[Programing] Create An Operating System!

To Make an Operating System, you will need:

An Assembler

• NASM -- If you're on Windows, use this. I will be using this for the tutorial.
  

A C/C++ Compiler

• Microsoft Visual C++ Express 2010 -- If you're on Windows, use this. I will be using this for the tutorial.
  

A Floppy Disk Emulation Program

• ImDisk Virtual Floppy Driver -- If you're on Windows, use this. I will be using this for the tutorial.
  

A Hex Editor

• HxD -- If you're on Windows, use this. I will be using this for the tutorial.
  

A PC Emulator

• Bochs -- If you're on Windows, use this. I will be using this for the tutorial.
  

I Will be updating this at a later time to include tools for OSX and several Linux Distros.

After Installing NASM, go to Control Panel -> System -> Advanced Settings -> Environment Variables
In that window you will see 2 List Boxes. The second one is the one you need to worry about. Scroll the box down until you find the "Path" variable. Double click it, and go to the end of the Text Box. At the end, put the location of where you installed NASM. This will allow you to use "nasm" instead of the full path of NASM at the command line.

After Installing these programs, feel congratulated! You have completed the first step to creating an Operating System. Now, It Gets Interesting - You are moving on to the Bootloader!!

Ok.. If you've made it this far.. well.. there's not really much to say. You installed a couple programs. Well, now you will use those programs to create a bootable program - the First part of Operating System Development.

A Bootloader takes a flat binary file and boots it. Simple as that. However, there are many types of Bootloaders. There are:

• Single Stage Bootloaders - These are small 512B files that are the Bootloader AND the Operating System Kernel.
  
• Dual Stage Bootloaders - These use a small 512B file to load the Operating System. The 512B file is the Bootloader.
  
• Tri Stage Bootloaders - These use a small 512B file to load another Bootloader. The second Bootloader can be any size, and it can prepare things for the Operating System. We will be making this kind of Bootloader.
  
• Generic Bootloaders - These are Bootloaders like NTLDR and GRUB.
  

Now that you know what kinds of Bootloaders there are, and which one we'll be using, it's time to start coding!

Ok.. Let's start with a Bootloader Template.

Code:

BITS 16
ORG 0x7C00

CLI
HLT

TIMES 510 - ($-$$) DB 0

DW 0xAA55

This Template starts off with BITS 16 - This defines to the Assembler that we are producing 16BIT output.

Next comes ORG 0x7C00 - This tells where to place the binary file in memory when booting. 0x7C00 is a hex number that points to where the BIOS gives control to a Bootloader.

Then comes CLI - This clears all interrupts. Interrupts are the processor's way of saying "Something Happened that Requires me to do Something." The operating System sends interrupts to the processor, and it pauses whatever it is doing and executes whatever is in the IVT (Interrupt Vector Table - 16BIT) or the IDT (Interrupt Descriptor Table - 32BIT). It then returns to whatever it was doing before.

After CLI comes HLT - This halts the Processor, which tells it to do nothing until a reboot. This insures that no random code will be executed afterwords.

Lower down we see TIMES 510 - ($-$$) DB 0 - This fills 510B of the program with 0's. This makes it so the program is exactly 512B in size. If it is not, it won't boot.

Lastly comes DW 0xAA55 - This is the Boot Signature. This is required by the BIOS to verify that it is a Bootloader. Without it, it won't boot.


Ok, now that you have that typed in, save it as BOOTFILE1.ASM.

Next, Open up CMD.EXE (MS-DOS Prompt). In it, type cd "Path of BOOTFILE1.ASM" (without the quotes). This will change your current directory to the one where you saved the code.

Next, type nasm -f bin BOOTFILE1.ASM -o BOOTFILE1.BIN into the command line. Hit enter. You have now assembled your first Bootloader. Next, inside the Bochs directory, you will find an application called bximage.exe. Run it, and type in the following (hit enter between every line)

Code:

fd
1.44
BOOTFILE1.IMG

Right click BOOTFILE1.IMG, and select Open With.... Select Choose Default Program.... Next click Browse.... Go to the folder where HxD was installed and select the HxD program. Hit OK on both windows.

After HxD Opens, do the same thing for BOOTFILE1.BIN.

Copy the first 512B from BOOTFILE1.BIN to BOOTFILE1.IMG. Save and close both windows.

Now you have a floppy disk to work with. Right click it and select Mount as ImDisk Virtual Disk.

Now go back to the Bochs directory. Inside it will be a file called bochsrc-sample.txt. Copy it and rename the copied file as bochsrc.txt. Open the file.

Edit the contents to look exactly like these:

Code:

romimage:    file="BIOS-bochs-latest", address=0xe0000
vgaromimage: file="VGABIOS-lgpl-latest"
floppya: 1_44=a:, status=inserted # Boot from drive A
log: OSDev.log # All errors and info logs will output to OSDev.log
error: action=report
info: action=report

Save that, and run Bochs. It will boot, and do nothing!

Congratulations! You made a bootable Program!!

Next step: Creating a bootable program that actually does something! This file you made will turn into a simple Bootloader that outputs text!