Mirrors/os-tutorial
1
0
Fork 0
mirror of https://github.com/cfenollosa/os-tutorial.git synced 2025-12-17 20:34:36 +03:00
Code Issues Actions Packages Projects Releases Wiki Activity

lesson 3, boot sector with memory addressing

Browse Source
This commit is contained in:
Carlos Fenollosa
2014-10-05 11:28:07 +02:00
parent 3a8b9086d4
commit 8211b950b1
3 changed files with 77 additions and 11 deletions
Download Patch File Download Diff File Expand all files Collapse all files

25
03-bootsector-memory/README.md
View File

@@ -1,13 +1,13 @@
*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 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.
I could just go ahead and tell you that it starts at `0x7C00`, but it's
better with an example.
The only goal of this lesson is to learn where the boot sector is stored
I could just go ahead and tell you that the BIOS places it at `0x7C00`, 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.
@@ -23,7 +23,7 @@ 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
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.
@@ -33,6 +33,21 @@ 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 file unless you have 100% understood
the boot sector offset and memory addressing.
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`
-----