Somebody asked me if ESX 3.0 can run 64-bit guests like Windows Server 2003 x64 or Windows Vista x64. In short, the answer is yes. The longer, more complete answer is that there is experimental support for these guests and that it depends on the server's CPU.

Clearly, you need a 64-bit CPU because most of the instructions are run directly on the CPU. VMware does not use emulation of some sorts. It would be technically possible to run a 64-bit guest on a 32-bit CPU but the required emulation would decrease performance significantly.

For Intel, you need a processor with EM64T. For AMD, you need an Opteron revision E or later. I only refer to Opteron here because that will be the most common AMD processor for running ESX 3.0. Other AMD processors like the Athlon64 and Turion also support 64-bit guests but you will more than likely run VMware Player or VMware Server on those processors.

In addition to EM64T support, you also need VT support in Intel processors. VT support (in this case) is used to provide isolation for guests. According to VMware, doing so without VT would mean that performance would not be optimal. For 32-bit guests, VT is not required because the current implementation provides good performance.

VT (Virtualization Technology) is an Intel-only feature. AMD has similar technology in the works, Pacifica. However, on AMD processors, Pacifica is not needed. Segmentation is used to provide isolation and that feature is available for Opterons starting from revision E.

To sum up: ESX 3.0 has experimental support for 64-bit guests. You need an Intel CPU with EM64T and VT support - or - an AMD Opteron revision E and up.