The AMD64 CPU architecture (codenamed "Hammer", and previously referred to as the x86-64 architecture) represents AMD's entry into the 64-bit microprocessor market.
AMD64 represents a break with AMD's past behavior of following Intel's standards, but follows Intel's earlier behavior of extending the so-called x86 architecture, from the 16-bit 8086 to the 32-bit 80386 and beyond, without ever removing backwards compatibility. The AMD64 architecture extends the 32-bit x86 architecture (IA-32) by adding 64-bit registers, with full 32-bit and 16-bit compatibility modes for earlier software. Even the 64-bit mode is largely backward-compatible, allowing existing tools targeting x86 (eg. compilers) to be retargeted to AMD64 with minimal effort.
This is in sharp distinction to Intel's competing IA-64 architecture, which makes a complete break from Intel's own x86 architecture.
AMD has announced that its first generation of AMD64 processors will be marketed under the names Athlon 64 and Opteron. The Athlon 64 (codenamed "ClawHammer") is designed for desktop and mobile computers, while the Opteron (codenamed "SledgeHammer") is intended for servers and workstations. The Opteron processor has been shipping since April 22, 2003, but no information about its real-world performance is available. Several companies are starting to ship Opteron-based servers.