Rick C. Hodgin
Programmer
extra guest said:
In order to change BIOS you need to have a BIOS image. If you have that, you can alter its contents and then flash the BIOS. Note, this is not a normal thing to do and is subject to all kinds of potential issues.
If you want to change just the boot sector so it displays a different message, then know that too might be difficult. The boot sector is 512 bytes, and it typically is crammed pack full with the code needed just to get the system up and running. There isn't typically much room for adding any code. You could find out where the boot sector's display message is, tweak that, and re-write it. That's a common thing to do, especially for OS authors who are testing their OS.
I needed to debug my OS boot process, so I modified my boot sector to load a 64 KB disassembler, and a 64 KB BIOS-based text-mode debugger at startup, called its equivalent of a main() function for it to hook into the interrupts it needed, and then issued INT 3 to cause it to single-step into my code. I was then able to debug the boot sector code with a real debugger that was very similar in appearance to CodeView 3.x. It was based on my 32-bit kernel debugger called Debi (which was graphical, see a video of it in operation here, use VLC if you can't view it).
Because it was written entirely in 80386 assembly, and because real-mode allows execution of 386-based opcodes, it didn't take very much to port it to text and make it a real-mode program as it was small and fit within a single 64 KB segment with room to spare.
It's possible to do the things you seek. They are tricky and hacky. It's not something to do lightly.
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Rick C. Hodgin