History
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 (which was an exception, in that it used the low order 24 bits of a word for addresses, resulting in a 16 MB address space size), 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 (4 MB) in installations that this was considered to be enough "headroom" for addressing. 4.29 billion addresses were considered an appropriate size to work with for another important reason: 4.29 billion integers are enough to assign unique references to most entities in applications like databases.
Some supercomputer architectures of the 1970s and 1980s used registers up to 64 bits wide. In the mid-1980s, Intel i860 development began culminating in a (too late for Windows NT) 1989 release. 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 GB 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, Silicon Graphics, and Hewlett Packard had developed 64-bit architectures for their workstation and server systems. A notable exception to this trend were mainframes from IBM, which then used 32-bit data and 31-bit address sizes; the IBM mainframes did not include 64-bit processors until 2000. During the 1990s, several low-cost 64-bit microprocessors were used in consumer electronics and embedded applications. Notably, the Nintendo 64 and the PlayStation 2 had 64-bit microprocessors before their 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 AMD released its first 64-bit x86-64 processor.
Read more about this topic: 64-bit Computing
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