Multiboot DOS

How to Use Multiboot to Start DOS

Starting DOS using a Multiboot-compliant loader involves creating a custom loader that can switch the CPU from protected mode (set up by the Multiboot-compliant bootloader) back to real mode (required by DOS).

Here’s how you can achieve this:

1. Understanding the Challenges

Mode Switching: DOS is a 16-bit real mode operating system, but Multiboot-compliant bootloaders like GRUB load the OS in protected mode (32-bit).
Memory Layout: DOS expects to be loaded at specific memory locations, typically starting at the real mode address 0x00007C00.
Boot Sector: DOS typically boots from a boot sector located at 0x00007C00, so your loader needs to emulate this process.

2. Creating a Multiboot-Compliant Loader

The goal is to create a loader that:

Complies with the Multiboot Specification: It must contain a Multiboot header so that it’s recognized by a Multiboot-compliant bootloader.
Switches from Protected Mode to Real Mode: This involves setting up the CPU to switch back to real mode.
Loads and Transfers Control to DOS: The loader must load DOS at the correct memory address and then jump to it.

3. Multiboot Header

Start by defining the Multiboot header in assembly, which the bootloader uses to verify that the kernel (loader) is Multiboot-compliant.

section .multiboot
align 4
    dd 0x1BADB002                ; magic number
    dd 0x00000003                ; flags (request memory map and video mode)
    dd -(0x1BADB002 + 0x00000003); checksum

4. Protected Mode to Real Mode Transition

The loader needs to switch the CPU from protected mode (32-bit) back to real mode (16-bit). Here’s how you can do it:

section .text
global start
start:
    cli                          ; Disable interrupts
    mov eax, cr0
    and eax, 0x7FFFFFFE          ; Clear the PE (Protection Enable) bit to exit protected mode
    mov cr0, eax
    jmp 0x0000:real_mode_start   ; Far jump to clear the instruction queue

real_mode_start:
    mov ax, 0x07C0               ; Set up segment registers to point to the boot sector area
    mov ds, ax
    mov es, ax
    mov ss, ax
    mov sp, 0x7C00               ; Set the stack pointer

    ; Load DOS boot sector or transfer control to it
    ; Assuming the DOS boot sector is loaded at 0x00007C00
    jmp 0x07C0:0x0000            ; Jump to the DOS boot sector entry point

5. Loading the DOS Boot Sector

The DOS boot sector should be loaded at address 0x00007C00, which is where DOS expects it to be. You can create a disk image with a DOS boot sector and use GRUB to load your Multiboot loader, which then jumps to the DOS boot sector.

6. Using GRUB to Boot the Loader

Create a GRUB Configuration:
- Create a grub.cfg file in your GRUB boot directory:

menuentry "Boot DOS" {
    multiboot /boot/multiboot_loader.bin
}

Create a Bootable ISO:
- Use grub-mkrescue to create an ISO image that contains your Multiboot loader:

mkdir -p isodir/boot/grub
cp multiboot_loader.bin isodir/boot/
echo 'menuentry "Boot DOS" { multiboot /boot/multiboot_loader.bin }' > isodir/boot/grub/grub.cfg
grub-mkrescue -o dos_boot.iso isodir

Test with QEMU or Real Hardware:
- Use QEMU to test the bootable image:

qemu-system-i386 -cdrom dos_boot.iso

7. Transferring Control to DOS

After switching to real mode, your loader will jump to the DOS boot sector’s entry point at 0x00007C00. DOS will then take over as if it were booted directly by the BIOS.

Summary

Using Multiboot to start DOS involves writing a custom Multiboot-compliant loader that:

Contains a Multiboot header for recognition by a Multiboot-compliant bootloader (like GRUB).
Switches the CPU from protected mode to real mode.
Loads DOS into memory and transfers control to it, emulating a traditional BIOS boot process.

This method leverages modern bootloaders like GRUB while maintaining compatibility with older operating systems like DOS. It requires a good understanding of both protected mode and real mode, as well as the ability to manipulate the CPU state directly using assembly language.

References

While specific implementations of using Multiboot to start DOS are rare due to the unique nature of DOS (which is typically booted directly by the BIOS), there are some resources and existing projects that demonstrate how to create custom bootloaders or switch from protected mode to real mode, which can be adapted for your needs. Here are some references that might help:

1. OSDev Wiki – Writing Your Own Bootloader

Link: OSDev Wiki – Bootloader
Description: This page provides a detailed guide on writing your own bootloader. It covers the basics of real mode, protected mode, and switching between the two, which are essential for creating a Multiboot-compliant loader that can boot DOS.

2. OSDev Wiki – Real Mode to Protected Mode and Back

Link: OSDev Wiki – Real Mode
Description: This article explains the process of switching between real mode and protected mode. It provides code examples that demonstrate how to switch back to real mode, which is crucial for booting DOS from a Multiboot-compliant loader.

3. GRUB Legacy and GRUB2 Source Code

Link: GRUB Git Repository
Description: GRUB’s source code can be a valuable resource for understanding how Multiboot works and how GRUB handles different operating systems. You can explore how GRUB sets up the environment for various Multiboot-compliant kernels, which can inspire your own implementation.

4. Simple Multiboot Kernel (Booting to Real Mode)

Link: GitHub – Multiboot Example
Description: This GitHub repository contains examples of bare-metal programs that are Multiboot-compliant. One of the examples includes a simple kernel that demonstrates switching back to real mode, which could be adapted for booting DOS.

5. FreeDOS Bootloader (Original Boot Process)

Link: FreeDOS GitHub Repository
Description: FreeDOS is an open-source DOS-compatible operating system. Although not Multiboot-compliant by default, its bootloader code may offer insights into how DOS expects to be loaded, which you can integrate with a Multiboot loader.

6. MiniOS (Minimal Operating System Example)

Link: MiniOS on GitHub
Description: MiniOS is a simple operating system that demonstrates basic OS concepts, including bootloading and mode switching. Though it’s not directly related to DOS, it can help you understand how to structure a minimal Multiboot-compliant OS.

7. GitHub – Multiboot Kernel Development Resources

Link: GitHub Search for Multiboot
Description: Searching GitHub for “Multiboot” will yield various projects and examples of Multiboot-compliant kernels. Browsing through these projects can provide inspiration and practical examples of how to implement your own Multiboot loader.

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