@ -22,9 +22,9 @@ Before we can move to the native Intel64 [Long Mode](http://en.wikipedia.org/wik
What is [protected mode](https://en.wikipedia.org/wiki/Protected_mode)? Protected mode was first added to the x86 architecture in 1982 and was the main mode of Intel processors from the [80286](http://en.wikipedia.org/wiki/Intel_80286) processor until Intel 64 and long mode came.
The main reason to move away from [Real mode](http://wiki.osdev.org/Real_Mode) is that there is very limited access to the RAM. As you may remember from the previous part, there are only 2<sup>20</sup> bytes or 1 Megabyte, sometimes even only 640 Kilobytes of RAM available in the Real mode.
The main reason to move away from [Real mode](http://wiki.osdev.org/Real_Mode) is that there is very limited access to the RAM. As you may remember from the previous part, there are only 2<sup>20</sup> bytes or 1 Megabyte, sometimes even only 640 Kilobytes of RAM available in Real mode.
Protected mode brought many changes, but the main one is the difference in memory management. The 20-bit address bus was replaced with a 32-bit address bus. It allowed access to 4 Gigabytes of memory vs the 1 Megabyte in real mode. Also, [paging](http://en.wikipedia.org/wiki/Paging) support was added, which you can read about in the next sections.
Protected mode brought many changes, but the main one is the difference in memory management. The 20-bit address bus was replaced with a 32-bit address bus. It allowed access to 4 Gigabytes of memory vs the 1 Megabyte in Real mode. Also, [paging](http://en.wikipedia.org/wiki/Paging) support was added, which you can read about in the next sections.
Memory management in Protected mode is divided into two, almost independent parts:
@ -33,7 +33,7 @@ Memory management in Protected mode is divided into two, almost independent part
Here we will only talk about segmentation. Paging will be discussed in the next sections.
As you can read in the previous part, addresses consist of two parts in real mode:
As you can read in the previous part, addresses consist of two parts in Real mode:
* Base address of the segment
* Offset from the segment base
@ -41,12 +41,12 @@ As you can read in the previous part, addresses consist of two parts in real mod
And we can get the physical address if we know these two parts by:
```
PhysicalAddress = Segment Selector * 16 + Offset
PhysicalAddress = Segment Base * 16 + Offset
```
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 the _Segment Descriptor_. The segment descriptors are stored in a data structure called the `Global Descriptor Table` (GDT).
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 the _Segment Descriptor_. These segment descriptors are stored in a data structure called the `Global Descriptor Table` (GDT).
The GDT is a structure which resides in memory. It has no fixed place in the memory so, its address is stored in the special `GDTR` register. Later we will see how the GDT is loaded in the Linux kernel code. There will be an operation for loading it into memory, something like:
The GDT is a structure which resides in memory. It has no fixed place in the memory, so its address is stored in the special `GDTR` register. Later we will see how the GDT is loaded in the Linux kernel code. There will be an operation for loading it from memory, something like:
```assembly
lgdt gdt
@ -57,7 +57,7 @@ where the `lgdt` instruction loads the base address and limit(size) of the globa
* the size(16-bit) of the global descriptor table;
* the address(32-bit) of the global descriptor table.
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:
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
@ -75,7 +75,7 @@ As mentioned above the GDT contains `segment descriptors` which describe memory
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:
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 the segment 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:
1. Limit[20-bits] is split between bits 0-15 and 48-51. It defines the `length_of_segment - 1`. It depends on the `G`(Granularity) bit.
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