One of the greatest benefits of the AMD Opteron processor is its binary compatibility with the existing 32-bit, x86 instruction set. The processor executes both 32-bit and 64-bit AMD64 code natively in the processor, eliminating overhead that can impair 32-bit application performance in other processors. To do this, the processor supports two modes:

Long Mode: Allows both 64-bit and x86 protected-mode 32-bit applications to run simultaneously. Any 64-bit applications see the full, extended register set of the 64-bit processor, while 32-bit applications see the familiar register set of x86-32 processors.

Legacy Mode: Supports 32-bit and 16-bit applications, including non-protected-mode x86 applications that won't run under long mode.

Servers based on the AMD Opteron processor run everything from 64-bit versions of the Sun Solaris OS, Linux, and Windows, to ancient 16-bit versions of DOS.

The most compelling aspect of 64-bit processors is the higher memory address ceiling, which breaks through the 4GB limit imposed by 32-bit architectures. Advanced server and workstation software, such as database caching and management, online transaction processing, electronic design automation (EDA), mechanical CAD (MCAD), and many scientific and mathematical applications, engage data sets in excess of 4GB. Photo, imaging, and video editing packages also demand access to very large data sets.