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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?

Bo Persson
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prinzdezibel
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    In winapi speak, word is still fixed to 16-bit Windows and thus 16-bit, even on architectures with a large machine size. So in general in Windows HLL programs, word is 16-bit, so it is definitely context sensitive. As Guffa already says, the same 16-bittisms still exist in x86 assembler. – Marco van de Voort May 24 '15 at 20:35

9 Answers9

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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
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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
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    You mean I can't use QWORD on 32-bit machine at all? – selurvedu Jul 08 '15 at 10:45
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    @selurvedu Old comment, but for people who may come across this in the future; you likely can. Intel processors tend to have 64-bit MMX registers which enable you to use QWORDs and you can always read/write 64-bits of data to/from memory (keeping endianness in mind). – Some Guy May 05 '17 at 14:26
  • With regard to @SomeGuy's point about endianness, [this post](https://stackoverflow.com/q/7865511/2636454) is helpful in understanding that. – GDP2 Jul 25 '17 at 06:14
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It depends upon the machine architecture. This document explains some basics about this.

Vinay
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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
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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
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16 bits (2 bytes) to a word is universal for x86.

Nick
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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.

Mihai Limbășan
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Peter Miehle
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    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 Mar 07 '09 at 11:13
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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
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  • Someone missing a sense of humour, or have to get rid of reputation points? Have fun, either way. – Pontus Gagge Mar 07 '09 at 11:04
  • 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! –  Mar 07 '09 at 11:06
  • 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 Mar 07 '09 at 11:11
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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
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