[syslinux] PXELINUX based kickstart query (probably OT)

[H. Peter Anvin]

Josh Lehan wrote:
> H. Peter Anvin wrote:
>> Can't be done.  Unfortunately the BIOS considers loading a boot sector 
>> (from floppy or from hard disk) to be "terminal" in the sense that it 
>> has now booted, and there is no going back.
>>
>> This is one of the most unfortunate aspects of the BIOS.
> 
> Hmm, would calling INT 18 work?
> 
> INT 18 seems to have the meaning of "there's nothing to boot here, move 
> along".
> 
> In the MBR, an INT 18 will return control to the BIOS.
> 
> A good BIOS hopefully will advance to the next device in the boot 
> sequence.  A bad BIOS will probably just hang then, with the unfortunate 
> "Missing operating system" error message.
> 
> It might be worth a try to overwrite some of the code in the MBR of your 
> hard drive, to force a call to INT 18, if you don't want to boot from 
> that drive.
> 
> This differs from INT 19, which should do a full reset of the computer, 
> entirely starting the boot sequence over from the beginning.  If INT 18 
> doesn't work, to advance to the next boot device, then maybe INT 19, to 
> go back to the beginning, might be another way to solve the problem.


When I've tried it, INT 18h generally results in printing a message and 
stopping.  The Compaq-Phoenix-Intel BIOS Boot Specification 
http://www.phoenix.com/NR/rdonlyres/56E38DE2-3E6F-4743-835F-B4A53726ABED/0/specsbbs101.pdf
does that that INT 18h is supposed to invoke the next boot device, and 
it might be worth trying (it might help the OPs problem.)

That is, replace the first two bytes in the MBR with CD 18 hex.  It will 
either work, or it won't.

I hadn't looked at the BBS for a while, and I clearly should have, 
because it actually provides a much richer API than I remember.  In 
particular, if the functions in Appendix B are widely implemented, then 
there might be a lot of things that are possible that I didn't quite 
realize.  Silly me, but this is paydirt!

	-hpa