64-Bit CPUs

Listen to the Computerworld TechCast: 64-Bit CPUs

Nowadays most computers use 32-bit processors (such as the Intel Pentium) running 32-bit operating systems (such as Windows XP, Mac OS, Unix or Linux). Some years back, desktop computers used 8-bit microprocessors (such as the Zilog Z80); then came 16-bit chips (the Intel 8086 and Motorola 68000). These bit numbers describe the length of the instruction word the CPU can handle in a single clock cycle. The next step in this evolution is the 64-bit CPU.

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Intel has been the microprocessor industry's 800-pound gorilla from the beginning. The company began 64-bit development in 1991, and the first systems with its 64-bit Itanium CPUs shipped in 2001.

Unfortunately, Intel developers early on opted for an architecture that's completely different from the common x86 (also known as IA-32) standard. The resulting platform has to resort to an inefficient emulation mode to run 32-bit applications.

The industry-leading vendor had stunningly misread what the market wanted, and the lack of true 32-bit compatibility caused the Itanium to languish. Approximately 5.3 million servers were shipped worldwide in 2003, and of those, 4.67 million (87%) had the 32-bit x86 architecture, according to analyst Mark Melanovsky at research firm IDC. Itanium CPUs were in just 19,000 servers.

A breakthrough came in April 2003, when Advanced Micro Devices Inc. in Sunnyvale, Calif., introduced its AMD64 platform and the Opteron series of 64-bit server CPUs. Unlike the Itanium, the Opteron chips could run 32-bit applications quickly and efficiently in addition to handling new 64-bit instructions. AMD's move led to faster, more cost-effective servers that didn't need to wait for the development of 64-bit applications.

AMD followed up the Opteron in September by announcing the Athlon 64 processor family for desktops and mobile computing. In 2003, some 35,000 Opteron-based servers (almost all of them dual-processor) were sold—nearly double the number of Itanium systems.

In response, Intel announced in February that within a few months it would ship new versions of its Xeon server CPUs (code-named Nocona and Prescott) that could handle 64-bit applications and operating systems. The new capability is being called Intel Extended Memory 64 Technology.

Analysts note, however, that the new Xeons aren't expected to offer the integrated memory controllers or HyperTransport links (a chip-to-chip interconnect technology that operates at memory speeds) of AMD64 chips. Intel's new CPUs are expected to be compatible with AMD's 64-bit instructions.

Why 64 Bits?

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