Is a machine WORD always the same or does it depend on the machine architecture? And is the meaning of the word WORD context sensitive or generally applicable?
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A:
A word is typically the "native" data size of the CPU. That is, on a 16-bit CPU, a word is 16 bits, on a 32-bit CPU, it's 32 and so on.
And the exception, of course, is x86, where a word is 16 bit wide (because x86 was originally a 16-bit CPU), a DWORD is 32-bit (because it became a 32-bit CPU), and a QWORD is 64-bit (because it now has 64-bit extensions bolted on)
jalf
2009-03-07 10:50:17
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My understanding is that a WORD is the amount of bits that can be shoved into the CPU with one action (on a particular machine), so in a 8bit-architecture it is 8 bits and on a modern 64-bit architecture it is 64 bits.
Peter Miehle
2009-03-07 10:51:34
In 8-bit land, we would never have said word when we meant byte. A common definition of word has been simply "two bytes", even on systems with 9-bit bytes and 18-bit words. The PDP-10 comes to mind there...
RBerteig
2009-03-07 11:13:27
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The machine word size depends on the architecture, but also how the operating system is running the application.
In Windows x64 for example an application can be run either as a 64 bit application (having a 64 bit mahine word), or as a 32 bit application (having a 32 bit machine word). So the size of a machine word can differ even on the same machine.
The term WORD has different meaning depending on how it's used. It can either mean a machine word, or a type with a specific size. In x86 assembly language WORD, DOUBLEWORD (DWORD) and QUADWORD (QWORD) are used for 2, 4 and 8 byte sizes, regardless of the machine word size.
Guffa
2009-03-07 10:53:24
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Yes.
Ok, let me be a bit clearer. Assuming we are talking about words of memory, there are two broad definitions.
First, a word is often the natural size of a single item that can be accessed atomically in the hardware. That is very much a platform dependent size, but is usually 16, 32, or 64 bits, but other sizes have been found in the wild.
Second, it is often used to specifically mean a 16-bit value. In that context, you will see DWORD used to mean a 32-bit value. This usage is common on x86 platforms, especially Windows, but was used on DEC PDP-11 and VAX, and Motorola 68000 descendants as well.
Telling which is the intended usage depends on context...
RBerteig
2009-03-07 10:54:19
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All you youngsters yappin' on about 32 bit thiss and 64 bit that: you know, there were and are other machine architectures than the x86 family. A PDP-11 had 40-bit words, f'rinstance.
But the simplest answer is just to search Wikipedia.
Pontus Gagge
2009-03-07 10:56:41
Someone missing a sense of humour, or have to get rid of reputation points? Have fun, either way.
Pontus Gagge
2009-03-07 11:04:55
The PDP-11 had 16-bit words. The PDP-10 (DEcSystem-10) had 36-bit words. I'm not aware of a DEC architecture with 40-bit words, but they had so many!
anon
2009-03-07 11:06:32
PDP-11's (at least those running RT-11, but I think it was used in other DEC OSs also) did use a strange character set known as Rad50 that allowed a 6.3 file name to fit in three words, with three characters per word. DEC liked quirky data packing ;-) (I have a running PDP-11 in my office.)
RBerteig
2009-03-07 11:11:38
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WORD is a Windows specific 16-bit integer type, and is hardware independent.
If you mean a machine word, then there's no need to shout.
Pete Kirkham
2009-03-07 10:57:20
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A "word" in small letters depends on the architecture.
A "WORD" in capital letters, as defined in Windows SDK, is 16 bits.
Similarly: "DWORD" - (double word) 32 bits. "QWORD" ... 64 bits.
Jimmy J
2009-03-07 10:58:53