Re: [ProgSoc] Boot Sector Writing / BSD Loader

[Paul Mclachlan <subtle@nospam.fwb.gulf.net>]

On Thu, 6 Aug 1998, Bryn ''Curious'' Davies wrote:

> For reasons connected with my intolerable lameness, I need to remove the
> BSD loader that was installed on my PC. Tragically, the BSD installation
> that put it there is no more. Does anyone know how to write a simple
> bootsector that will just tell the computer to mount the first disk and
> run with it?
> 
> I seem to remember that being very easy on the Amiga ;)

I've been playing with bootsectors a bit recently, so this is all very new
and guessish, buuut...

If you just want to boot into DOS/Windows, load DOS and "fdisk /mbr" will
erase the bootsector & put the DOS one there.  If you're loading into
linux, just re-run lilo (after changing the config if you need to).

I disassembled the DOS bootsector and had a look through it, and there's
actually quite a bit to be done.  It seems to (somehow) work out which
partition is active (I couldn't follow that bit, okay?) and then it goes
to the FAT sector on that drive to find the COMMAND.COM style files, loads
them into memory (at a fixed location, it seems), and jumps to that code.

I guess the thing is, on the IBM you've got a tiny little BIOS with some
utility routines for outputting to the screen, on my old '500 you had 512k
worth of ROM code, graphics libraries, math libraries, etc, etc.  On the
Amiga, the only thing you had to do in the boot sector was say 'yes, boot
now, please' (or words to that effect).  I think the bootsector on the
Amiga was primarily there to allow non-OS things to boot - there was
certainly no need for it to actually bootstrap anything.

Just my guesses. :)

Paul

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