[syslinux] PXELINUX based kickstart query (probably OT)

[Richard L. James]

Hi all,
   
  > This differs from INT 19, which should do a full reset of the computer, 
Sorry but this is wrong - Interrupt 19h doesn't do a full reset of the computer.  Interrupt 19h preserves the contents of memory +  Interrupt vector table and simply calls the boot strap loader.  Indeed to demo this simply create a 2 byte com program and run it from native DOS as follows (note this isn't a debug script I writing this from memory):
   
  debug
  n lmrbt.com
  a
  int 19
   
  rcx
  2
  w
  q
   
  http://www.delorie.com/djgpp/doc/rbinter/ix/19.html
   
   
  As for the the reliability of calling interrupt 18h (originally the call for loading built in IBM ROM BASIC) well it depends on the model of the machine on which it has been executed.  Note comments: http://www.delorie.com/djgpp/doc/rbinter/id/50/22.html
   
  I personally lack usage experience of syslinux/etherboot at the moment.  However I would have thought that it may be possible to do a double boot (something I was working on for a utility to aid projects like the flopper project).  In effect you let the PC boot from a boot sector, copy the interrupt vector table elsewhere to memory with a CRC and then calling interrupt 19h to boot again to DOS allowing the initial stored interrupt vector table to be retrieved without much hassle in a safe and clean way.
   
  There is also a DOS utility around called NOBOOT (the US "PC magazine" util by this name appears to do something else) that modified floppy disk images so that if a disk was left in the drive the disks bootsector would be executed and then it would simply "reboot" from the MBR instead.  I don't have this util to hand but I got it from a PC mag in the early 1990's and from memory it written by a reader of the mag which did something similar to what is also being discussed although obviously the intention of that utility was to boot first from a floppy and then swap to booting from a harddisk.
   
  Regards Richard
  
"H. Peter Anvin" <hpa at zytor.com> wrote:
  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

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