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lesson 6, segmentation

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Carlos Fenollosa 2014-10-05 20:41:18 +02:00
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05-bootsector-functions-strings/README.md
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We are close to our definitive boot sector.
In lesson 6 we will start reading from the disk, which is the last step before
In lesson 7 we will start reading from the disk, which is the last step before
loading a kernel. But first, we will write some code with control structures,
function calling, and full strings usage. We really need to be comfortable with
those concepts before jumping to the disk and the kernel.

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06-bootsector-segmentation/README.md Normal file
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*Concepts you may want to Google beforehand: segmentation*
**Goal: learn how to address memory with 16-bit real mode segmentation**
If you are comfortable with segmentation, skip this lesson.
We did segmentation
with `[org]` on lesson 3. Segmentation means that you can specify
an offset to all the data you refer to.
This is done by using special registers: `cs`, `ds`, `ss` and `es`, for
Code, Data, Stack and Extra (i.e. user-defined)
Beware: they are *implicitly* used by the CPU, so once you set some
value for, say, `ds`, then all your memory access will be offset by `ds`.
[Read more here](http://wiki.osdev.org/Segmentation)
Furthermore, to compute the real address we don't just join the two
addresses, but we *overlap* them: `segment << 4 + address`. For example,
if `ds` is `0x4d`, then `[0x20]` actually refers to `0x4d0 + 0x20 = 0x4f0`
Enough theory. Have a look at the code and play with it a bit.
Hint: We cannot `mov` literals to those registers, we have to
use a general purpose register before.

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06-bootsector-segmentation/boot_sect_segmentation.asm Normal file
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mov ah, 0x0e ; tty
mov al, [the_secret]
int 0x10 ; we already saw this doesn't work, right?
mov bx, 0x7c0 ; remember, the segment is automatically <<4 for you
mov ds, bx
; WARNING: from now on all memory references will be offset by 'ds' implicitly
mov al, [the_secret]
int 0x10
mov al, [es:the_secret]
int 0x10 ; doesn't look right... isn't 'es' currently 0x000?
mov bx, 0x7c0
mov es, bx
mov al, [es:the_secret]
int 0x10
jmp $
the_secret:
db "X"
times 510 - ($-$$) db 0
dw 0xaa55