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Snow Leopard 32- vs 64-bit kernel, how to boot it, supported hardware

zen

zen

macrumors 68000
Original poster
Jun 26, 2003
1,713
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Okay, this is seriously confusing me.

I have been reading a lot about Snow Leopard in prep for tomorrow, but getting an answer for this seems pretty difficult. I apologise if this has been covered in one of the big Snow Leopard threads, but every time I look at them they gain 10 pages of posts and it's hard to find anything.

So, with Snow Leopard, there is a 32-bit kernel, and a 64-bit kernel. Correct?

As I understand it, it will boot into the 32-bit kernel by default, but if your hardware is supported, you can select the 64-bit kernel.

My question is - where is the list of supported hardware, and how do you select the 64-bit kernel?

My confusion comes from several posts and various articles which all say the first-gen aluminium iMac has a 32-bit EFI, and therefore can only boot into the 32-bit kernel. But I've just downloaded an app called Startup Mode Selector, which shows you your system config, and it says I have a 64-bit EFI.

Assuming this app is reporting correctly, that means my iMac can boot the 64-bit kernel, and that at least three supported hardware lists that I have seen are incorrect.

Can anyone help out here and clear up the confusion?
 
I thought the installer would test the hardware and install the right kernel?

Not very Apple like to let users choose the right kernel themselves :s

I hope someone clears this up, good thread!
 
I think it pretty much happens automatically. I'm in Snow Leopard now, on 64-bit hardware and I didn't select any option or anything to achieve native 64-bit support. The only thing I am notified by is when something is running in 32-bit mode, like certian preference panes and older apps without updates.

Bear in mind that the Growl preference pane is 32-bit, so every time you access it System Preferences has to be relaunched into compability mode before you can access it.
 
Heilage said:
I think it pretty much happens automatically. I'm in Snow Leopard now, on 64-bit hardware and I didn't select any option or anything to achieve native 64-bit support. The only thing I am notified by is when something is running in 32-bit mode, like certian preference panes and older apps without updates.

Bear in mind that the Growl preference pane is 32-bit, so every time you access it System Preferences has to be relaunched into compability mode before you can access it.
Click to expand...

One can hope all devs update there software to support 64-bit, I think they will.
 
MAYBE apple hardware will have drivers for x64, but I seriously doubt 3rd party devices will, since SL is the first OSX to have an x64 kernel

There's more harm in running an x64 kernel than there is benefit!
 
We've gotten some explanation from Apple on the issue at hand. The deal is, Mac desktops and laptops will indeed boot with a 32-bit kernel, while only the Xserve will be 64-bit. But this won't matter to the consumer. Our Apple contact explained:

The 32-bit kernel fully supports 64-bit applications, all system libraries that 64-bit applications use are fully 64-bit, and 64-bit applications have a full 64-bit virtual address space of 16 exabytes available to them on Mac OS X. The primary benefit of a 64-bit kernel is to improve the efficiency of accessing over 32GB of RAM.
Click to expand...

http://feeds.gawker.com/~r/gizmodo/full/~3/7HZUI2ad2ws/snow-leopard-currently-restricts-64+bit-booting-to-newer-macs
 
Yeah, you might be right - I just found some discussion regarding 32-bit kext files. While 32-bit and 64-bit processors can run 32-bit and 64-bit applications, if an application uses a kext, it must match the kernel. I think a lot of kexts are currently 32-bit only, so if you boot into the 64-bit kernel and try to use an app with a 32-bit kext, the app just won't launch.

Interesting, and a little confusing. I imagine most kexts will be updated by developers over time for a 64-bit kernel, so perhaps a future Snow Leopard update will boot the 64-bit kernel by default.
 
fishmoose said:
I thought the installer would test the hardware and install the right kernel?

Not very Apple like to let users choose the right kernel themselves :s

I hope someone clears this up, good thread!
Click to expand...

Apple does choose the right kernel. All users get the 32 bit kernel, unless they're running a server.
 
Heilage said:
I think it pretty much happens automatically. I'm in Snow Leopard now, on 64-bit hardware and I didn't select any option or anything to achieve native 64-bit support. The only thing I am notified by is when something is running in 32-bit mode, like certian preference panes and older apps without updates.
Click to expand...

Catfishman is right. You are running the 32-bit kernel, not 64-bit like you think.
 
Okay, so no particular advantage to booting the 64-bit kernel, it will likely cause app problems due to 32-bit kexts, and it's really nothing to worry about.

Problem solved and confusion cleared, thank you!
 
zen said:
Okay, so no particular advantage to booting the 64-bit kernel, it will likely cause app problems due to 32-bit kexts, and it's really nothing to worry about.

Problem solved and confusion cleared, thank you!
Click to expand...

Beyond the incompatible kernel extensions, for me, at least, the 64-bit kernel was SLOWER (by a lot) than the 32-bit kernel. The machine felt at least 50% faster under the 32-bit kernel for applications like Mail and Safari.


