Most CPUs are designed so that the contents of a single integer register can store the address (location) of any datum in the computer's virtual memory. Therefore, the total number of addresses in the virtual memory — the total amount of data the computer can keep in its working area — is determined by the width of these registers. Beginning in the 1960s with the IBM System/360, then (amongst many others) the DEC VAX minicomputer in the 1970s, and then with the Intel 80386 in the mid-1980s, a de facto consensus developed that 32 bits was a convenient register size. A 32-bit address register meant that 232 addresses, or 4 GB of RAM, could be referenced. At the time these architectures were devised, 4 GB of memory was so far beyond the typical quantities (16 MB) available in installations that this was considered to be enough "headroom" for addressing. 4 GB 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.
Some supercomputer processor architectures of the 1970s and 80s used registers up to 64 bits wide. However, 32 bits remained the norm until the early 1990s, when the continual reductions in the cost of memory led to installations with quantities of RAM approaching 4 GB, and the use of virtual memory spaces exceeding the 4-gigabyte ceiling became desirable for handling certain types of problems. In response, MIPS and DEC developed 64-bit microprocessor architectures, initially for high-end workstation and server machines. By the mid-1990s, HAL Computer Systems, Sun Microsystems, IBM and Hewlett Packard had developed 64-bit architectures for their workstation and server systems. A notable exclusion to this trend were mainframes from IBM, which remained 32-bit. During the 1990s, several low-cost 64-bit microprocessors were used in consumer electronics and embedded applications. Notably, the Nintendo 64 and PlayStation 2 both had 64-bit microprocessors before its introduction in personal computers. High-end printers and network equipment, as well as industrial computers also used 64-bit microprocessors such as the Quantum Effect Devices R5000. 64-bit computing started to drift down to the personal computer desktop from 2003 onwards, when some models in Apple's Macintosh lines switched to PowerPC 970 processors (termed "G5" by Apple) and the launch of AMD's 64-bit x86-64 extension to the x86 architecture, processors based on this architecture becoming common in high-end PCs.
The emergence of the 64-bit architecture effectively increases the memory ceiling to 264 addresses, equivalent to approximately 17.2 billion gigabytes, 16.8 million terabytes, or 16 exabytes of RAM. To put this in perspective, in the days when 4 MB of main memory was commonplace, the maximum memory ceiling of 232 addresses was about 1,000 times larger than typical memory configurations. Today, when over 2 GB of main memory is common, the ceiling of 264 addresses is about ten trillion times larger, i.e., ten billion times more headroom than the 232 case.