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32 lines
1.4 KiB
32 lines
1.4 KiB
*Concepts you may want to Google beforehand: GDT*
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**Goal: program the GDT**
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Remember segmentation from lesson 6? The offset was left shifted
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to address an extra level of indirection.
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In 32-bit mode, segmentation works differently. Now, the offset becomes an
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index to a segment descriptor (SD) in the GDT. This descriptor defines
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the base address (32 bits), the size (20 bits) and some flags, like
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readonly, permissions, etc. To add confusion, the data structures are split,
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so open the os-dev.pdf file and check out the figure on page 34 or the
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Wikipedia page for the GDT.
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The easiest way to program the GDT is to define two segments, one for code
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and another for data. These can overlap which means there is no memory protection,
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but it's good enough to boot, we'll fix this later with a higher language.
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As a curiosity, the first GDT entry must be `0x00` to make sure that the
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programmer didn't make any mistakes managing addresses.
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Furthermore, the CPU can't directly load the GDT address, but it requires
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a meta structure called the "GDT descriptor" with the size (16b) and address
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(32b) of our actual GDT. It is loaded with the `lgdt` operation.
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Let's directly jump to the GDT code in assembly. Again, to understand
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all the segment flags, refer to the os-dev.pdf document. The theory for
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this lesson is quite complex.
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In the next lesson we will make the switch to 32-bit protected mode
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and test our code from these lessons.
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