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Is it possible to use the true hibernate system functionalities of Win 7 and OS X to create a kind of instant switching between the two? Imagine this when reading/writing the sleep images to a quick SATA-3 SSD.

Hibernate-corruption-fix: If this is made possible, make sure that your different partitions are not write-enabled from eachother. (e.g. make it so that Boot Camp Windows 7 cannot write to OS X partition and vice versa). This safety measure should make it impossible for your different systems to corrupt eachother when one is suspended into hibernation.

Edit: I realized that I'm not sure that this is how partitions work on a disk. The hibernated OS X kernel could have references to blocks on the disk which it will continue to write to when it is woken up again. Are disk blocks like these contained within a partition or are they assigned to a partition after they have been written on? Is it different on HDD and SSD? Does this affect the above hibernate-corruption-fix?

Ok let's move on:

Windows 7 already has an option "Hibernate" which allows you to boot back to your OS X partition, but OS X does not exactly offer the same.

With OS X it is possible to hibernate by changing the hibernatemode system variable.

Pre OS X 10.7

sudo pmset -a hibernatemode 2

OS X 10.7

sudo pmset -a hibernatemode 25

This makes your Mac go into hibernate when you put it to sleep. You can put it to sleep with:

sudo shutdown -s now

Ok, so far so good. So we can put both Windows 7 and OS X Lion into hibernation at will. But wait! There's more problems up ahead.

When you put OS X into hibernate it will automatically boot back into OS X when you power your Mac on again. This makes it impossible for us to use it for "hotswitching" between partitions.

Possible fixes:
If you install rEFIt without rEFItBlesser you will get the rEFIt boot menu after OS X hibernation. On OS X 10.7 Lion however, this does not fully work, instead you get a black screen and have to hard shutdown your Mac. But hey! At least it interrupts the automatic OS X boot, right? Maybe it can be tweaked further from there.

As mentioned on earlier OS X:s the rEFIt boot menu actually does come up after hibernation shutdown + power on. This allows you to e.g. boot your Boot Camp Windows 7 partition instead. There are problems here as well though. Apparently the OS X partition loses its hibernation flag if you follow this method, which means that the next time you boot into OS X, it will not return from its hibernation state. Instead it will do a clean startup. Why is this?
Does OS X look for the existence of a sleep image to determine wether to wake up from hibernation or to do a clean start or does it have some kind of system variable/flag set?

Summary

When Win 7 goes into hibernate it shuts down completely and you can then boot into OS X on startup. On OS X however, hibernate forces you to wake up into OS X. Can you hack this so that you're allowed to select boot partition after OS X hibernates and still be able to return later to OS X and wake it up from its hibernate?

Let's see what we collectively can come up with!

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see superuser.com/questions/59188/does-mac-os-x-support-hibernation for hibernating in OSX ( = your 2) - note the default is as per your 3 as you have seen –  Mark Jun 5 '12 at 10:59
    
I tried those solutions before but they do not change anything for me. My Mac still goes into Safe Sleep regardless of what setting I use or if i use the Deep Sleep Widget mentioned. It wakes up instantly without showing the loading bar at first which I assume means that is was never in real hibernation mode? –  Willem Jun 5 '12 at 11:09
    
After I asked my question on Ask Different, I was told that the reason the feature was dropped from Mac OS is because it's actually dangerous. When your computer wakes up, it expects the environment to be identical to when it went to sleep. Suppose you have a file open on your Windows partition, sleep Mac OS, and delete that file from Windows. You might be in for some trouble. Since the ability to run Windows on a Mac is a big feature of Intel Macs, Apple probably chose to make Windows-like hibernation difficult to protect novice users. –  zneak Jun 6 '12 at 14:04
    
I actually proposed a solution to that problem in this text. Look near the beginning. –  Willem Jun 6 '12 at 22:15
    
What's your question again? It looks like a feature request to Apple (?) interspersed with questions that are unrelated to each other. –  Daniel Beck Jun 7 '12 at 9:08

1 Answer 1

up vote 1 down vote accepted

It is not possible to do this the way you are thinking. The problem lies in the tight integration of Apple's firmware and OS X. OS X and the firmware work with each other to determine the sleep state of the computer.

When Windows hibernates, it dumps the contents of RAM to C:\Hiberfil.sys and sets a flag in the registry that the machine is hibernated. When you first boot a Windows machine, the boot sector code loads the BCD file, which loads that portion of the registry very early in the boot process and sees that the system is hibernated. After performing a basic sanity check, it loads hiberfil.sys back into memory. The important thing here is that all all of this is contained within the filesystem. This is why you can freely boot to OS X, then boot again back to Windows and it will continue to resume from the hibernation file.

The same is not true of OS X. When OS X hibernates, it dumps the contents of RAM to /var/vm/sleepimage the same way Windows does. But it saves the hibernation flag in the PRAM, not the filesystem (the setting is called IORegistryCurrentSleepMode if you're interested). When you turn a Mac back on, the values in PRAM are read before an attempt is even made to boot to the OS. If the flag indicates that the system is hibernated, the first thing it does is flip it back to a normal status. The firmware then immediately boots the system and ignores the Startup Disk preference and any attempts to Option+boot. You don't even get a startup chime. On a Mac, the firmware contains all the logic needed to inspect the filesystem and boot the OS. It does not need boot code the way Windows does.

When you throw rEFIt into the mix, it inserts itself into the process. It replaces the normal /System/Library/CoreServices/boot.efi (which is OS X's boot loader), with its own boot loader file. This is where things get fuzzy for me because this is all Apple proprietary stuff, but the bottom line is that when the firmware is booting OS X, it passes whatever arguments needed to load /var/vm/sleepimage instead of the normal Darwin kernel. rEFIt does not do this properly with Lion and later. But regardless of whether it's an older version of OS X or a newer one, the firmware has already flipped the hibernation bit before rEFIt even loads. This is why resuming OS X from hibernate is no longer possible after the first power-on.

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Wow. Finally an answer and a good one at that! (Even though the news is kind of sad...) Would it be possible to solve this by having Windows set the PRAM flag as it goes into hibernation? –  Willem Sep 25 at 21:21
    
If you could find a windows-based program for modifying PRAM values (similar to OS X's nvram tool) I think it'd be possible. You could set it as a scheduled task to trigger when the workstation sleeps. But I don't know if such a program exists. If you run across one I'd love to hear about it. –  Wes Sayeed Sep 25 at 21:57
    
Maybe someone here on Stackoverflow is interested in writing one if we highlight it. I believe the possibility of switching "instantly" between OS X and Windows is sought after by many. –  Willem Sep 25 at 23:13
1  
I posted the question here because I'm interested too. It might not be possible though. The firmware provides a Compatibility Support Module (CSM) which emulates a PC BIOS so Windows can boot. Windows may not be able to see through that CSM. Your screen brightness, keyboard backlight brightness, and audio volume saves to the PRAM as well. But I notice when I change those things from Windows they do not get retained on the next reboot the way they do when I change them from OS X. –  Wes Sayeed Sep 25 at 23:35
    
Alright nice, let's hope it's possible. I'm accepting your answer in the meantime. Let's comment here if we find anything. –  Willem Sep 26 at 7:10

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