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AMD64 is a 64-bit processor architecture that was developed by Advanced Micro Devices (AMD) to add 64-bit computing capabilities to the x86 architecture.  It is sometimes referred to as x86-64, x64, and Intel 64.  With its enhanced computing capabilities, it supports vastly more virtual memory and physical memory than the 32-bit version, which allows programs to store more data in memory.  AMD64 is backward compatible, which is highly beneficial in the real world because it means that 64-bit applications can coexist with 16-bit and 32-bit applications.   It is famous for its ability to support simultaneous 32-bit and 64-bit computing without performance penalties.

AMD64 was built as an alternative to the IA-64 architecture designed by Intel and Hewlett Packard.  AMD chose an evolutionary strategy, adding 64-bit capabilities to existing x86 architecture, whereas Intel created a completely new 64-bit architecture for their IA-64 offering.  The first AMD64-based processor (Opteron) shipped in 2003.

Linux was the first OS kernel to run the AMD64 architecture in long mode (the intended, primary mode of operation), beginning in 2001 before the hardware was even available for it, and it also provides backward compatibility for 32-bit executables.  SUSE Linux Enterprise 15 supports 32-bit runtimes on AMD64 architectures, which may be needed to run tools to set up software or hardware, that are still compiled as 32-bit binaries.