QuestioniMacs and the Quad-core Mac Pro’s support up to 8gb of ram. And the 8-core Mac Pro supports up to 32GB. BTW I’m also use Photoshop. — LordAboveAll, Guru3D

Answer: That’s only for physical addressing for being able for the operating system to use 4 GB or more. Each software still can’t use more than 4 GB. Hence me saying that PAE actually works properly on Macs.

Photoshop on Macs unfortunately is a 32-bit application so if you’re rendering an image that is large enough that Photoshop needs to use more than 4 GB of its virtual addressing, it’ll either lag or crash out since even if PAE is enabled, it can only do the physical addressing and not the virtual addressing that software relies on. It won’t matter if you have 8 GB or not by then. You may be able to shove more software into it, but if you account for physical and pagefile (or swap space since MacOS X is a UNIX system), it may still exceed the 4 GB virtual barrier which the software relies on. The software doesn’t know how much RAM you really have. On Windows, it only gets 2 GB to work with on 32-bit systems (The other 2 GB goes to system and Windows itself). On UNIX machines like MacOS X, the operating system and other software get their own 4 GB addressing and won’t partition it from the software like Windows. Unless you’re running 64-bit, no software can use more than 4 GB, esp. in the virtual space. Hence, PAE is only a temporary solution.

The only reason why MacOS X can use more than 4 GB is through the technique which is mentioned called PAE which stands for Physical Address Extension. This means, through some workaround, it can extend physical addressing. However, it doesn’t do virtual addressing which software relies on. Since there’s no virtual address extensions, it won’t extend pass the 4 GB barrier. Such is life on the 32-64-bit transition phase.

By shoving more software, I mean that the “Virtual Memory Manager” which handles both physical memory and swap space (this system exist in all modern operating systems) will take that 4 GB allocation in virtual address and shove it inside that physical memory. It’s called memory mapping. That 4 GB in the virtual address can no way be expanded since there’s no workaround for that. Think like hosting space. You have an 1 TB drive on your server and you have a set limit of 200 MB per hosting account (and that’s the only space you may be able to set it due to some strange theoritical limit if it exists). Even though you can buy a larger drive, those hosting accounts still can’t be expanded, your customers still have that 200 MB limit even if you got a 2 TB drive. This is the same idea with the virtual memory manager and the 32-bit virtual address limit.

Question: The unix kernal Mac os X uses is not like a windows kernal. The unix kernal does not have memory limitations like windows kernals. It could address more than 4gb of ram before, and right now it’s using a hybrid kernal. For photoshop and such, 8gb of ram is nice. But if you’re just playing games and such, then keep at around 4gb to save money. — vbetts, Guru3D

Answer: Not quite, MacOS X doesn’t use a UNIX kernel at all. It uses a kernel developed by the University of Carnegie Mellon called Mach which is a microkernel kernel. Albeit, the kernel used in MacOS X is heavily modified to what is known as a hybrid kernel mixing the philosophies of monolithic and microkernels. This is true with Windows as well (as a hybrid kernel). Linux is an example of a monolithic kernel but with modules. Probably it’s the interesting design of Mach is the reason Apple can run 64-bit applications on a 32-bit kernel. This is probably due to the microkernel side of the design.

The only part that is UNIX 03 certified is the BSD userland that was infused with the Mach-based kernel space. You can run UNIX software on the operating system, but it’s not UNIX in the traditional sense. This infusion makes up the opensource part of MacOS X called Darwin which is released under the Apple Public Software License (APSL). You can download the source code and look through what makes up the base of MacOS X. The kernel package is called XNU as that is the official name of the MacOS X/Darwin OS kernel.

Like other 32-bit operating systems, UNIX or not, MacOS X can only give the kernel space and every other applications 4 GB to use. Funny thing is that all major operating systems in the world have their design roots in UNIX. The only “big” operating system I can think of that isn’t is the IBM i5/OS (which large conglomerates love due to its amazing reliability … more stable than most UNICES and anything Microsoft can come up with).

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