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XNU
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===K32/K64=== XNU in [[Mac OS X Snow Leopard]], v10.6, ([[Darwin (operating system)|Darwin]] version 10) comes in two varieties, a [[32-bit]] version called ''K32'' and a [[64-bit computing|64-bit]] version called ''K64''.<ref>[https://arstechnica.com/apple/reviews/2009/08/mac-os-x-10-6.ars/5 Mac OS X 10.6 Snow Leopard: the Ars Technica review, page 5]</ref> K32 can run 64-bit applications in [[Userland (computing)|userland]].<ref>{{cite web|url=https://arstechnica.com/apple/2009/08/mac-os-x-10-6/5/|title=Mac OS X 10.6 Snow Leopard: the Ars Technica review|last=Siracusa |first=John |date=August 31, 2009|website=[[Ars Technica]]|quote=Finally, this is worth repeating: please keep in mind that you do not need to run the 64-bit kernel in order to run 64-bit applications or install more than 4 GB of RAM in your Mac. Applications run just fine in 64-bit mode on top of the 32-bit kernel, and even in earlier versions of Mac OS X it's been possible to install and take advantage of much more than 4GB of RAM.}}</ref> What was new in Mac OS X 10.6 was the ability to run XNU in 64-bit [[kernel space]]. K32 was the default kernel for 10.6 ''Server'' when used on all machines except [[Mac Pro]] and [[Xserve]] models from 2008 onwards<ref>[http://support.apple.com/kb/HT3770 Mac OS X Server v10.6: Macs that use the 64-bit kernel]</ref> and can run 64-bit applications. K64 has several benefits compared to K32:<ref>{{Cite web |url=https://developer.apple.com/mac/library/releasenotes/MacOSX/WhatsNewInOSX/Articles/MacOSX10_6.html |title=What's New in Mac OS X: Mac OS X v10.6 |website=Apple Developer |publisher=Apple, Inc.}}</ref> * Can manage more than 32 GB RAM, as the [[memory map]] would consume a disproportionately large area of the 32-bit kernel space. * Cache buffer sizes can be larger than what the 32-bit kernel space allows, potentially increasing I/O performance. * Performance is increased when using high-performance networking devices or multiple [[graphics processing unit]]s (GPUs), as the kernel can map all of the devices in 64-bit space even if several have very large [[direct memory access]] (DMA) buffers. Booting while holding down ''6'' and ''4'' forces the machine to boot K64 on machines supporting 64-bit kernels.<ref>[http://support.apple.com/kb/HT3773 Mac OS X Server v10.6: Starting up with the 32-bit or 64-bit kernel]</ref> K64 will run 32-bit applications but it will not run 32-bit [[Loadable kernel module|kernel extensions]] (KEXTs), so these must be ported to K64 to be able to load. XNU in [[OS X Mountain Lion]] (10.8) and later only provides a 64-bit kernel.
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