where the O/S has been installed, boot.ini is also used in dual boot NT systemsBOOT.INI

PC File Systems O/S FAT16FAT32NTFS DOS <=6.22  DOS >6.22  Windows 98/ME  Windows NT4  Windows 2000  Windows XP 

MS DOS - Boot Sequence The BIOS (Basic Input / Output System) performs a system check and configures logical (COM and LPT ports) and plug-and-play (PnP) devices. The BIOS searches for a disk with a boot sector and loads that boot sector's volume boot code. (All formatted hard disks, floppy disks, and Zip and Jaz disks and some CD-ROMs have boot sectors, but not all disks contain operating system files.) The volume boot code examines the structures on the disk it is booting. If there is a problem, the boot process ends in an error. The volume boot code searches the root directory of the disk being booted for operating system files (for DOS these are IO.SYS, MSDOS.SYS, and COMMAND.COM). If OS files are not found the volume boot code displays the error message "Non-system disk or disk error--Replace and press any key when ready." (At this point the computer has already loaded and executed any code in the boot sector, even if the disk contains no operating system.) If OS files are found, the volume boot code transfers control to the OS (DOS). DOS processes the Windows registry, which sets up the system configuration for Windows. DOS runs CONFIG.SYS, which sets up the system configuration for DOS. DOS executes the commands in AUTOEXEC.BAT (erasing / setting temp directory, etc.). DOS starts Windows.

Windows NT/2000 Boot Files The initial Power On Self Test (POST) initialises hardware components then the system BIOS identifies the boot device. Typically, this is a floppy disk or a hard disk. In the case of the hard disk, the BIOS reads the first physical sector on the disk, called the Master Boot Sector, and loads an image of it into memory. The BIOS then transfers execution to that image of the Master Boot Sector then called the Master Boot Record (MBR). The Master Boot Record contains the partition table and a small amount of executable code. The executable code examines the partition table and identifies the active (or bootable) partition. The Master Boot Record then finds the active partition's starting location on the disk and loads an image of its first sector, called the Boot Sector, into memory. The Master Boot Record then transfers execution to that Boot Sector image. The Boot Sector is responsible for locating the executable file, NTLDR.EXE, which continues the boot process. The following files are essential to boot windows NT:  NTLDR.EXE – starts the boot process and reads BOOT.INI to build a boot loader menu  NTLDR.EXE then loads the OS that the user chooses  NTDETECT.COM – runs if the user chooses Windows NT/2000 and examines available hardware then builds a hardware list  NTLDR.EXE runs NTOSKRNL.EXE to begin the Windows NT load phases that initialise NT OTHER FILES  NTBOOTDD.SYS - (Used if you are using SCSI disks with the SCSI BIOS disabled)  BOOTSECT.DOS – (Used if the user chooses another OS other than Windows NT

Installing MSDOS Installation of DOS starts with the preparation of the hard disk using the FDISK utility. The PC is booted first from a (bootable) floppy disk Once the hard disk is partitioned, it is formatted and the necessary boot files are installed on it in the root directory All other external DOS command files and drivers are normally stored in the C:\DOS directory Any configurations to the settings are made within the config.sys and autoexec.bat files (e.g. you may want to run a menu system when the PC boots)

Installing Windows 98/ME All versions of Windows other than NT rely on MSDOS Therefore DOS needs to be installed first then Windows afterwards Later versions can be installed from a bootable CD without the need for DOS installation, it is more user friendly but still installs DOS files such as COMMAND.COM, IO.SYS and MSDOS.SYS Command line files are now stored in the C:\WINDOWS\COMMAND directory

Installing Windows NT/2000/XP The major difference is that NT and others based on NT do not rely on DOS although can run DOS programs To install, MS supply an install boot floppy disk set (4 disks) This boots to an NT environment and sets up your hard disk (like FDISK) then copies the necessary setup files to it The Windows directory is named WINNT instead of WINDOWS (except for XP which uses WINDOWS as default) Command.com has now gone and any command prompt commands such as DIR and COPY are provided from CMD.EXE in the C:\WINNT\system32 directory which sets up a “pseudo” DOS environment All hard disk operations such as partitioning and formatting are performed within the GUI using disk manager

Microsoft Windows ® Early versions of Windows were semi graphical/text based systems Windows 3.0 was the first version to get the industry interested in using Windows rather than DOS Windows 3.11 (for workgroups) fixed many bugs, was more stable and added networking features

MS Windows ® versions Windows 286 – 1988/89 Windows Windows 3.1, 3.11 (for workgroups) – 1991/92 Windows 95 – 1994/95 Windows 98, 98SE /98 Windows ME – 1999/2000/2001 Windows NT 3.51, 4.0 / Workstation Windows 2000 Pro / Server – 1999/2000/2001 Windows XP – Oct 2001/2002

Windows XP Desktop

XP “Classic” Start Menu