The Booting Process 
===================

Turning on the Computer's power causes a "Cold Boot". The Processor
enters a reset state, clears all memory locations to zero, performs
a parity check of memory, and sets the CS register to segment address
FFFF[0]H and IP register to offset zero. The CS refers to Code Segment
register and IP refers to Instruction Pointer regsiter. The first 
Instruction to execute therefore, is at the address formed by CS:IP
pair, which is FFFF0H, the entry point to BIOS in ROM.

The BIOS routine beginning at FFFF0H checks various ports to identify
and initialize devices that are attached to the Computer. BIOS then
establishes two data areas:

1. An Interrupt Service Table that begins in low memory at location 0
   and contains addresses for interrupts that occur.

2. A BIOS data area beginning at location 40[0], largely concerned with
   attached devices.


			Map of Physical Memory


  Start		Address		         Purpose
  Dec		Hex		|=============================|
				|			      |
				|    64K Base System ROM      |
  960K		F0000		|                             | 
				|=============================|
				|                             |
				|    192K Memory Expansion    |
				|    area (ROM)               |  
  768K		C0000		|                             |
				|=============================|
				|                             |
				|    128K Video Display       |
				|    area (RAM)               |
  640K		A0000		|                             |  
				|=============================|
				|                             |
				|    640K Memory (RAM)        |
				|                             |
  Zero		00000		|                             |
				|=============================|


The ROM Basic Input/Output System (BIOS) begins at 768K and handles I/O
devices, such as a Hard Disk controller. ROM beginning at 960K controls
the Computer's basic functions, such as the Power-On-Self-Test, dot 
patterns for Graphics, and the Disk Self Loader. When you switch on the 
Power, ROM performs various check-outs and loads special data from disk 
in RAM.

BIOS next determines whether a disk containing the Operating System 
Files is present and if so, it accesses the Bootstrap Loader from the 
Disk. When the DOS Operating System is found in the Disk, then this 
program loads system files IO.SYS and MSDOS.SYS from disk into memory 
and transfers control to the entry point of IO.SYS, which contains 
device drivers and other hardware-specific code. IO.SYS relocates 
itself in memory and transfers control in its turn to MSDOS.SYS. This 
module initializes internal DOS tables and the DOS portion of
the interrupt table. Finally the control is passed to COMMAND.COM, 
which processes the AUTOEXEC.BAT file, displays its prompt, and 
monitors the keyboard for input.



- by Suresh (suresh_cprog@softcodez.com).
For any comments or errors please feel free to e-mail me.


DISCLAIMER:
===========
I verify the information contained in this file to the best of my 
ability, but I cannot be held responsible for any problems caused 
by use or misuse of the information.