Write Your Half BIOS

What is BIOS? BIOS is shorten from Basic Input Output System. Don't doubt it. That is really meaning of BIOS. Someone says that the purpose of BIOS is to boot OS. I don't think so. OS should be optional. What is Input? The keyboard and mouse are input devices. What is Output? The monitor and port 80 are output devices. If you write a program that enables input and output devices and your program is performed by CPU at reset vector, your program is BIOS.

I'm glad to see the page has the same idea of writing your BIOS. The page was written by Harrison Hsieh who is my former colleague.
http://biosengineer.blogspot.tw/2007/12/x86-bios4.html

My page just describes how to write a Half BIOS, a program that is performed by CPU at reset vector but it doesn't enable input and output devices. I also demonstrate how to run the Half BIOS with Bochs.

The following are source code of Half BIOS.

HalfBios.asm

Please follow the steps to build it by MASM and run it with Bochs.

Step 1. Build it with MASM611 or above.

> masm HalfBios.asm

The HalfBios.obj is generated.

Step 2. Update bochsrc.bxrc to change romimage file to HalfBios.bin

# romimage: file=../BIOS-bochs-latest
romimage: file=../HalfBios.bin

Step 3. Manually generate HalfBios.bin

> copy BIOS-bochs-latest HalfBios.bin

Please manually copy the 32 bytes with the signature "########" from HalfBios.obj to the offset 1FFE0h in HalfBios.bin. The content of 32 bytes is the program of Half BIOS.

Please make sure that there are EB F3 machine code at the offset 1FFF0h where is reset vector.

Step 4. Run Half BIOS with Bochs in debug mode.

C:\Program Files\Bochs-2.6.6\dlxlinux>..\bochsdbg.exe -f bochsrc.bxrc

We see that CPU performs the Half BIOS at the reset vector, f000:fff0, and then jump POST entry point.

-Count

Why I Like Bochs?

What is computer? I consider computer is a machine that quickly and automatically runs algorithm. The computer of Chinese text is 電 (electrical) 腦 (brain). Our brains are computers but not quick and correct. For example, I am an BIOS engineer. How do I proof it? I have blurred BIOS algorithm in my mind but it cannot be correctly run in my brain. I can tell you the algorithm but not the whole. I need to write BIOS program and run it in a machine to make sure that I know something in BIOS.

The machine, my brain, is not smart. I admire mathematicians who can present their theories in boards with very long formulas without computers. For me, a computer is necessary to help me thinking. I need burn BIOS in PC and see the process so that I can learn more.

But I don't want to use my PC to update BIOS. That has risk than using my brain. The idea is to use PC to emulate PC. I use a PC emulator to run a simple BIOS without breaking my PC at home. That is why I use Bochs, a PC emulator.

http://bochs.sourceforge.net/

-Count

Use Bochs to Lean BIOS Entry Point

It's Sunday in autumn. Really nice weather with wind, not hot. I can see sun, white cloud, and a small far island on the sea out the window. I want to be outdoor to enjoy the weather, but there is a more interesting thing I cannot bear to skip it. That is to use Bochs to trace BIOS entry point. You should be interested in the task if you are/will be/was a BIOS engineer.

Bochs is a PC emulator. I've introduced it in my blog page.
http://countchu.blogspot.tw/2014/09/bochs-pc-emulator.html

Bochs reads a file, BIOS-bochs-latest file, to be a BIOS of the PC emulator. We know that CPU considers the address f000:fff0 as a BIOS entry point when the PC is power on. So we open the  BIOS file to get the last line.

We can find that there are machine codes and a date signature, 08/02/13, that I guess as the released date of the BIOS. What do the machine codes mean? Le's use debug command in prompt.

> copy BIOS-bochs-latest test.bin
> debug test.bin

So the the assembly language of the machine codes is JMP F000:E05B. Le's show the machine codes at F000:E05B.

The code started at XOR is POST Entry Point. Please refer the link to know what it is.

http://bochs.sourceforge.net/cgi-bin/lxr/source/bios/rombios.c

Above is a way to manually trace the BIOS entry point. How to use Bochs to easily trace it? Just run bochsdbg.exe, a debug mode of Bochs.

>  bochsdbg.exe

It is just like the DOS debug command, it is broken at the line of <bochs:1> to wait for user command. I send the help command to display how to use it. Then I select the n command to trace BIOS entry point step by step. It displays in assembly language and teaches us how BIOS entry point works. Is it interesting?

-Count

Bochs - A PC Emulator

Bochs is a PC emulator. What is my expectation to Bochs? I want to write a simplest BIOS and burn it in the emulator without breaking my real PC. I'm too excited to read Bochs document. Let's download it, run it, and try to change the default BIOS of the emulator.

Here is the Bochs official web page.
http://bochs.sourceforge.net/

Please download Bochs-2.6.6.exe in the web page in your Windows environment and install it. Then, you can find Bochs 2.6.6 in your Windows start menu.

Just click Bochs 2.6.6 icon to start the PC emulator. You can see a window and a dialog.

No doubt, just click the Start button in the dialog to turn on the PC. I'll explain the Load button later. You can see another window that displays as a monitor of the PC. Another dialog is also popped up because a fatal error, no-bootable-device, occurs.

We can see a default BIOS, Bochs BIOS - build: 08/02/13, in the window. The question is how to replace the default BIOS? Do you remember the Load button? If you click it, a dialog, Load Bochs Config File, displays. It loads a *.bxrc file that you specify. You should be interested in the file. Please search them.

C:\Program Files\Bochs-2.6.6>dir *.bxrc /s
Directory of C:\Program Files\Bochs-2.6.6\dlxlinux
2014/09/13  ?? 10:08             1,533 bochsrc.bxrc

You can find the bochsrc.bxrc. Please open it and search BIOS or rom keyword.

# filename of ROM images
romimage: file=../BIOS-bochs-latest
vgaromimage: file=../VGABIOS-lgpl-latest

You can change the romimage config to specify your BIOS.

Another way is to run the following command in the prompt.
C:\Program Files\Bochs-2.6.6>bochs -f dlxlinux\bochsrc.bxrc -q

What does the command mean? Please see the help text by running "bochs -h".
C:\Program Files\Bochs-2.6.6>bochs -h

The below page shows you how to write the simplest BIOS and burn it with Bochs.

http://countchu.blogspot.tw/2014/10/write-your-half-bios.html

-Count