Bootload Your Message

Bootload Your Message: A Minimal x86 Experiment

The power of a boot loader is undeniable. It’s the first code your computer executes, setting the stage for everything that follows. But what if you could write your own boot loader, displaying a custom message as the first thing your computer sees? In this article, we’ll explore how to create a minimal x86 boot loader using the NASM assembler and delve into its functionalities.

The Minimal Boot Loader:

Our goal is to create a boot loader that displays a simple message upon booting. Here’s a breakdown of the key elements:

1. Boot Sector:

The boot sector is a special area on the disk containing the first instructions executed during boot. We’ll assemble our code into this sector using NASM.

2. Interrupt Service Routines (ISRs):

ISRs are routines that handle hardware interrupts. We’ll use the int 0x10 interrupt to access the BIOS and display text on the screen.

3. VGA Text Mode:

The VGA text mode is a common way to display text on the screen. We’ll use the mov ah, 0x02 instruction to set the display mode and then int 0x10 with the ah register set to 0x00 to write characters to the screen.

Example Code:

BITS 16

org 7c00h

segment .text
global _start

_start:
	mov ah, 0x02
	mov al, 'H'
	int 0x10
	mov ah, 0x02
	mov al, 'e'
	int 0x10
	mov ah, 0x02
	mov al, 'l'
	int 0x10
	mov ah, 0x02
	mov al, 'o'
	int 0x10
	jmp $

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

Understanding the Code:

  • The BITS 16 directive specifies that the code is in 16-bit mode.
  • The org 7c00h directive sets the starting address of the code to 7c00h, the beginning of the boot sector.
  • The global _start directive declares the _start function as the entry point.
  • The _start function:
    • Sets the ah register to 0x02, indicating VGA text mode.
    • Writes each letter of the message using int 0x10.
    • Jumps to the end of the code.
  • The remaining space in the boot sector is filled with zeros.
  • The dw 0xaa55 instruction writes the magic word 0xaa55 to the end of the boot sector, indicating its valid signature.

Compiling and Running:

  1. Save the code as boot.asm.
  2. Compile it with nasm -f bin boot.asm -o boot.bin.
  3. Copy boot.bin to the floppy disk’s boot sector (usually track 0, sector 1).
  4. Reboot your computer.

Conclusion:

This experiment demonstrates the power of writing your own boot loader. With a few lines of code, you can customize the first experience your computer presents. While this is a simple example, the possibilities are endless. You can explore other functionalities like loading and executing code from a disk or displaying more complex graphics. The realm of boot loaders offers endless opportunities for exploration and experimentation.