x86-64 is a 64-bit microprocessor architecture and corresponding instruction set; it is a superset of the Intel x86 architecture, which it natively supports. It was designed by Advanced Micro Devices (AMD), who have since renamed it AMD64. This architecture has also been adopted by Intel under the name Intel EM64T or Extended Memory 64 Technology. This leads to the common use of the names x86-64 or x64 as more vendor-neutral terms to collectively refer to the two nearly identical implementations. Microprocessors based on the x86-64 technology can natively support both 32-bit and extended 64-bit operating systems. Depending on the operating system loaded, the microprocessor can be switched to operate in either of the modes. This provides a smooth upgrade path for end users from a 32-bit operating environment to a 64-bit environment when they are ready. It also proivdes users with future proof technologies and investment protection. The end user can just load a new operating system and they have a 64-bit environment on the same hardware. x86-64 microprocessors introduced a whole new set of enhancements to the proven x86 architecture. The most significant of these enhancements is support for larger addressable memory - up to 256 tebibytes of virtual address space (248 bytes). This limit can be raised in future implementations to 16 exbibytes (264 bytes). This is compared to just 4 gibibytes for 32-bit x86. This means that very large files can be operated on by mapping the entire file into the process' address space (which is generally faster than working with file read/write calls), rather than having to map regions of the file into and out of the address space. Other enhancements include:

• 64-bit integers
• Additional, wider registers
• Additional SSE registers for multimedia and vector processing
• NoeXecute bit
• Wider pointers

For more information, please check the official Intel website and AMD website. For more information, check the AMD whitepaper website.