Required Knowledge

In this entry we will learn about two basic things that are important when developing kernels, we'll see what memory is and what does a computer do when booting.

Memory

Memory is a piece of hardware that allows computers to store information, RAM (Random Access Memory) is the main memory of your computer, programs can store and read information there.

ROM memory (Read-Only Memory) is a form of memory that allows to only read its information, it cannot be modified.

Something very important to know about memory, is that, information is accessed in a linear way, for example, if you tell a program "read the information in address 0xff781a", the program will read the information that is stored in 0xff781a, if you ask the program to read what's stored in address 0x0af9, it will read what's stored there, information is written in a linear way as well.

Even though memory is read from and written to in a linear way, when the processor access it and reads what's stored there, it will do it in a different way as the processor has different memory scheme, but, when we read directly from the memory, we will read in a linear way.

RAM Memory

In RAM you can write and read information, but RAM can only be used as temporary storage, once your computer shuts down, its content will be flushed, it will be gone.

ROM memory

In ROM memory, you cannot write information - through conventional manners -, but this is permanent, that is, even if you turn off your computer there will be the information.

This kind of memory, is generally used to store permanent programs that will never change, for example, embedded devices or your computer's BIOS.

Boot process

Let's take a look at the steps your computer take before executing your operating system:

• First, BIOS is executed from ROM memory in which it is stored.
• Then, the BIOS will look for an operating system in all the storage devices connected to the PC, and once it finds it, it will load it in memory address 0x7c00.
• The bootloader of the operating system loads the kernel into memory and then, it is executed.

What is a bootloader?

A bootloader is a small program, that loads the kernel of an operating system into memory.

When the computer boots, the processor will be in a compatibility state called "Real Mode" and this gives us access to 1 MiB of memory and also, executes only 16-bit code. In this mode, we will be fairly limited, but then our bootloader will switch to a mode called "Protected Mode" and this one will give us access to 4 GiB of memory and we will be executing 32-bit code.

Regarding BIOS…

BIOS is executed directly from ROM, therefore, once we turn on the computer, the BIOS will load itself into memory - so it is faster - and it is executed then.

BIOS is also in charge of initialise important hardware, this will provice functions to communicate with the hard disks, print text to the screen, draw graphics, etc.

When the BIOS is finished initialising the computer's hardwarwe, it will look in all the connected hard disks that the last 2 bytes of the first sector are 0x55AA, once it finds these bytes in a disk, it means it has an operating system and BIOS will load it to memory address 0x7c00.

Something that is pretty important to know, is that BIOS is almost a kernel itself, it has several routines that will help our bootloader to execute our kernel, it executes 16-bit code and these routines are generic and part of a standard.