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

[Gleb Natapov]

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.

> 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. 

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

--
                        Gleb.