COMPUTER ARCHITECTURE AND ASSEMBLY LANGUAGE COURSE
COMPUTER ARCHITECTURE AND ASSEMBLY LANGUAGE COURSEINSTRUCTOR
:SIR AAZAM KHAN
MADE BY:SHAHZEB PIRZADA(5701)MUHAMMAD FAIZ(55903)
BOOTABLE OPERATING SYSTEM
BOOT DISK
A boot disk is actually not a computer disk in the shape of a
boot. If it was, most disk drives would have a difficult time
reading it. Instead, a boot disk is a disk that a computer can
start up or "boot" from. The most common type of boot disk is an
internal hard drive, which most computers use to start up from. The
operating system installed on the hard drive is loaded during the
boot process.
BOOT DISKHowever, most computers allow you to boot from other
disks, including external Firewire hard drives, CD-ROMs, DVD-ROMs,
and floppy disks. In order to function as boot disks, these disks
need to have an operating system installed that is understandable
by the computer. This can either be a full-blown operating system
like Windows or Mac OS X, or a small utility operating system, such
as Norton Utilities or Disk Warrior.
BOOT DISKCD and DVD boot disks are often used to start up a
computer when the operating system on the internal hard drive won't
load. This can happen when bad data blocks or other errors occur on
the disk. By running a disk repair utility from the CD or DVD, you
can often fix the hard drive and restart from it, using the full
operating system.
What is Boot Loader
Boot loaderis a program situated at the first sector of the hard
drive; and it is the sector where the boot starts from. BIOS
automatically read all content of the first sector to the memory
just after the power is turned on, and jump to it. The first sector
is also calledMaster Boot Record.Actually it is not obligatory for
the first sector of the hard drive to boot something. This name has
been formed historically because developers used to boot their
operating systems with such mechanism.
How system boots
In order to solve our task we should recall how the system is
booting.Lets consider briefly how the system components are
interacting when the system is booting (see Fig.1).
After the control has been passed to the address
0000:7C00,Master Boot Record (MBR)starts its work and triggers the
Operating System boot.
Lets code
In the next sections we will be directly occupied with the
low-level programming we will develop our own boot loader.
Source Code of BOOT LOADER
[BITS 16] ; 16 bit code generation[ORG 0x7C00] ; Origin
location; Main programmain:; Label for the start of the main
program mov ax,0x0000; Setup the Data Segment register; Location of
data is DS:Offset mov ds,ax; This can not be loaded directly it has
to be in two steps.; 'mov ds, 0x0000' will NOT work due to
limitations on the CPU mov si, HelloWorld; Load the string into
position for the procedure. call PutStr; Call/start the
procedurejmp $; Never ending loop
Source Code of BOOT LOADER; ProceduresPutStr:; Procedure
label/start ; Set up the registers for the interrupt call mov
ah,0x0E; The function to display a chacter (teletype) mov bh,0x00;
Page number mov bl,0x07; Normal text attribute.nextchar; Internal
label (needed to loop round for the next character) lodsb; I think
of this as Load String Block ; (Not sure if that's the real meaning
though); Loads [SI] into AL and increases SI by one ; Check for end
of string '0' or al,al; Sets the zero flag if al = 0 ; (OR outputs
0's where there is a zero bit in the register) jz .return; If the
zero flag has been set go to the end of the procedure.; Zero flag
gets set when an instruction returns 0 as the answer. int 0x10; Run
the BIOS video interrupt jmp .nextchar; Loop back round tothe
top.return; Label at the end to jump to when complete ret; Return
to main program
Source Code of BOOT LOADER; DataHelloWorld db
'C:\Windows',13,10,0; End Mattertimes 510-($-$$) db 0; Fill the
rest with zerosdw 0xAA55The Boot Loader that we have made Assembly
Is totally depend on the above Piece of codenasm -f bin -o
floppy.img new.asm
Assembly languages Assembly languages are a type of low-level
languages for programming computers, microprocessors, micro
controllers, and other (usually) integrated circuits. They
implement a symbolic representation of the numeric machine codes
and other constants needed to program a particular CPU
architecture. This representation is usually defined by the
hardware manufacturer, and is based on abbreviations (called
mnemonics) that help the programmer remember individual
instructions, registers, etc. An assembly language family is thus
specific to a certain physical (or virtual) computer architecture.
This is in contrast to most high-level languages, which are
(ideally) portable.
Explanation of Source Code
[BITS 16] [ORG 7C00h]------- An operating System can be 16bit ,
32 bit 64bitwe have made the boot loader of 16BIT operating
System
--------This is the Address where we want to Run the Operating
System. Origin, tells the compiler where the code is going to be in
memory after it has been loaded. (hex number)
Command Discription
mov ah,0Eh ------ These are the TeleType Functions.. It will
print What is in RegisterMov:: The mnemonic "mov" represents the
opcode 10110000 which actually copies the value in the second
operand into the register indicated by the first operandAH -
0x0E
