Project Documentation

Step 1

Setup

Installed required packages on Ubuntu Linux.

• NASM: Netwide Assembler — converts assembly code into executable machine code.
• QEMU: System emulator used to run and test the operating system.

sudo apt install nasm qemu-system-x86

Core Concepts

• Assembler: Translates assembly code into binary instructions that the CPU can execute.
• BIOS (Basic Input/Output System): Firmware stored on a motherboard chip that initializes hardware before any OS loads. Resides in ROM, unlike RAM which is user-installed.

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Step 2

Goal

Create a file that the BIOS interprets as a bootable disk.

Concepts

• Boot Sector: The first 512 bytes of a storage device (hard drive, floppy disk, USB stick, etc.).
• BIOS Load: At startup, the BIOS reads the first 512 bytes from a disk into RAM at address 0x7C00.
• Boot Check: The BIOS checks bytes 511 and 512 for the value 0xAA55 to verify if the disk is bootable.
• Padding: The boot sector must be exactly 512 bytes. We pad unused bytes with zeros so the final two bytes can store 0xAA55.

Assembly Code

loop:
    jmp loop

times 510-($-$$) db 0
dw 0xaa55

Explanation

• times: Repeat the following instruction a specified number of times.
• 510: Target size before the two signature bytes.
• $: Current address/position.
• $$: Start of the section.
• $ - $$: Represents the current offset (end - start).
• db 0: Define one byte with value 0.

Calculation:

• 3 bytes for the infinite loop
• 507 bytes of zeros
• 2 bytes for the boot signature
• 512 total bytes

Building the Binary

Created a new file boot_sect_simple.asm and compiled it:

nasm -f bin boot_sect_simple.asm -o boot_sect_simple.bin

Verification

Checked the beginning and end of the generated .bin file:

xxd boot_sect_simple.bin | head
xxd boot_sect_simple.bin | tail

Step 3: Printing Hello World Program

What I Learned

• CPU Registers: Tiny storage inside CPU (ax, ah, al)
• Interrupts: Way to call BIOS functions (int 0x10 = video services)
• Teletype mode: 0x0e in ah = print character mode

Code Structure

mov ah, 0x0e    ; Set function: teletype
mov al, 'H'     ; Set character
int 0x10        ; Call BIOS to print

Actual Code

mov ah, 0x0e ; tty mode
mov al, 'H'
int 0x10
mov al, 'e'
int 0x10
mov al, 'l'
int 0x10
int 0x10 ; 'l' is still on al, remember?
mov al, 'o'
int 0x10

Result

Bootloader printed "Hello" to screen!

Stack code below