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7 years ago | |
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README.md | 10 years ago | |
boot_sect_memory.asm | 7 years ago | |
boot_sect_memory_org.asm | 7 years ago |
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 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:
mov al, the_secret
mov al, [the_secret]
mov al, the_secret + 0x7C00
mov al, 2d + 0x7C00
, where2d
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
[1] This whole tutorial is heavily inspired on that document. Please read the root-level README for more information on that.