5 ways to reboot a PC. None of them reliable.

http://mjg59.livejournal.com/137313.html

by beza1e1 • 15 years ago

124 33 15 years ago

mjg59.livejournal.com

It amazes me that this sort of behavior still hasn't been standardized and agreed upon by all PC and component manufacturers.

mjg594• 15 years ago

Reality is that in areas of poor standardisation, vendors will test against Windows and ship it if it works. You could argue for a cross-vendor compliance testing suite, but if the behaviour of Windows varied from that then people would still need to match the Windows behaviour because that's where the market is.

So for all practical purposes, the behaviour is standardised. It's just embodied in the behaviour of Windows rather than anywhere useful.

checker• 15 years ago

The behavior is standardized to a degree that it works for Windows, but according to your article Windows does its own poking around for the reset. It would be nice if there was one established protocol for resets rather than any poking around at all.

I guess this was the point of your article though.

xpaulbettsx2• 15 years ago

FWIW, the fact that it's standardized against Windows makes Windows kernel developers' lives suck too - any change can break some dumb device that only actually worked purely by coincidence against XP SP1 or some ancient version

cube13• 15 years ago

I think that the other issue is the culture around(and design) *nix and BSD systems, where a reboot is generally only done when very bad things happen to the system, and nothing else can correct it. Often, there are a lot of other things going on that prevent the system from gracefully rebooting.

So hardware vendors might test a "standard" reboot, where nothing is broken, and the OS is in a stable state. Unfortunately, that probably isn't the case where most required reboots occur.

lebug• 15 years ago

Just type SYS 64738.

michael_dorfman• 15 years ago

Wow, that takes me back. Nicely done.

acqq• 15 years ago

My machine used RAND USR 0

jbronn• 15 years ago

PR#6

wzdd• 15 years ago

Method 6: return to 16-bit mode, call interrupt 0x19 (not if you're on efi).

Method 7: return to 16-bit mode, set cs to 0xf000 and jump to 0xfff0.

Linux used to use method 6, but it wasn't reliable either. Something tells me returning to 16-bit mode might not be possible for some hardware / driver combinations.

mjg594• 15 years ago

Method 7 is available for 32-bit systems, and actually pretty reliable. But going from 64-bit to 16-bit just to reboot is more pain than anyone really wanted to go through.

jgershen• 15 years ago

Could you explain in more detail how these trigger a reboot? From writing a 32-bit kernel I understand how the triple fault method is supposed to work, for example, but would love to know a little more about these methods.

mjg594• 15 years ago

int 19 should just execute code in the BIOS that triggers a reboot. Setting CS and jumping is re-entering the BIOS at the same entry point as at power-on. The BIOS should then reinit all the hardware.

JoeAltmaier• 15 years ago

Jumping to the BIOS is not a reliable reboot method either (method 7). Because the BIOS assumes hardware in a hardware-reset state, and the OS left it in some other state.

Tried to get an OS on a PC to soft-reboot, failed, because it is not actually possible to return all hardware to the reset state in some cases - they have an initial power-on state and once programmed can never return there unless hard-reset.

And why does the BIOS require hardware in the hard-reset state? Because a BIOS is never tested in a soft-reset scenarios, because Windows doesn't require it. And "if you haven't tried it, it doesn't work".

yuhong5• 15 years ago

Note that this way of rebooting was common in DOS though.

chadgeidel• 15 years ago

Wow - that's very interesting. So, what do most operating systems (aside from Apple presumably) normally do to get a consistent reboot?

Are they probing the hardware at install time? Do they just have a "table of things to try" the first time you reboot and then use the "successful thing" from then on?

mjg594• 15 years ago

Most operating systems fail on various bits of hardware. Windows uses the ACPI vector, then tries the keyboard controller, then the ACPI vector again, then the keyboard controller again, and then just stops.

rbanffy3• 15 years ago

Was there ever a computer as ugly as the IBM-PC?

I imagine even the Commodore 16 could reboot itself reliably.

smallblacksun• 15 years ago

An actual IBM-PC could reliably be rebooted using the keyboard controller (as mentioned in the article). The issues come with IBM-PC "compatible" machines. I'm sure commodore would have the same problem if anyone had bothered to clone them.

yuhong5• 15 years ago

To be more precise, an IBM PC-AT. It is closely related to the A20 line enable.

BrandonM• 15 years ago

I don't know if anyone else remembers, but I think it was either Windows 95 or Windows 98 which, after selecting shut down, the eventual result would be a message that said, "It is now safe to turn off your computer." I remember thinking it was magic when the computer would actually turn itself off at this point.

This article provides a good explanation for why that behavior was necessary.

yuhong5• 15 years ago

Power off uses APM or ACPI and is not related.

notYoursAtAll• 15 years ago

"How to shut down a PC" no mention of Linux, but apparently that is the topic.

But since it's not mentioned in the topic, it's going to come up. Personally I use a batch file and SysInternals to shutdown (after forcing bad-behavior apps to exit).

Sounds like you need a SysInternals for Linux??

quux• 15 years ago

Yup, I remember that. IIRC when this screen was displayed, the system was actually at a DOS prompt, just not in character mode. You could type "win" to restart windows for instance, and I think there was some magic command to reset the display adapter and get a working DOS prompt.

kronusaturn• 15 years ago

No, that was a completely different issue. Before ATX the power supply could not be controlled by software at all—it only had a mechanical switch, often near the back of the machine rather than in front, because that's where the power supply was.

This article is about rebooting, as in resetting all the hardware without turning off the power. The original IBM PC had a well defined way to do this, but after decades of clones and clones of clones there's no longer any one method that's guaranteed to work on any machine.

xpaulbettsx2• 15 years ago

I'm surprised the triple fault method doesn't reliably work - I thought that processors were guaranteed to restart if you do this; I suppose it doesn't guarantee that other components get reset in the same way.

sliverstorm• 15 years ago

Man, I love my ARM Cortex chips. I can just call NVIC_SystemReset()

5 ways to reboot a PC. None of them reliable.

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