From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mailman by lists.gnu.org with tmda-scanned (Exim 4.43) id 1NDGrc-0001Bt-88 for qemu-devel@nongnu.org; Wed, 25 Nov 2009 07:20:52 -0500 Received: from exim by lists.gnu.org with spam-scanned (Exim 4.43) id 1NDGrX-0001Ac-2s for qemu-devel@nongnu.org; Wed, 25 Nov 2009 07:20:51 -0500 Received: from [199.232.76.173] (port=41366 helo=monty-python.gnu.org) by lists.gnu.org with esmtp (Exim 4.43) id 1NDGrW-0001AZ-Tr for qemu-devel@nongnu.org; Wed, 25 Nov 2009 07:20:46 -0500 Received: from mx1.redhat.com ([209.132.183.28]:1026) by monty-python.gnu.org with esmtp (Exim 4.60) (envelope-from ) id 1NDGrW-0005NZ-Lg for qemu-devel@nongnu.org; Wed, 25 Nov 2009 07:20:46 -0500 Date: Wed, 25 Nov 2009 14:20:39 +0200 From: Gleb Natapov Subject: Re: [Qemu-devel] Re: POST failure (loop) with isapc and seabios Message-ID: <20091125122039.GM2999@redhat.com> References: <20091122153809.GL3193@redhat.com> <8A56C1F849F34911ABF51DE049CA3D24@FSCPC> <20091123111137.GF2999@redhat.com> <11090DAABE9449F7B5D1415C45F8F411@FSCPC> <20091123194307.GC13854@redhat.com> <2D659FA33BF64C419243FD76049AFD33@FSCPC> <20091124062810.GZ2999@redhat.com> <20091124143812.GA27783@shareable.org> <20091124144044.GJ2999@redhat.com> <20091125060951.GA17203@shareable.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20091125060951.GA17203@shareable.org> List-Id: qemu-devel.nongnu.org List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Jamie Lokier Cc: Kevin O'Connor , qemu-devel@nongnu.org, Sebastian Herbszt On Wed, Nov 25, 2009 at 06:09:51AM +0000, Jamie Lokier wrote: > Gleb Natapov wrote: > > > But QEMU is used to run old OSes too. > > > > > That's OK. I don't expect BIOS to be reloaded if OS restart by jumping > > to BIOS reset code. > > That's good then. > > What about DOS and DOS-extender programs which do a soft reset by > triple-faulting the CPU (see Sebastian's notes on i440FX behaviour), > and asking the keyboard controller? > > Both of those methods are used by DOS and DOS-extender programs to > switch from protected mode to real mode. Keyboard controller was used > originally, but then someone figured out that triple fault can be used > (on most PCs) and is faster. > > The switch to real mode is done by writing somewhere the BIOS checks, > so the BIOS just branches back to the application. > If offset 0x0f in CMOS contains 0x0a then BIOS jumps to address stored in memory address 0x467. > I think that may imply it has to be a "soft reset" as described by > Sebastian in the i440FX description, and I would think the BIOS must > not be reloaded. Reading ich9 spec I see that on this chipset it is possible to configure what kind of reset triple fault generates. Make it not very reliable. Was this triple fault trick only needed on 286 anyway? > > But the BIOS must be reloaded from ROM, I'm guessing, if the keyboard > controller method is used and the word asking for a branch back to the > application has not been set. Because that's how a modern OS (if not > using ACPI) asks for a system reset. > > Do you think the above is (a) correct, and (b) what's implemented? > Do different things during reset depending on CMOS values doesn't sound right to me. I don't know what is implemented right now. I thought that we reload BIOS on reset. -- Gleb.