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

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

**Goal: Learn how the computer memory is organized**

Please open page 14 [of this document](
http://www.cs.bham.ac.uk/~exr/lectures/opsys/10_11/lectures/os-dev.pdf)<sup>1</sup>
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:
```nasm
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:

```nasm
[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`

-----

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