[syslinux] unsubscribe

[Tung Ying Chang]

----- Original Message ----- 
From: "Richard L. James" <richard_l_james at yahoo.co.uk>
To: "H. Peter Anvin" <hpa at zytor.com>; "Josh Lehan" <jlehan at scyld.com>
Cc: "Geert Stappers" <stappers at stappers.nl>; <syslinux at zytor.com>
Sent: Wednesday, October 18, 2006 5:28 PM
Subject: Re: [syslinux] PXELINUX based kickstart query (probably OT)


> 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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