IMO, it's not what anyone else might think. SQL, Exchange, and Web Services get all the hype, but I think Terminal Services will get the most immediate benefit from the backwards-compatible nature of the x64 architecture. Let's look at some of the benefits that the platform provides over x86 and IA-64:
- Huge VA memory space for the whole system. The current limits for x86 TS implementations is limted kernel memory space. Once you run out, that's it...no more users. What surprises many people is that it often happens well before 4GB of RAM gets exhausted. Forget about /PAE, that just makes the problem worse.
- Full-speed execution of 32bit x86 code, with full Win32 support. This means no one has to port business apps to see the benefit of the new platform. You can move to it on your own schedule. This is the way most Enterprises want to work.
- Shared pages for 32bit code. Besides the slow emulation solution for current IA-64 platforms, there's also a problem in that there are no shared code pages, due to the page size difference from x86 to IA-64 (4kb for x86 vs. 8kb for IA-64). That means every copy of Outlook needs 150MB, whereas on a shared memory system, many of those pages could be used by different processes for hundreds of users.
- No Execute (NX) means that there's an added level of security that wasn't available previously. This might cause heartache for some in-house apps, but thanks to the configurable nature of the NX support, most companies can use this feature to their benefit, without having to purchase all new workstations.
I don't have numbers, but I expect that you would get FAR more users onto a 4-proc (Opteron) 4GB or 8GB machine running 64bit Server 2003, than the same machine running 32bit Server 2003.
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