Previously it was believed that all Intel Pentium 4 processors “Prescott” in LGA775 packaging would sport EM64T, but Intel denied such claim. Intel said it would ship Prescott processors with 64-bit capability for 1P applications only to system integrators requesting such microprocessors for their servers and workstations. Although all Prescott CPUs, including Intel Pentium 4 and Celeron D, are 64-bit from micro-architectural standpoint, processors supplying for retail channels as well as for typical desktops have their 64-bit capability disabled.

It is unclear, how Intel suspends 64-bit capability on its chips and whether it is possible to enable it on the level of BIOS, a small program that commands computers’ mainboards.

SRC: http://www.xbitlabs.com/news/cpu/display/20050104133604.html