lesson 6, segmentation

06-bootsector-segmentation/README.md

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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.

06-bootsector-segmentation/boot_sect_segmentation.asm

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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