Legacy Mode includes no 64-bit support at all, effectively closing off access to the upper half of the processor's registers. It is completely compatible with 32-bit x86 software.

Long mode is made up of two sub modes, Compatibility mode and 64-bit mode. Compatibility mode is designed for a 64-bit operating system such as Microsoft's impending 'x64 edition' of Windows XP and Microsoft Server 2003 running 32-bit software. The beta version of 64-bit XP supports 32-bit executables through the use of this mode.

When running in this mode, each 32-bit application is still limited to 4GB of memory, but it can have all of that 4GB to itself with no overhead for the operating system (assuming that there is more than 4GB of memory installed).

The OS will use the memory space over 4GB which cannot be addressed by 32-bit apps. This provides a great shot in the arm for servers using demanding 32-bit applications, like databases, which cannot be easily changed over to 64-bit operation. 64-bit mode is intended for a pure 64-bit environment, both operating system and software. The AMD64 architecture allows a processor running a 64-bit operating system to switch seamlessly between these two sub modes.

Inside the Athlon 64 processor

In September of 2003, the AMD Athlon 64/FX line of processors brought the flexibility of the AMD Opteron processor to the home and enthusiast markets.

As a fully functional 64-bit release of Windows XP or Server is still a year or more off, and the next version of Windows is farther still, 32-bit compatibility is a good thing. Likewise for Linux enthusiasts who now had the hardware to exploit its full capabilities within their reach.

Still, people wondered if the AMD Athlon 64 would bring anything to the table besides 64 bits of potential for the future. They needn't have worried. The architecture of the Athlon 64 is radically different from conventional 32-bit processors, and the changes that AMD made benefit 32-bit performance as well as enabling 64-bit operations.

The first of the two main changes was the relocation of the memory controller off the motherboard (where it traditionally resides in the Northbridge chipset) and into the processor die itself. Thus the processor gained the advantage of unrestricted access to memory.

In previous designs, data traveling to and from the memory controller and the memory itself had to pass through the Northbridge chip and across the bus linking it to the processor. This same path is shared by data from every other peripheral attached to the system. As you can imagine, information bottlenecks often resulted.