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@@ -41,7 +41,7 @@ As you can read in the previous part, addresses consist of two parts in real mod
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And we can get the physical address if we know these two parts by:
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And we can get the physical address if we know these two parts by:
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```
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```
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-PhysicalAddress = Segment * 16 + Offset
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+PhysicalAddress = Segment Selector * 16 + Offset
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```
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```
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Memory segmentation was completely redone in protected mode. There are no 64 Kilobyte fixed-size segments. Instead, the size and location of each segment is described by an associated data structure called _Segment Descriptor_. The segment descriptors are stored in a data structure called `Global Descriptor Table` (GDT).
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Memory segmentation was completely redone in protected mode. There are no 64 Kilobyte fixed-size segments. Instead, the size and location of each segment is described by an associated data structure called _Segment Descriptor_. The segment descriptors are stored in a data structure called `Global Descriptor Table` (GDT).
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@@ -135,11 +135,12 @@ As we can see the first bit(bit 43) is `0` for a _data_ segment and `1` for a _c
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8. D/B flag(bit 54) - Default/Big flag represents the operand size i.e 16/32 bits. If it is set then 32 bit otherwise 16.
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8. D/B flag(bit 54) - Default/Big flag represents the operand size i.e 16/32 bits. If it is set then 32 bit otherwise 16.
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-Segment registers don't contain the base address of the segment as in real mode. Instead they contain a special structure - `Segment Selector`. Each Segment Descriptor has an associated Segment Selector. `Segment Selector` is a 16-bit structure:
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+Segment registers contain segment selectors as in real mode. However, in protected mode, a segment selector is handled differently. Each Segment Descriptor has an associated Segment Selector which is a 16-bit structure:
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```
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```
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+15 3 2 1 0
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-----------------------------
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-----------------------------
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-| Index | TI | RPL |
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+| Index | TI | RPL |
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-----------------------------
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-----------------------------
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```
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```
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@@ -213,8 +214,8 @@ memcpy(&boot_params.hdr, &hdr, sizeof hdr);
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```
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```
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So,
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So,
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-* `ax` will contain the address of the `boot_params.hdr` in bytes
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-* `dx` will contain the address of `hdr` in bytes
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+* `ax` will contain the address of the `boot_params.hdr`
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+* `dx` will contain the address of `hdr`
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* `cx` will contain the size of `hdr` in bytes.
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* `cx` will contain the size of `hdr` in bytes.
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`memcpy` puts the address of `boot_params.hdr` into `si` and saves the size on the stack. After this it shifts to the right on 2 size (or divide on 4) and copies from `si` to `di` by 4 bytes. After this we restore the size of `hdr` again, align it by 4 bytes and copy the rest of the bytes from `si` to `di` byte by byte (if there is more). Restore `si` and `di` values from the stack in the end and after this copying is finished.
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`memcpy` puts the address of `boot_params.hdr` into `si` and saves the size on the stack. After this it shifts to the right on 2 size (or divide on 4) and copies from `si` to `di` by 4 bytes. After this we restore the size of `hdr` again, align it by 4 bytes and copy the rest of the bytes from `si` to `di` byte by byte (if there is more). Restore `si` and `di` values from the stack in the end and after this copying is finished.
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Matthieu Tardy