Windows XP Boot Sequence

WINDOWS XP BOOT SEQUENCE

As with other Windows Operating Systems, when you turn on your PC, it goes through an elaborate boot up process. It begins when the computer performs the POST (power-on self test), followed by the POST for each adapter card that has a BIOS, for example, your video card. The BIOS then reads the MBR (Master Boot Record) which is in the first sector of the first hard disk and transfers control to the code in the MBR which is created by the XP Setup. This is where Windows takes over the startup process.
What comes next? Here's what happens:
1. The MBR reads the boot sector which is the first sector of the active partition.This sector contains the code that starts Ntldr which is the boot strap loader for Windows XP. The first role of Ntldr is to allow full memory addressing, start the file system, read boot.ini and put up the boot menu. IMPORTANT: Ntldr must be located in root folder of the active partition along with Ntdetect.com, boot.ini, bootsect.dos (for dual booting) and Ntbootdd.sys (needed with some SCSI adapters).
2. Selecting XP from the boot menu causes Ntldr to run Ntdetect.com to get information about installed hardware. Ntldr then uses the ARC path specified in the boot.ini to find the boot partition. The one where Windows XP is installed. It might look like this:
· default=multi (0)disk (0)partiton (2) \WINDOWS
· [operating systems]
· multi(0)disk(0)partiton(2)\WINDOWS="Microsoft Windows XP Home" /fastdetect
Ntldr, then, loads the two files that make up the core of XP: Ntoskrnl.exe and Hal.dll. These files must be located in the %SystemRoot%\System32 folder.
3. Ntldr reads the registry files, selects a hardware profile, control set and loads device drivers, in that order.
4. Then, Ntoskrnl.exe takes over and starts Winlogon.exe which starts Lsass.exe (Local Security Administration), this is the program that displays the Welcome screen (If Professional Edition-the Windows Log On dialog box), and allows the user to log on with his/her user name and password.



Windows 95/98--BOOT SEQUENCES
The Windows 95 boot sequence is as follows:
5. When you boot the machine, POST (Power-On Self Test) occurs just as it does for BIOS (Basic Input/Output System) that is not Plug and Play.
6. The Plug and Play(PnP) BIOS begins by looking at the hardware devices on the system and figuring out which ones are PnP compliant. The BIOS first enables the devices that are not Plug and Play, and then tries to make the PnP devices use the leftover resources.
7. The Basic Input/Output System looks for devices containing the Operating System (OS) and loads Windows 95, making information about the current allocation of resources available to the OS.
8. Just as with DOS, the Master Boot Record (MBR) executes the boot record on the hard drive, which looks for the initial hidden files of Windows 95, called IO.SYS.
9. Again, just as with DOS, IO.SYS loads. If fact, IO.SYS is really a small core DOS module. IO.SYS looks for CONFIG.SYS file, and, if found, the CONFIG.SYS file executes. The CONFIG.SYS file is not required for Windows 95. Many of its functions have been eliminated and incorporated into Windows 95. but you can use the CONFIG.SYS file to load device drivers, if you want. However, the driver loaded in CONFIG.SYS will be a 16 bit driver and it will be slower that the 32 bit drivers that Windows 95 uses.
10. After CONFIG.SYS is complete, IO.SYS searches for MSDOS.SYS. The role of MSDOS.SYS differs greatly in Windows 95 from that in DOS. In Windows 95, MSDOS.SYS is a hidden file with settings used to customize the boot process.
11. The functions of the first few entries in MSDOS.SYS look like the following:
· WinDir= (Location of the Windows 9x directory)
· WinBootDir= (Location of the Windows 9x startup files)
· HostWinBootDrv= (Drive that is the Windows boot drive)
· BootGUI= (BootGUI=1, automatic graphical startup into Windows 9x is enabled. BootGUI=0, the system boots to a command prompt)
12. Next, COMMAND.COM loads just as with DOS. COMMAND.COM is used to provide a command interface for users and to execute an AUTOEXEC.BAT file, if it is present.
13. If AUTOEXEC.BAT is found, it now executes.
14. The heart of Windows 95 now loads, providing a desktop from which you can execute application software.
WINDOWS 98 BOOT SEQUENCE:
Just as with Windows 95, Windows 98 goes through a boot sequence in the order listed above.
1. BIOS runs POST
2. BIOS loads a small DOS core
3. The DOS core loads Windows 98
Windows 98 has made some some minor changes in what happens during startup, in order to speed up the boot process.
· Windows 95 waits 2 seconds, while "Starting Windows 95" is displayed so that you can press a key to alter the boot process.
· Windows 98 eliminated the 2 second wait and, in its place, allows you to hold down the CTRL key as it loads. If you do that, you will see the Startup Menu display.


WHAT IS THE DOS BOOT SEQUENCE?
WHAT IS THE DOS BOOT SEQUENCE??
There is a short answer to this and much longer explanation on this subject.
First, the short one. The computer runs the Power On Self Test (POST)which checks BIOS,CPU, RAM, Video, Keyboard, drives, etc. Once POST is completed, the next step is to load the Operating System (OS). Most often, the OS is loaded from the hard drive. The minimum information required on the hard drive is a small program at the very beginning called the Master Boot Program that is needed to locate the beginning of the OS on the drive. Next, it needs the Partition Table. At the beginning of the boot drive the DOS boot record that loads the first program file of the OS. This is IO.SYS. MSDOS.SYS is needed next, followed by the COMMAND.COM. These two files plus the IO.SYS are the core components of DOS. It briefly looks like this:
POST-->OS-->MBR-->Partition Table-->DOS Boot Record (IO.SYS)-->MSDOS.SYS-->COMMAND.COM.
This is the longer version of what happens when DOS is loaded as the OS.
15. BIOS locates the Master Boot Record (MBR) on the hard drive.
16. The partition table find the physical location of the logical boot drive and turns to the boot record of that logical drive
17. The boot record (a very short program) loads two hidden files into memory. These files are IO.SYS and MSDOS.SYS
· The IO.SYSfile contains more BIOS software
· The MSDOS.SYScontains software to manage files, run applications software and interface with hardware
18. Once these two files are loaded, the boot record program is longer needed and turns control over to a file stored on MSDOS.SYS
19. This program looks on the hard drive for a file named CONFIG.SYS. This is the first OS file that you, the user, can change. This file contains commands that tell DOS how many files it can open at any one time (FILE=) and how many file buffers (a temporary holding area for a file) to create (BUFFERS=). It also contains the commands to load device drivers (small programs that tell your computer how to communicate with devices such as printers) (DRIVERS=) and other information. Several drivers can be loaded into memory and CONFIG.SYS puts them anywhere it wants unless the program requests a certain memory location.
20. When CONFIG.SYS is done, MSDOS looks for another file called COMMAND.COM. This file consists of 3 parts: more code to manage Input/Output (I/O), internal DOS commands such as COPY and DIR, and a short program that looks for AUTOEXEC.BAT.
21. AUTOEXEC.BAT stands for "automatically executed batch" program. This file holds a list of DOS commands that are automatically executed each time DOS loads. Two of these commands are:
· PROMPT $P$G this instructs DOS to display the current directory name and the current drive name as part of the prompt. C:\Windows instead of C:>.
· PATH Tells DOS where to look for program files. Example: C:\Windows\Driver Cache\Fonts
· AUTOEXEC.BAT also loads TSRs (terminate and stay resident programs).
22. The boot process is completed after AUTOEXEC.BAT has finshed executing. At this point, COMMAND.COM is in charge and you have the command Prompt (C:>).