What is a “64-bit” computer? The word size of a computer generally indicates the largest integer it can process in a single instruction, and the size of a memory address, which is usually, but not necessarily the same as the integer size.

The main indication of the word size is how much memory the processor can address. The maximum amount of memory a processor is capable of accessing is known as the address space.

A processor with a 32-bit address, such as an Intel Pentium or AMD Athlon, can only represent 2³² distinct memory addresses. Hence, if each address contains one byte, then the computer can have at most 2³² bytes, or 4 gibibytes (a little more than 4 billion bytes) of memory. This is more than enough RAM for most purposes, but some applications can benefit from using much more, so 64-bit processors and cheap RAM are blessings to some types of computing.

An AMD64 processor, also known as x86_64, has a 64-bit memory address, and uses byte-addressable memory (one byte of memory per address). Hence, the memory address space is 2⁶⁴ bytes (16 exbibytes, or roughly 16 billion gibibytes). Recall that a gibibyte is 2³⁰ bytes, or a little more than a gigabyte (10⁹). This is far more memory than any single computer is ever likely to have. Most PCs have between a few gibibytes and a few hundred gibibytes.