OBJECT:TO DESIGN AND DEVELOPED A PASSWORD ROUTINE WHICH ACTIVATE AT THE START OF THE BOOTING OF A COMPUTER.
Department of Computer ScienceUniversity of Karachi-Pakistan
Dos Boot ProcessWhen 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
Flags All cleared
IP register 0000h
CS register FFFFh
DS register 0000h
SS register 0000h
ES register 0000hAt 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.SYSIt comprises the Dos kernel routine providing the essential Dos service that remains memory resident.
CUSTOMIZING BOOT STRAP LOADERNaturally 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:
The BIOS strap differs from conventional assembly language code in the following points: