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by millstone 4764 days ago
The key phrase is "either of which could run 32-bit and 64-bit applications." OS X supported 64-bit apps on a 32-bit kernel. This enabled users to run 64 bit apps, and use >4 GB address space, without any device driver incompatibilities.

I think Solaris (and Windows and Linux) require a 64 bit kernel to run 64 bit apps. Correct me if I'm wrong.
1 comments
binarycrusader 4764 days ago
Fair enough.

With that said, OS X's hack support for 64-bit applications on a 32-bit kernel certainly wasn't at the same performance level as a 64-bit kernel as they claim. It's a dubious achievement at best given the performance tradeoffs.

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millstone 4764 days ago
It seems like you are casting aspersions. In what way was it a "hack" and what performance tradeoffs were there? Apple's claim is that 64 bit apps ran at native speeds, which is true, since they just used the native ISA.

Apple lists a few benefits of K64 at https://developer.apple.com/library/mac/#documentation/MacOS... , specifically more efficient support for systems with lots of RAM, a larger buffer cache, and better support for multiple video cards with over 2 GB of RAM. These are pretty specialized, and I certainly don't remember my Mac getting a lot faster when it switched to K64. (I sure wish it had!)

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binarycrusader 4764 days ago
I don't need to cast aspersions.

Apple's claims about performance primarily applied to OS X when run on a specific version of the PowerPC processor:

http://www.ece.uprm.edu/~nayda/Courses/Inel4215F03/power64.p...

(See pages 304-306.)

But, there was a cost to that dependent on architecture. If you read the document I linked above, you'll see that there is a cost to this 64-bit translation -- the performance is not completely equivalent to a 64-bit application running on a 64-bit kernel where there is no need for translation.

And yes, there were performance tradeoffs. A 64-bit application running on a 32-bit OS still had all of the 32-bit limitations enforced (limit on maximum process size, number of file descriptors, etc.).

And once Apple made the transition to x86 from PowerPC they lost the built-in hardware advantage. x86 can also run 64-bit applications on a 32-bit kernel, but the cost of doing that hardware switch is not as cheap as it was on PowerPC. I also think it's very telling that Solaris, Linux, and Windows opted to never do this as well.

And now that OS X no longer offers a 32-bit kernel, this all seems moot anyway.

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