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@@ -60,19 +60,19 @@ where the `lgdt` instruction loads the base address and limit(size) of the globa
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As mentioned above the GDT contains `segment descriptors` which describe memory segments. Each descriptor is 64-bits in size. The general scheme of a descriptor is:
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```
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- 63 56 51 48 45 39 32
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+ 63 56 51 48 45 39 32
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------------------------------------------------------------
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| | |B| |A| | | | |0|E|W|A| |
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| BASE 31:24 |G|/|L|V| LIMIT |P|DPL|S| TYPE | BASE 23:16 |
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-| | | D | | L | 19:16 | | | | 1 | C | R | A | |
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-| --- |
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+| | |D| |L| 19:16 | | | |1|C|R|A| |
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+------------------------------------------------------------
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- 31 16 15 0
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+ 31 16 15 0
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------------------------------------------------------------
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| BASE 15:0 | LIMIT 15:0 |
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-| | |
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-| --- |
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+| | |
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+------------------------------------------------------------
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```
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Don't worry, I know it looks a little scary after real mode, but it's easy. For example LIMIT 15:0 means that bits 0-15 of Limit are located at the beginning of the Descriptor. The rest of it is in LIMIT 19:16, which is located at bits 48-51 of the Descriptor. So, the size of Limit is 0-19 i.e 20-bits. Let's take a closer look at it:
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@@ -98,10 +98,11 @@ To determine if the segment is a code or data segment, we can check its Ex(bit 4
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A segment can be of one of the following types:
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```
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-| Type Field | Descriptor Type | Description |
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-| --------------------------- | --------------- | ---------------------------------- |
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-| Decimal | |
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-| 0 E W A | |
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+--------------------------------------------------------------------------------------
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+| Type Field | Descriptor Type | Description |
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+|-----------------------------|-----------------|------------------------------------|
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+| Decimal | | |
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+| 0 E W A | | |
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| 0 0 0 0 0 | Data | Read-Only |
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| 1 0 0 0 1 | Data | Read-Only, accessed |
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| 2 0 0 1 0 | Data | Read/Write |
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@@ -110,7 +111,7 @@ A segment can be of one of the following types:
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| 5 0 1 0 1 | Data | Read-Only, expand-down, accessed |
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| 6 0 1 1 0 | Data | Read/Write, expand-down |
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| 7 0 1 1 1 | Data | Read/Write, expand-down, accessed |
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-| C R A | |
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+| C R A | | |
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| 8 1 0 0 0 | Code | Execute-Only |
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| 9 1 0 0 1 | Code | Execute-Only, accessed |
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| 10 1 0 1 0 | Code | Execute/Read |
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@@ -119,6 +120,7 @@ A segment can be of one of the following types:
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| 14 1 1 0 1 | Code | Execute-Only, conforming, accessed |
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| 13 1 1 1 0 | Code | Execute/Read, conforming |
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| 15 1 1 1 1 | Code | Execute/Read, conforming, accessed |
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+--------------------------------------------------------------------------------------
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```
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As we can see the first bit(bit 43) is `0` for a _data_ segment and `1` for a _code_ segment. The next three bits (40, 41, 42) are either `EWA`(*E*xpansion *W*ritable *A*ccessible) or CRA(*C*onforming *R*eadable *A*ccessible).
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@@ -143,8 +145,8 @@ Segment registers contain segment selectors as in real mode. However, in protect
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```
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15 3 2 1 0
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-----------------------------
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-| Index | TI | RPL |
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-| ----- |
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+| Index | TI | RPL |
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+-----------------------------
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```
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Where,
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Yaroslav Pronin