Config: Mac Pro (2008) with dual 2.8GHz processors, 14 GB of RAM, ATI 4780 card.
 
monksealpup said:
Beyond the incompatible kernel extensions, for me, at least, the 64-bit kernel was SLOWER (by a lot) than the 32-bit kernel. The machine felt at least 50% faster under the 32-bit kernel for applications like Mail and Safari.


Config: Mac Pro (2008) with dual 2.8GHz processors, 14 GB of RAM, ATI 4780 card.
Click to expand...

Hm... that's odd. Shouldn't be true. A little more ram used by the kernel, but that shouldn't matter with 14GB. I wonder what's wrong there.
 
zen said:
Yeah, you might be right - I just found some discussion regarding 32-bit kext files. While 32-bit and 64-bit processors can run 32-bit and 64-bit applications, if an application uses a kext, it must match the kernel. I think a lot of kexts are currently 32-bit only, so if you boot into the 64-bit kernel and try to use an app with a 32-bit kext, the app just won't launch.

Interesting, and a little confusing. I imagine most kexts will be updated by developers over time for a 64-bit kernel, so perhaps a future Snow Leopard update will boot the 64-bit kernel by default.
Click to expand...

Its not the developers you need to worry about, but device manufacturers. If I booted a 64bit kernel, I couldn't use:

HP PSC 1350 Printer
HP 710 Digital Camera
Wacom graphire 4 drawing tablet
Logitech USB headset

Just to name a few things. The manufactures has no incentive to make drivers for old products, they want you to upgrade! So sticking to the 32 bit kernel ensures my drivers will work
 
Catfish_Man said:
Hm... that's odd. Shouldn't be true. A little more ram used by the kernel, but that shouldn't matter with 14GB. I wonder what's wrong there.
Click to expand...

FWIW, under the development builds, the 32- and 64-bit kernels felt like they were more or less the same speed. Not sure what changed between that and the GM build.
 
What advantages does 64-bit kernel give? I'm also confused because of this 32-bit and 64-bit kernel thing. I know you can change it to 64-bit with little file modification but isn't that dangerous and not recommended? Thanks for any input
 
The primary benefit of a 64-bit kernel is to improve the efficiency of accessing over 32GB of RAM.
Click to expand...

Booting in 64-bit kernel, you might run into problems with driver/kext support. That is why Apple has decided all consumers will boot in 32-bit kernel, and only Xserves will boot in 64-bit kernel, since Xserves are much less likely to have legacy hardware that only have 32-bit kexts.
 
Hellhammer said:
I just downloaded that and it says I have 64-bit CPU and EFI, so 64-bit kernel should work? Is there any other benefits than +32GB RAM? Because if there isn't, I'm going to stay with 32-bit.
Click to expand...

It could be slightly faster at some things. My intent is to do some benchmark(et)ing once the NDA is lifted, but we'll see if I can find time; my only 64 bit kernel machine is at work, so it's naturally mostly busy with other things.
 
Hellhammer said:
I just downloaded that and it says I have 64-bit CPU and EFI, so 64-bit kernel should work? Is there any other benefits than +32GB RAM? Because if there isn't, I'm going to stay with 32-bit.
Click to expand...

I've got pretty much the same iMac as yours minus the graphics card. I've been running 64 bit for two days now, my printer works fine (Canon MX310), my Miglia TV Card works. Don't really have much else connected.

Cant really say I notice any difference. Geekbench is still slower on 64 bit SL than 32 bit Leopard.
 
Catfish_Man said:
It could be slightly faster at some things. My intent is to do some benchmark(et)ing once the NDA is lifted, but we'll see if I can find time; my only 64 bit kernel machine is at work, so it's naturally mostly busy with other things.
Click to expand...

Put me in waiting line :p

paulbeattie87 said:
I've got pretty much the same iMac as yours minus the graphics card. I've been running 64 bit for two days now, my printer works fine (Canon MX310), my Miglia TV Card works. Don't really have much else connected.

Cant really say I notice any difference. Geekbench is still slower on 64 bit SL than 32 bit Leopard.
Click to expand...

Okay. I think I'll stick with 32-bit for just incase.

Thank you both :cool:
 
Hellhammer said:
What advantages does 64-bit kernel give? I'm also confused because of this 32-bit and 64-bit kernel thing. I know you can change it to 64-bit with little file modification but isn't that dangerous and not recommended? Thanks for any input
Click to expand...

64bit allows to access more than 4GB of RAM.

if you're doing heavy PS work on large files than it will be a big speed booster. same for FCP
 
alent1234 said:
64bit allows to access more than 4GB of RAM.

if you're doing heavy PS work on large files than it will be a big speed booster. same for FCP
Click to expand...

64-bit apps allow access to more RAM, more than 2GB. 64-bit kernel allows access to more than 64GB of RAM.

Strange, but Mac OS X doesn't seem to have an API to allow a 32-bit program to access greater than 2GB RAM, like Windows does?
 
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