. A 32-bit register meant that 2[to the power of]32 addresses, or 4 gigabytes of RAM, could be referenced. At the time these architectures were devised, 4 gigabytes of memory was so far beyond the typical quantities available in installations that this was considered to be enough "headroom" for addressing. 4-gigabyte addresses were considered an appropriate size to work with for another important reason: 4 billion integers are enough to assign unique references to most physically countable things in applications like databases.
However, with the march of time and the continual reductions in the cost of memory (see Moore's Law), by the early 1990s installations with quantities of RAM approaching 4 gigabytes began to appear, and the use of virtual memory spaces exceeding the 4-gigabyte ceiling became desirable for handling certain types of problems. In response, a number of companies began releasing new families of chips with 64-bit architectures, initially for supercomputers and high-end workstation and server machines. 64-bit computing has gradually drifted down to the personal computer desktop, with Apple Computer's Mac Pro desktop line (as well as the Power Mac G5 before it) using a 64-bit processor (for the Mac Pro, the Intel Xeon), and AMD's "AMD64" architecture (implemented by Intel as "EM64T") becoming common in high-end PCs. The emergence of the 64-bit architecture effectively increases the memory ceiling to 2[to the power of]64 addresses, equivalent to 17,179,869,184 gigabytes or 16 exabytes of RAM
