A 64-bit processor is one that generally supports a word size of 264, where the constant "maxint" has the value 264-1, or 18,446,744,073,709,551,615. The signed version of this number can express a range of -18,446,744,073,709,551,615..0 and 0..18,446,744,073,709,551,615. The unsigned version of this number can express a range of 0..18,446,744,073,709,551,616. It also means that programs and data spaces can be much larger than on the older 32-bit and 16-bit machines.
Examples of 32-bit processors include
- Newer versions of the X86 microcomputer, Itanium and newer, AMD64 (WINTEL architecture)
Currently, 64-bit processors (X86) are enhancements of 32-bit processors, and generally can run 32-bit and 16-bit applications. The reverse, however, is not true; 64-bit code cannot run on a machine smaller than 64 bits.