How many bits is a word

Question

This is from the book Assembly Language Step By Step  Jeff Duntemann      Here   s the quick tour  A bit is a single binary digit  0 or 1  A byte   is 8 bits side by side  A word is 2 bytes side by side  A double word   is 2 words side by side  A quad word is 2 double words side by side    And this is from the book Principles of Computer Organization and Assembly Language  Using the Java Virtual Machine  Patrick Juola      For convenience  8 bits are usually grouped into a single block    conventionally called a byte  The next-largest named block of bits is   a word  The definition and size of a word are not absolute  but vary   from computer to computer  A word is the size of the most convenient   block of data for the computer to deal with    So is a word 2 bytes  16 bits   or is it the most convenient block of data for the computer to deal with   I am also not sure what this means

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most convenient block of data  probably refers to the width  in bits  of the WORD  in  correspondance to the system bus width  or whatever underlying  bandwidth  is available  On a 16 bit system  with WORD being defined as 16 bits wide  moving data around in chunks the size of a WORD will be the most efficient way   On hardware or  system  level    With Java being more or less platform independant  it just defines a  WORD  as the next size from a  BYTE   meaning  full bandwidth   I guess any platform that s able to run Java will use 32 bits for a WORD

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The second quote is correct  the size of a word varies from computer to computer  The ARM NEON architecture is an example of an architecture with 32-bit words  where 64-bit quantities are referred to as  doublewords  and 128-bit quantities are referred to as  quadwords       A NEON operand can be a vector or a scalar  A NEON vector can be a 64-bit doubleword vector or a 128-bit quadword vector    Normally speaking  16-bit words are only found on 16-bit systems  like the Amiga 500

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This is from the book Hackers  Heroes of the Computer Revolution by Steven Levy         the memory had been reduced to 4096  words  of eighteen bits each     A  bit  is a binary digit  either a 1 or 0  A series of binary   numbers is called a  word      As the other answers suggest  a  word  does not seem to have a fixed length

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On x86 x64 processors  a byte is 8 bits  and there are 256 possible binary states in 8 bits  0 thru 255   This is how the OS translates your keyboard key strokes into letters on the screen   When you press the  A  key  the keyboard sends a binary signal equal to the number 97 to the computer  and the computer prints a lowercase  a  on the screen   You can confirm this in any Windows text editing software by holding an ALT key  typing 97 on the NUMPAD  then releasing the ALT key   If you replace  97  with any number from 0 to 255  you will see the character associated with that number on the system s character code page printed on the screen   If a character is 8 bits  or 1 byte  then a WORD must be at least 2 characters  so 16 bits or 2 bytes   Traditionally  you might think of a word as a varying number of characters  but in a computer  everything that is calculable is based on static rules   Besides  a computer doesn t know what letters and symbols are  it only knows how to count numbers   So  in computer language  if a WORD is equal to 2 characters  then a double-word  or DWORD  is 2 WORDs  which is the same as 4 characters or bytes  which is equal to 32 bits   Furthermore  a quad-word  or QWORD  is 2 DWORDs  same as 4 WORDs  8 characters  or 64 bits   Note that these terms are limited in function to the Windows API for developers  but may appear in other circumstances  eg  the Linux dd command uses numerical suffixes to compound byte and block sizes  where c is 1 byte and w is bytes

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I m not familiar with either of these books  but the second is closer to current reality   The first may be discussing a specific processor   Processors have been made with quite a variety of word sizes  not always a multiple of 8   The 8086 and 8087 processors used 16 bit words  and it s likely this is the machine the first author was writing about     More recent processors commonly use 32 or 64 bit words   In the 50 s and 60 s there were machines with words sizes that seem quite strange to us now  such as 4  9 and 36   Since about the 70 s word size has commonly been a power of 2 and a multiple of 8

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In addition to the other answers  a further example of the variability of word size  from one system to the next  is in the paper Smashing The Stack For Fun And Profit by Aleph One      We must remember that memory can only be addressed in multiples of the   word size   A word in our case is 4 bytes  or 32 bits   So our 5 byte buffer   is really going to take 8 bytes  2 words  of memory  and our 10 byte buffer   is going to take 12 bytes  3 words  of memory

[byte] How many bits is a "word"?

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