Re: [Qemu-devel] Re: POST failure (loop) with isapc and seabios

[Jamie Lokier]

Gleb Natapov wrote:
> On Sun, Nov 29, 2009 at 10:53:40PM +0000, Natalia Portillo wrote:
> > >>
> > >We already concluded that "return to PM by triple fault" is not
> > >something
> > >we want to support. It was needed only on 286 and QEMU doesn't even
> > >support 286 cpu emulation.
> > 
> > It is used by a whole kind of operating systems (all the 16-bit OS/2
> > tree) and who knows by how many DOS extenders.
> I suppose none. But if you know any please mansion it by name here. Note
> that is should support i386 CPU since QMEU does not emulate i286.
> 
> I386 has native way to switch to PM, no need triple fault trick and it
> was introduced by Intel in 1985. For those who wanted to be compatible
> with 286 there was a trick invented back then to switch to PM in a
> portable way between i386 and i286:
> http://www.rcollins.org/ftp/source/3fault/reset.asm
> 
> I doubt that OS/2 was not updated to use something like this.

I can't test 16-bit OS/2, so if someone would like to give it a try
that'd be great.  It'd be interesting to see what method it uses.

I will try 16-bit Windows 3, and MS-DOS with HIMEM.SYS and Turbo C++ to
see if (a) they still work and (b) what method they use.

> > It is supported by Pentium processors, so it should be by QEMU, no
> > matter it does not emulate the 286 but a later CPU.
> Most chipsets can be configured to do full reset on CPU shutdown cycle.
> You simple can't rely on certain behaviour of triple fault. 

I wouldn't be surprised if, for ancient 286 app compatibility, real
BIOSes always set the chipset to do a soft reset, but I've no idea
really what they do.

286 era PCs didn't do much on soft reset :-)

The triple fault and keyboard reset methods, when BIOS is told it's
for PM->RM switch, don't disturb the video output, lose keyboard input
or have to do a slow reinitialisation of hard disk or memory, because
those things all carry on working.

The PM->RM switch is used as a slow context switch or method for
calling the BIOS in multitasking environments.  Even though it's slow
(especially when done through a keyboard controller request), it's
fast enough to use for context switching.

> BTW QEMU today does full reset on triple fault and I haven't noticed any
> complains.

Those OSes we've just talked about don't care about the ACPI (etc)
tables getting wiped in the BIOS, do they? :-)

I'm getting the impression all the BIOS self-modification is in areas
which aren't used by ancient OSes using the triple fault or keyboard
methods for PM->RM switch.  Is that right?

It would still be unfortunate if you could boot DOS, run a few things
that use the old PM->RM methods, then run LOADLIN to boot a modern
Linux (from DOS) and find Linux needs something that's messed up in
the BIOS.

-- Jamie