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Summary
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The Power Supply is
Switched On.
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The power supply will
performs a self-test.
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The signal PWR_OK is
sent to the Mobo.
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The CPU starts
executing the ROM BIOS code at address FFFF:0000
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A jump instruction
then points to the actual address of the ROM BIOS code.
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The ROM BIOS performs
a basic test of hardware to verify that basic functionality is
indeed present.
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The ROM BIOS Code
will "initialize only" several core motherboard components and
peripherals.
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The BIOS then
searches for adapters that may need to load their own ROM BIOS
routines
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The ROM BIOS then
executes what is called a full POST (Power On Self Test).
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Next the BIOS Checks
the Boot Order, Defined by You in the CMOS.
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Having identified
its target boot drive, via the CMOS settings, the BIOS looks for
boot information to start the
operating
system
boot process.
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It looks for a
master boot record (MBR) at cylinder 0, head 0, sector 1 (the
first sector on the disk).
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At this point, the
code in the boot sector of the hard drive takes over from the
BIOS.
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The partition loader
(or Boot Loader) examines the partition table
for a partition marked as active indicated by the boot flag.
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The
partition loader then searches that very first sector of that
partition for a
Boot Record.
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The
Boot Record is also 512 bytes and it too contains a table that
describes the characteristics of that partition
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The
Boot Record also contains the jump code that locates the
beginning of the operating system files.
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The
BIOS reads the first physical sector ("Sector 0") on the active
disk, called the Master Boot Sector, and loads
an image of it into
memory.
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The BIOS then
transfers execution to that image of the Master Boot Sector on
that disk.
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This program locates
the NTLDR file on the active partition and passes control to it.
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The loading of
Windows OS is then controlled by the file NTLDR.
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NTLDR will continue
to load XP. Control is now passed to NTLDR.
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NTLDR is composed of
two modules: The StartUp module and the OS Loader module.
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The StartUp module
switches the CPU from Real Mode to Protected mode.
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The StartUp module
then loads and launches OS loader.
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OS loader includes
basic functionality to access disks formatted with a recognized
file system.
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The boot loader then
reads the contents of the boot.ini file to locate even more
information about the system volume.
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NTLDR Now runs a
Program Called NTDetect.exe
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NTdetect.com gathers
basic information about the computer's hardware as reported by
the BIOS.
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This information is
then passed onto Ntoskrnl.exe, The Windows Kernel.
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The Windows kernel
Ntoskrnl.exe and the Hardware Abstraction Layer ( hal.dll ) are
read into memory.
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With the kernel in
memory, boot-time device drivers are loaded (but not yet
initialized).
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This information
(along with information on all detected hardware and Windows
Services) is stored in the registry ControlSet
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All drivers are now
fully loaded.
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The Windows Logo
Screen – Finally
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Once all the Boot
and System drivers have been loaded, the kernel starts the
Session Mgr Subsystem.
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Win32
subsystem which switches Windows into graphical mode.
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Creates virtual
memory paging files.
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Starts the Windows
Logon Manager (winlogon.exe).
