- Micro-architecture improvements: wider general-purpose registers, 8 additional 64-bit registers, and 8 additional SSE (XMM) registers.
- New 64-bit instructions
- New Long mode that includes two sub-modes:
- 64-bit mode: in this mode the CPU can execute only 64-bit applications.
- Compatibility mode: In this mode the CPU can execute existing 32-bit and 16-bit applications, while running under control of 64-bit operating system.
To operate in the Long mode a 64-bit Operating System is required.
- Better performance due to wider register width, bigger number of general and SSE registers, and cleaner architecture.
- Larger size of addressable memory. 32-bit microprocessors cannot address more than 4 GB of physical memory. K8-based microprocessors (Athlon 64, Opteron, etc) in 64-bit mode can access up to 1 TB of memory, and K10-based processors (Phenom, Third Generation Opteron) can access up to 256 TB of memory.