Recently, the subject of AMD 64-bit extensions came up among some colleagues and we debated the merits of 64-bit computing. But over the Christmas weekend, Valve software unveiled a 64-bit version of their flagship game Half-life 2. What was interesting was that no where in the announcement did the word Intel come up even though all of the current model Server and Desktop Intel CPUs (not mobile) support AMD's 64-bit extensions.
64-bit extensions was invented by AMD a few years ago as an add-on to their existing 32-bit CPUs to ease the transition from 32 to 64 bit computing while Intel refused to enter the 32/64 bit market and focused on their pure 64-bit Itanium CPUs. Since the Itanium had no legacy circuitry and relied on software emulation for 32-bit compatibility, its 32-bit performance was lackluster while true 64-bit software was at best rare and at worst nonexistent. The success of AMD64 and the weak sales of Itanium forced Intel to abandon their pure 64-bit strategy for their Itanium chip and adopt AMD's 64-bit extensions for their own Pentium 4 and XEON line of processors. Since Cross licensing arrangements between AMD and Intel allow AMD to use Intel technology and vice versa, Intel simply added AMD64 instruction capability to their CPUs which they refer to as EM64T. The same arrangement allowed AMD to implement Intel's MMX and SSE/SSE2 instruction in their CPUs which is commonly used in today's software while 64-bit extensions are not. Microsoft simply refers to Intel's EM64T and AMD's AMD64 technology as x64 for their desktop/server operating systems.
Most technically savvy people I know have the perception that only AMD produced these hybrid 32/64 bit CPUs, but I have to wonder why Intel is failing to market their own 64-bit capabilities and allowing AMD to gain so much market share. Intel losing the prestigious benchmark wars in both single-core and dual-core competition certainly has something to do with Intel's slide in market share, but the 64-bit factor played a very large role in public perception even though it has no current practical significance for the vast majority of the computing population. The truth of the matter is, almost no one uses 64-bit extensions today. Even if you manage to install the x64 version of Windows XP and all 64 bit drivers, you're limited to two 64-bit games like Farcry and Half-life 2 along with a handful of x64 applications. On the performance front, there is no real difference in speed between most 32 and 64 bit software. 64-bit computing begins to makes sense when more than 4 GBs of RAM is needed but the reality is that most people today don't need more than 4 GBs of RAM. This means 64-bit is only significant from a marketing perspective and Intel has practically handed the 64-bit marketing advantage to AMD.