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I see talk of CPU's being either 32-bit or 64-bit processors. Information which is often required on download pages

But what is that feature officially called.

i.e What's the inverse of saying "I have a 64-bit processor"?

I want to say: The ??? of my processor is 64 bit

What is the correct term to use for ???

I have looked at a random product on the Intel site and I suspect the correct word for this is "Instruction Set", but I'm not sure.

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Thanks for your answers so far. Note: I have updated the question to add a bit more context "Information which is often required on download pages". Sorry if this alters relevance of the, great, answers already given. –  JW01 Jan 10 '11 at 6:01
It isn't as much a 'feature' as an underlying architectural difference. In its essence, 32-bit/64-bit describes how 'big' the CPU variables (registers) are in bits. This is why you have different OS versions, as they need to be re-coded/compiled to take advantage of these bigger CPU 'variables'. –  Dan McGrath Jan 10 '11 at 6:27
Yes - I think the small print on the Intel site (ark.intel.com/Product.aspx?id=40200) explains that 64-bit is only really meaningful when looking at the system as a whole - "64-bit computing on Intel® architecture requires a computer system with a processor, chipset, BIOS, operating system, device drivers and applications enabled for Intel® 64 architecture. Processors will not operate (including 32-bit operation) without an Intel 64 architecture-enabled BIOS. Performance will vary depending on your hardware and software configurations. Consult with your system vendor for more information." –  JW01 Jan 10 '11 at 20:12
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5 Answers

up vote 16 down vote accepted

As mentioned by Twisol, you are probably looking for "word-size", although what you should really be saying is:

"The registers (integer/address) of my CPU are 64-bits wide"*

You are definitely not looking for Instruction Set, as this is simply the actual set of instructions that a CPU understands. Your 64-bit CPU will have a 64-bit instruction set, but this is an effect, not a cause of the CPU being 64-bit (which as above, is to do with the register word-size).

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Some that come to mind are; registers, ALU, operand width, address mode

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The bit width generally refers to the native word size (i.e., the width of general-purpose integer registers) of the processor.

On x86 processors specifically, 64-bit mode is referred to in the documentation as long mode (which enables the 64-bit general-purpose registers), in contrast to 32-bit legacy mode, 32-bit protected mode (which is the native mode for 32-bit x86 processors), and the various 16-bit modes.

Note that this has little to do with the floating-point unit (which is 80-bit on x86) or the vector unit (which uses 128-bit registers).

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I use the term "bit width." According to Wikipedia, it's can be referred to as "word size," "bit width," "data path width," or "integer precision" when dealing with strictly integer numbers (as opposed to floating point). Wikipedia reference

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this sounds very odd to me. afaik a bit is the smallest unit a digital device uses to store information, and it's state is either on or off.. using 'bit width' to me kinda sounds like it's about the width of a bit. –  stijn Jan 10 '11 at 9:18
I think it's more about "how many bits wide" and not "how wide is a bit." –  pwc Jan 10 '11 at 15:41
The widest bit I've ever seen was as big as a beachball! I've had sleepless nights about that shocking incident ever since. ;o) –  JW01 Jan 10 '11 at 23:51
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I believe that would be "word size", but I'm not intimately familiar with the details at a hardware level.

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