Simple Boot Loader in Assembly

Terminal Commands:

nasm boot1.asm -f bin -o boot1.bin
dd if=boot1.bin bs=512 of=myhd.img
qemu myhd.img


Help for boot1.asm:

'INT 0x10' is a BIOS video interrupt. All video related calls are made through this interrupt.
To use this interrupt, we need to set the values of some registers as follows.

    AL => With ASCII value of character to display
    AH => With 0x0E ;
          Teletype mode (This will tell bios that we want to print one character on screen)
    BL => With Text Attribute (This will be the fore ground and background color
            of character to be displayed. 0x07 in our case.)
    BH => With Page Number (0x00 for most of the cases)

    Once all the registers are filled with appropriate values, we can call interrupt.

Implementation in Assembly

[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
call PrintString
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                   ;0x07 is lightgrey font on black background        

int 0x10                           ;Call video interrupt
ret                                    ;Return to calling procedure

PrintString:

next_character:

mov al, [si]
inc si
or al, al
jz exit_function
call PrintCharacter
jmp next_character

exit_function:

ret

HelloString db 'Nice Work Dude......................',0
times 510-($-$$) db 0                    ;Fill the rest of sector with 0
dw 0xAA55                                    ;Add boot signature at the end of bootloader



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