My OSDev Journey - 01

May 31, 2023

Why am I building this?

  • As a precursor to anything that follows this in this post, I would like to declare that this is completely a hobby project. I’d say its more of a learning journey than anything else.

  • This project is a result of nothing but a spontaneous decision of wanting to build an operating system.

  • As for the OS itself, here is where you can find it: https://github.com/Icarus131/IcarOS

  • For the first part of the log, I’ll be going over the bootloader, how it works and how we can implement our own bootloader.

  • Now before we get hands on, there are a few things we must understand thoroughly.

    • Bitwise operators
    • Using binary to perform arithmetic operations
    • Assembly
    • Memory allocation
    • Registers

  • The bootloader performs a very important task of allowing the computer to recognize the drive that we are running the operating system in as bootable

  • The boot sector allows the computer to determine the start point of the operating system itself.

  • The boot sector is a 512 byte long binary code that ends with 55AA or AA55 to correct for little endian format.

  • This is how we start writing the bootloader

  • First lets start by hanging the bootloader and in essence creating a loop by jumping to the current address.

  • Now, because we are not occupying 512 bytes with the jmp statement, we will need to add some extra bytes to maintain the boot signal of 512 bytes.

  • Let’s add padding bytes using db 0. db stands for define byte.

  • From here we can calculate the required number of bytes and add it.

  • Finally, we can add he two numbers.

  • We are using ORG to set the origin point to BIOS. 0x7c00 is a magic address.

  • BITS 16 sets the bootloader to real mode.

    BITS 16
ORG 0x7c00                  ;Setting origin point to BIOS set MBR origin point [Magic Address]
jmp $
times 510-($ - $$) db 0     ; Adding extra bytes as boot sig needs to be 512dw 0xAA55

  • First to compile:

    • nasm -f bin ./main.asm -o ./main.bin

  • Then to run in qemu

    • qemu-system-x86_64 -hda ./main.bin

  • If you see this, then everything is right so far!

Boot

What’s Next?

  • From here, in the next post, I will start with how we can print text from the bootloader onto the screen.
  • We will learn how to make our own print function in assembly