Unlike Microsoft’s Windows, Apple’s Mac OS X does 64-bit right
Tuesday, August 26, 2008 - 12:20 PM EST "Even with the 64-bit Power Mac G5 hardware, there were still software limitations in 2003's Mac OS X Panther; the 32-bit OS allowed the system to support more than 4GB of memory but still corralled each application into its own 32-bit, 4GB space. With 2005's Mac OS X Tiger, Apple enabled desktop apps to spin off processes and servers that could handle enormous memory addressing of their own: up to a theoretical 16 EB of 64-bit virtual memory and a conceptual 42-bits or 4TB of physical RAM, although shipping Macs still could only support 8GB of RAM," Prince McLean reports for AppleInsider."To enable this, Tiger supplied a 64-bit version of libsystem, the system library handling most of its Unix APIs. This followed the LP64 model to allow broad compatibility with 64-bit versions of Linux and commercial Unix. It also delivered a 64-bit PowerPC ABI (application binary interface) for accommodating native 64-bit apps on the G5. Tiger still used a 32-bit kernel (although it was not limited to 32-bit memory addressing, so it could actually make use of the 8GB of RAM installed in G5s), and it was also still missing a 64-bit version of the Cocoa or Carbon APIs, which meant apps with a user interface had to be 32-bit," McLean reports.
"However, a 32-bit graphical app on Tiger could spin off a faceless 64-bit background process to perform number crunching on a vast data set requiring a 64-bit memory space, which could then communicate the results back to the 32-bit foreground app running in parallel. Apple also delivered a mechanism for deploying applications using a bundle of both 64-bit and 32-bit code, allowing the system to automatically run the appropriate version for the Mac hardware in use. Tiger itself also supplied both 32- and 64-bit underpinnings, allowing one OS to run on any Mac," McLean reports. "This has made it easier for Apple to rapidly migrate Mac users toward 64-bit hardware."
"When Apple began migrating to Intel in 2006 it actually had to take a step backward, as it only initially supported 32-bit Intel systems with the Core Solo and Core Duo CPUs. Apple had to cope with the same 32-bit PC limitations Microsoft had been dealing with. in the Intel transition, Mac developers lost the features supplied by PowerPC, including its liberal supply of registers. However, Intel's new 32-bit Core Duo was fast enough in other areas to skirt around the problem, particularly in laptops where the aging G4 was holding Macs back," McLean reports.
"By the end of the year Apple had widened support to include the 64-bit x64 PC architecture in the new Mac Pro and Xserve, and subsequent desktop Macs using the Core 2 Duo also delivered 64-bit hardware support. With updates to Tiger, Apple delivered the same level of 64-bit support for x64 Intel processors as it had for the PowerPC G5," McLean reports. "Within the course of one year, Apple had not only adroitly moved its entire Mac product line to Intel but also paved the way forward to rapidly push its users to 64-bits, narrowly escaping the disaster of being left the last member of the desktop PowerPC party. In its spare time, the company also threw the iPhone together while also working to develop its next jump in 64-bit operating system software."
"In contrast, a separate 64-bit version of Windows is required to run 64-bit Windows apps on 64-bit x86 PCs, and any 32-bit apps have to run in a special compatibility environment (below). There is no slick mechanism for deploying bundles of mixed code that "just work" on both architectures, and 64-bit Windows itself lacks the ability to run on either type of PC. This has had a chilling effect on the popularity of and the momentum behind 64-bit Windows that parallels the problems with Vista," McLean reports. "Software compatibility, a lack of drivers, and other problems have also complicated the move to 64-bit Windows, leaving mainstream Windows users stuck at 32-bits. Windows 7 was initially supposed to move users to 64-bits in perhaps 2010, but reports indicate that it too will be delivered in separate 32- and 64-bit versions."
Much more in the full article, "Road to Mac OS X 10.6 Snow Leopard: 64-Bits" — highly recommended — here.

Wow!
What can I say?