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26 lines
1021 B
Markdown
26 lines
1021 B
Markdown
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*Concepts you may want to Google beforehand: segmentation*
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**Goal: learn how to address memory with 16-bit real mode segmentation**
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If you are comfortable with segmentation, skip this lesson.
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We did segmentation
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with `[org]` on lesson 3. Segmentation means that you can specify
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an offset to all the data you refer to.
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This is done by using special registers: `cs`, `ds`, `ss` and `es`, for
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Code, Data, Stack and Extra (i.e. user-defined)
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Beware: they are *implicitly* used by the CPU, so once you set some
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value for, say, `ds`, then all your memory access will be offset by `ds`.
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[Read more here](http://wiki.osdev.org/Segmentation)
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Furthermore, to compute the real address we don't just join the two
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addresses, but we *overlap* them: `segment << 4 + address`. For example,
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if `ds` is `0x4d`, then `[0x20]` actually refers to `0x4d0 + 0x20 = 0x4f0`
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Enough theory. Have a look at the code and play with it a bit.
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Hint: We cannot `mov` literals to those registers, we have to
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use a general purpose register before.
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