OS BOOTLOADER

Starting OS system.

1. Creating Boot loader.

Program.

Hello World Boot loader

boot.asm

[BITS 16] ;Tells the assembler that its a 16 bit code
[ORG 0x7C00] ;Origin, tell the assembler that where the code will
    ;be in memory after it is been loaded

MOV SI, HelloString ;Store string pointer to SI
CALL PrintString ;Call print string procedure
JMP $   ;Infinite loop, hang it here.


PrintCharacter: ;Procedure to print character on screen
 ;Assume that ASCII value is in register AL
MOV AH, 0x0E ;Tell BIOS that we need to print one charater on screen.
MOV BH, 0x00 ;Page no.
MOV BL, 0x07 ;Text attribute 0x07 is lightgrey font on black background

INT 0x10 ;Call video interrupt
RET  ;Return to calling procedure



PrintString: ;Procedure to print string on screen
 ;Assume that string starting pointer is in register SI

next_character: ;Lable to fetch next character from string
MOV AL, [SI] ;Get a byte from string and store in AL register
INC SI  ;Increment SI pointer
OR AL, AL ;Check if value in AL is zero (end of string)
JZ exit_function ;If end then return
CALL PrintCharacter ;Else print the character which is in AL register
JMP next_character ;Fetch next character from string
exit_function: ;End label
RET  ;Return from procedure


;Data
HelloString db 'Hello World', 0 ;HelloWorld string ending with 0

TIMES 510 - ($ - $$) db 0 ;Fill the rest of sector with 0
DW 0xAA55   ;Add boot signature at the end of bootloader

Install Qemu and nasm in linux

sudo apt-get install build-essential qemu nasm

Compiling / Assembling

 nasm boot.asm -f bin -o boot.bin

Making a virtual floppy drive.

dd status=noxfer conv=notrunc if=boot.bin of=myfirst.flp

Virtual emulator

qemu -fda myfirst.flp

Make ISO image using following command

mkisofs  -no-emul-boot -boot-load-size 4 -boot-info-table -o myfirst.iso -b myfirst.flp boot/