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lesson 3, boot sector with memory addressing

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Carlos Fenollosa 2014-10-05 11:30:57 +02:00
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03-bootsector-memory/README.md
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@ -6,12 +6,14 @@ and look at the figure with the memory layout.
The only goal of this lesson is to learn where the boot sector is stored 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 I could just bluntly tell you that the BIOS places it at `0x7C00` and
example with wrong solutions will make things clearer. 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 We want to print an X on screen. We will try 4 different strategies
and see which ones work and why. 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: First, we will define the X as data, with a label:
```nasm ```nasm
the_secret: the_secret:
@ -33,7 +35,7 @@ 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 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. by counting the bytes, and replace `0x2d` with the new one.
Please don't continue onto the next file unless you have 100% understood Please don't continue onto the next section unless you have 100% understood
the boot sector offset and memory addressing. the boot sector offset and memory addressing.
@ -47,7 +49,7 @@ us define a "global offset" for every memory location, with the `org` command:
[org 0x7c00] [org 0x7c00]
``` ```
Go ahead and open `boot_sect_memory_org.asm` and you will see the canonical 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 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. and run it, and you will see how the `org` command affects each previous solution.