Date: Fri, 21 Apr 1995 12:37:56 -0700 (PDT) From: "Rodney W. Grimes" <rgrimes@gndrsh.aac.dev.com> To: bde@zeta.org.au (Bruce Evans) Cc: current@FreeBSD.org Subject: Re: Fix for motherboards that don't reboot. Message-ID: <199504211937.MAA09159@gndrsh.aac.dev.com> In-Reply-To: <199504190115.LAA15783@godzilla.zeta.org.au> from "Bruce Evans" at Apr 19, 95 11:15:30 am
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>
> >I spent some time talking with a BIOS engineer today about the problem
> >of FreeBSD not rebooting on some motherboards and have come up with
> >the following patch that fixes the problem on the board I have here.
>
> > void
> > cpu_reset() {
> >+
> >+ /* Attempt to do a CPU reset via the keyboard controller */
> >+ outb(IO_KBD + 4, 0xFE);
>
> This is what I implemented for Minix about (sigh) 6 years ago. I found
> that outputting 0xFC works better. AFAIR 0xFE strobes the CPU reset
> line and 0xFC additionally strobes GateA20. Both methods worked in
> protected mode on the machines that I had access to, but in real mode,
> (with A20 forced to 0) several machines hung, apparently due to a bad
> fetch of the first instruction after reset. The first instruction was
> usually invalid and jumped to the BIOS invalid instruction handler,
> which was brain damaged and looped. Trapping it with a debugger and
> jumping to the correct reset vector worked. I suspect the reset worked
> better in protected mode only because the invalid instruction handler
> was invalid and the system eventually got reset better after a triple
> fault.
I have gotten back one test report on my patch, it did not work for
the person and he got the:
Keyboard reset did not work, attempting CPU shutdown
message I put in there. I had him change the 0xFE to 0xFC and it
caused a processor exception (gating A20 off without the reset
occurring sent the machine to god knows where) and did not output the
above message. This is clearly either a defective MB or a bad
design. But given the 2 values it looks as if 0xFE is safer since
it allows us to at least attempt the CPU shutdown if the reset
failed.
The person I got the information from about doing this says that
in fact 0xFE is the better value to use (this person writes BIOS'es
for a living) and is suppose to dig out the official reason for
me today. Once I here from him I will be committing my patch with
additional comments with the 0xFE value.
--
Rod Grimes rgrimes@gndrsh.aac.dev.com
Accurate Automation Company Custom computers for FreeBSD
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