cfenollosa_os-tutorial/03-bootsector-memory/README.md

Concepts you may want to Google beforehand: memory offsets, pointers

The only goal of this lesson is to learn where the boot sector is stored

Please open page 14 of this document¹ and look at the figure with the memory layout.

I could just go ahead and tell you that it starts at 0x7C00, but it's better with an example.

We want to print an X on screen. We will try 4 different strategies and see which ones work and why.

First, we will define the X as data, with a label:

the_secret:
    db "X"

Then we will try to access the_secret in many different ways:

1. mov al, the_secret
2. mov al, [the_secret]
3. mov al, the_secret + 0x7C00
4. mov al, 2d + 0x7C00, where 2d is the actual position of the X in the binary

Take a look at the code and read the comments.

Compile and run the code. You should see a string similar to 1[2¢3X4X, where the bytes following 1 and 2 are just random garbage.

If you add or remove instructions, remember to compute the new offset of the X by counting the bytes, and replace 0x2d with the new one.

[1] This whole tutorial is heavily inspired on that document. Please read the
root-level README for more information on that.