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A term describing a computer architecture based around an ALU, registers and data bus which are 64 bits wide.

64-bit processors were quite common in 1996, e.g. Digital Alpha, versions of Sun SPARC, MIPS, IBM AS/4000. the PowerPC and Intel were expected to move to 64 bits at their next generation - PPC 620 and Intel P7.

A 64-bit address bus allows the processor to address 18 million gigabytes as opposed to the mere 4 gigabytes allowed with 32 bits. There were in 1996 already hard disks which can hold over 4GB. Floating point calculations can also be more accurate.

A 64-bit OS is needed as well to take advantage of the CPU. In 1996 there were only a few 64-bit operating systems, including OS/400, Digital Unix, Solaris (partialy). A 32-bit OS can run on a 64-bit CPU.
This article is provided by FOLDOC - Free Online Dictionary of Computing (foldoc.org)

64-bit computing

CPUs that process 64 bits as a single unit, compared to 8, 16 or 32. Today's desktop and laptop computers are mostly 64-bit machines; however, there are countless 32-bit, 16-bit and 8-bit microprocessors manufactured every year (see microcontroller).

A 64-bit computer is not twice as fast as its 32-bit predecessor. The 64 bit "word size" is only one aspect of internal processing. The CPU's clock speed, along with the speed of storage, RAM and input/output (the peripheral bus) all play roles in a computer's performance (see throughput). In many cases, the perceived difference between the same brand of a 32-bit and 64-bit computer can be negligible. See word.

More Memory
The major advantage of a 64-bit computer is its larger address bus, which supports considerably more RAM than its 32-bit counterpart. A 32-bit computer is typically limited to 4GB, whereas a 9th-generation 65-bit Intel i9 CPU can handle 128GB and a third-generation 64-bit Mac Pro supports 1.5TB (that's 1.5 terabytes of RAM, not storage).

A Lot of 32-Bit Software
Although CPUs migrated to 64-bits years ago, many 32-bit applications still run in 64-bit computers. The Mac ran 32-bit and 64-bit applications until the Catalina version of macOS in 2019, which supports only 64-bit programs.

Windows comes in both 32-bit and 64-bit versions. The 64-bit version also runs 32-bit apps, and there are many in existence. However, people still run old 16-bit DOS and 16-bit Windows apps that only 32-bit Windows supports. See Intel 64, AMD64, Opteron and Athlon. See 8-bit computing, 16-bit computing, 32-bit computing, 128-bit computing and bit specifications.

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