Intel has released their ”64-bit technology“ in order to compete with AMD’s 64-bit technology. This technology, officially called EM64T (Extended Memory 64 Technology), is present on all Pentium 4 6xx models, on Pentium 4 5x1 models (like 541, 551, 561, and so on) and also on Celeron D 3x1 and 3x6 models (331, 336, 341, 346 and so on). This shows us that Intel will add this technology to all new CPU that they will launch now on. In this article we will explain what is this technology and how it works.

If you want to compare Intel’s implementation to AMD’s , read our article AMD 64-bit architecture (x86-64).

CPUs with this technology have a new operating mode, called IA32E, which has two sub-modes:

• Compatibility mode: Allows 64-bit operating systems to run 32-bit and 16-bit software unmodified. The operating system can have 64-bit (in 64-bit mode), 32-bit and 16-bit (both on compatibility mode) programs running at the same time. However, 32-bit programs will run as if they were running on a 32-bit CPU, i.e., will access only up to 4 GB of RAM. The same idea goes to 16-bit program, that will still access only up to 1 MB of RAM.
• 64-bit mode: Allows 64-bit operation systems to use the new 64-bit addressing space provided by this technology.

As you can see above, EM64T technology can only be used by 64-bit operating systems, like the forthcoming Windows 64. 32-bit operating systems, like Windows XP, will continue to run under regular IA32 mode, i.e., using the regular 32-bit addressing space – in order words, can access only up to 4 GB RAM.

In plain English, Windows nowadays cannot take advantage of EM64T technology since it is still a 32-bit operating system.