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64 lines
2.1 KiB
64 lines
2.1 KiB
*Concepts you may want to Google beforehand: memory offsets, pointers*
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**Goal: Learn how the computer memory is organized**
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Please open page 14 [of this document](
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http://www.cs.bham.ac.uk/~exr/lectures/opsys/10_11/lectures/os-dev.pdf)<sup>1</sup>
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and look at the figure with the memory layout.
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The only goal of this lesson is to learn where the boot sector is stored
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I could just bluntly tell you that the BIOS places it at `0x7C00` and
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get it done with, but an example with wrong solutions will make things clearer.
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We want to print an X on screen. We will try 4 different strategies
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and see which ones work and why.
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**Open the file `boot_sect_memory.asm`**
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First, we will define the X as data, with a label:
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```nasm
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the_secret:
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db "X"
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```
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Then we will try to access `the_secret` in many different ways:
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1. `mov al, the_secret`
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2. `mov al, [the_secret]`
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3. `mov al, the_secret + 0x7C00`
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4. `mov al, 2d + 0x7C00`, where `2d` is the actual position of the 'X' byte in the binary
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Take a look at the code and read the comments.
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Compile and run the code. You should see a string similar to `1[2¢3X4X`, where
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the bytes following 1 and 2 are just random garbage.
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If you add or remove instructions, remember to compute the new offset of the X
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by counting the bytes, and replace `0x2d` with the new one.
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Please don't continue onto the next section unless you have 100% understood
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the boot sector offset and memory addressing.
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The global offset
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-----------------
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Now, since offsetting `0x7c00` everywhere is very inconvenient, assemblers let
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us define a "global offset" for every memory location, with the `org` command:
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```nasm
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[org 0x7c00]
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```
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Go ahead and **open `boot_sect_memory_org.asm`** and you will see the canonical
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way to print data with the boot sector, which is now attempt 2. Compile the code
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and run it, and you will see how the `org` command affects each previous solution.
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Read the comments for a full explanation of the changes with and without `org`
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-----
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[1] This whole tutorial is heavily inspired on that document. Please read the
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root-level README for more information on that.
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