We started to dive into linux kernel internals in the previous [part](linux-bootstrap-1.md) and saw the initial part of the kernel setup code. We stopped at the first call of the `main` function (which is the first function written in C) from [arch/x86/boot/main.c](https://github.com/torvalds/linux/blob/master/arch/x86/boot/main.c). Here we will continue to research the kernel setup code and see what `protected mode` is, some preparation for the transition into it, the heap and console initialization, memory detection and much much more. So, Let's go ahead.
We started to dive into linux kernel internals in the previous [part](linux-bootstrap-1.md) and saw the initial part of the kernel setup code. We stopped at the first call to the `main` function (which is the first function written in C) from [arch/x86/boot/main.c](https://github.com/torvalds/linux/blob/master/arch/x86/boot/main.c).
In this part we will continue to research the kernel setup code and
* see what `protected mode` is,
* some preparation for the transition into it,
* the heap and console initialization,
* memory detection, cpu validation, keyboard initialization
Before we can move to the native Intel64 [Long mode](http://en.wikipedia.org/wiki/Long_mode), the kernel must switch the CPU into protected mode.
Before we can move to the native Intel64 [Long Mode](http://en.wikipedia.org/wiki/Long_mode), the kernel must switch the CPU into protected mode.
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 is only 2^20 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 memory management. The 24-bit address bus was replaced with a 32-bit address bus. It allows access to 4 gigabytes of physical address space vs 1MB of 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 allows access to 4-gigabytes of physical address space vs 1MB of 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:
kittyhawk
9 years ago