cfenollosa_os-tutorial/03-bootsector-memory

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

Goal: Learn how the computer memory is organized

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

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

I could just bluntly tell you that the BIOS places it at 0x7C00 and get it done with, but an example with wrong solutions will make things clearer.

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

Open the file boot_sect_memory.asm

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' byte 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.

Please don't continue onto the next section unless you have 100% understood the boot sector offset and memory addressing.

The global offset

Now, since offsetting 0x7c00 everywhere is very inconvenient, assemblers let us define a "global offset" for every memory location, with the org command:

[org 0x7c00]

Go ahead and open boot_sect_memory_org.asm and you will see the canonical way to print data with the boot sector, which is now attempt 2. Compile the code and run it, and you will see how the org command affects each previous solution.

Read the comments for a full explanation of the changes with and without org

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