The AMD64 is, according to AMD’s official white paper on the subject, a “64-bit extension of the 486 architecture”. It merely translates the standard x86 commands into their 64-bit equivalents. Aside from expanding the general register size to 64 bits, it also doubles the number of general purpose registers and SMID streaming registers. It is fully backwards compatible with the x86 architecture, and can run in 32 or 16 bit modes.

The AMD64 architecture has largely set the standard for mainstream 64-bit computing. It is the first time in the history of the Intel vs AMD competition that AMD has led the market to the development of a new standard, with Intel following.

Source: http://www.amd.com/us-en/assets/content_type/white_papers_and_tech_docs/x86-64_overview.pdf

AMD first used their AMD64 architecture on their Opteron server processors. They now use the architecture in all of their new home, business, laptop, and server processors. Unlike Intel, who chose to discreetly add 64-bit technology into their processors, the 64-bit functionality has long been advertised as a hallmark feature of AMD’s desktop processors. They even chose to include the number 64 in their desktop and laptop processor names. This advertising has largely been misleading, as even to this day very few computers with AMD 64-bit processors installed are able to take advantage of this functionality.

We listened to enterprise customers and businesses that want to migrate to 64-bit computing but have an enormous investment in 32-bit software and can't afford to migrate without compatibility," Hector Ruiz, AMD president and CEO.