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SYSTEM SECURITY
OBJECT:TO DESIGN AND DEVELOPED A PASSWORD ROUTINE WHICH ACTIVATE AT THE START OF THE
BOOTING OF A COMPUTER.
BRAIN DEBUGGERS
Department of Computer Science
University of Karachi-Pakistan
Brain.debuggers@usa.net
Dos Boot Process
When a PC is turned on, the CPU begins executing
instructions from a location fixed when the chip was designed. i.e during the Power up
sequence, the 8086/8088 microprocessor, by virtue of its design, automatically executes
the instruction located at physical address FFFF0h, as in the table mention below:
STATE OF THE 8068/8088 ON POWER ON OR RESET
COMPONENTS STATUS
Flags All cleared
IP register 0000h
CS register FFFFh
DS register 0000h
SS register 0000h
ES register 0000h
At FFFF0h, IBM BIOS contain a jump instructon to
a routine that performs several hardware tests. When these test conclude the SOFT RESET
routine checks for a diskette in drive number 0 and, if present, read the diskettes side
0, track 0, and sector no 1 (called the Boot Sector) into memory at logical address
0000:7c00h.
Finally Boot transfers execution to the instruction location at the
address, upto this part the programmer cannot interfere with the power-up sequence, that
of replacing the BIOS.
ROM BOOTSTRAP LOADER:
The Rom Bootstrap loader is the heart of the boot
sequence. After peripherals identified in Rom Bios have been identification tested and
initialization the Boot Sector and system files are loaded into memory at physical address
0000:7c00h
By this ROM Bootstrap Loader. These programmes are too large or too
variable (depending upon the System Configuration) or too infrequently used to be included
in ROM. Since the operating System is not yet available, the loader of the boot programme
must be done at a very basic level only what is known about the disk in ROM BIOS can be
used.
Boot Programme:
The boot sector contains information about the system and
disk organization (BPB) as well as that programmme (the boot program or disk bootstrap
loader). The boot program uses BIOS service available in ROM and information from the BPB
to load the IO.SYS file, which contains the additional BIOS routine and the sys files
modules that controls the rest of the system restart.
IO.SYS
It contains various BIOS routine not supplied with ROM, usually
unique to a particular manufacturer or system configuration. IO.SYS contains sysinit,
which relocates MSDOS.SYS the DOS Kernel to its final location. Bootstrap loader loads
IO.SYS, which in turns loads MSDOS.SYS.
MSDOS.SYS
It comprises the Dos kernel routine providing the essential Dos
service that remains memory resident.
CUSTOMIZING BOOT STRAP LOADER
Naturally the boot record on the diskeete
corresponding to the MSDOS operating system, which takes over execution at this time. For
many applications that runs under MSDOS this sequence of events in quite convinience. The
programme manual for the application usually includes instructions for incorporating DOS
into the program or vice versa. Boot circumstances arise. That requires by passing this
conventional sequence of operation in situations like the following:
- In creating system program that receive control directly from BIOS, as would be a new
DOS .
- In executing programs do not requires the services provided by MS DOS or that can profit
from not considering with MS DOS in one or more of the following ways.
- Greater diskette stroage space
- Greater RAM avaibility
- Greater software independence
The BIOS strap differs from conventional assembly language code in the following
points:
- The routine is automatically loaded from disk 0, track 0, and sector number 1 to the
absolute address 0000:7c00h
- The routine cannot exceed one standard disk sector that is 512.
- CS: IP cannot be specified in the service.
- Segment fix ups cannot be present. The routine must executes at the absolute address
7c00h and cannot be loaded or relocated by the DOS command processor.
- The bootstrap programme can use BIOS services, since BIOS has been initialized at this
time but not the service provided by the DOS.
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