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Fix tables and pseudographics formatting

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pull/607/head
Yaroslav Pronin 6 years ago committed by GitHub
parent 84e07ff6c9
commit 3af6133f05
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1 changed files with 15 additions and 13 deletions
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Booting/linux-bootstrap-2.md
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@ -60,19 +60,19 @@ where the `lgdt` instruction loads the base address and limit(size) of the globa
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:
```
63 56 51 48 45 39 32
63 56 51 48 45 39 32
------------------------------------------------------------
| | |B| |A| | | | |0|E|W|A| |
| BASE 31:24 |G|/|L|V| LIMIT |P|DPL|S| TYPE | BASE 23:16 |
| | | D | | L | 19:16 | | | | 1 | C | R | A | |
| --- |
| | |D| |L| 19:16 | | | |1|C|R|A| |
------------------------------------------------------------
31 16 15 0
31 16 15 0
------------------------------------------------------------
| | |
| BASE 15:0 | LIMIT 15:0 |
| | |
| --- |
| | |
------------------------------------------------------------
```
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:
@ -98,10 +98,11 @@ To determine if the segment is a code or data segment, we can check its Ex(bit 4
A segment can be of one of the following types:
```
| Type Field | Descriptor Type | Description |
| --------------------------- | --------------- | ---------------------------------- |
| Decimal | |
| 0 E W A | |
--------------------------------------------------------------------------------------
| Type Field | Descriptor Type | Description |
|-----------------------------|-----------------|------------------------------------|
| Decimal | | |
| 0 E W A | | |
| 0 0 0 0 0 | Data | Read-Only |
| 1 0 0 0 1 | Data | Read-Only, accessed |
| 2 0 0 1 0 | Data | Read/Write |
@ -110,7 +111,7 @@ A segment can be of one of the following types:
| 5 0 1 0 1 | Data | Read-Only, expand-down, accessed |
| 6 0 1 1 0 | Data | Read/Write, expand-down |
| 7 0 1 1 1 | Data | Read/Write, expand-down, accessed |
| C R A | |
| C R A | | |
| 8 1 0 0 0 | Code | Execute-Only |
| 9 1 0 0 1 | Code | Execute-Only, accessed |
| 10 1 0 1 0 | Code | Execute/Read |
@ -119,6 +120,7 @@ A segment can be of one of the following types:
| 14 1 1 0 1 | Code | Execute-Only, conforming, accessed |
| 13 1 1 1 0 | Code | Execute/Read, conforming |
| 15 1 1 1 1 | Code | Execute/Read, conforming, accessed |
--------------------------------------------------------------------------------------
```
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).
@ -143,8 +145,8 @@ Segment registers contain segment selectors as in real mode. However, in protect
```
15 3 2 1 0
-----------------------------
| Index | TI | RPL |
| ----- |
| Index | TI | RPL |
-----------------------------
```
Where,

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