What's the difference between a word and byte?

Question

I've done some research. A byte is 8 bits and a word is the smallest unit that can be addressed on memory. The exact length of a word varies. What I don't understand is what's the point of having a byte? Why not say 8 bits?

I asked a prof this question and he said most machines these days are byte-addressable, but what would that make a word?

Answer 1

The exact length of a word varies. What I don't understand is what's the point of having a byte? Why not say 8 bits?

Even though the length of a word varies, on all modern machines and even all older architectures that I'm familiar with, the word size is still a multiple of the byte size. So there is no particular downside to using "byte" over "8 bits" in relation to the variable word size.

Beyond that, here are some reasons to use byte (or octet¹) over "8 bits":

1. Larger units are just convenient to avoid very large or very small numbers: you might as well ask "why say 3 nanoseconds when you could say 0.000000003 seconds" or "why say 1 kilogram when you could say 1,000 grams", etc.
2. Beyond the convenience, the unit of a byte is somehow as fundamental as 1 bit since many operations typically work not at the byte level, but at the byte level: addressing memory, allocating dynamic storage, reading from a file or socket, etc.
3. Even if you were to adopt "8 bit" as a type of unit, so you could say "two 8-bits" instead of "two bytes", it would be often be very confusing to have your new unit start with a number. For example, if someone said "one-hundred 8-bits" it could easily be interpreted as 108 bits, rather than 100 bits.

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¹ Although I'll consider a byte to be 8 bits for this answer, this isn't universally true: on older machines a byte may have a different size (such as 6 bits. Octet always means 8 bits, regardless of the machine (so this term is often used in defining network protocols). In modern usage, byte is overwhelmingly used as synonymous with 8 bits.

Answer 2

Whatever the terminology present in datasheets and compilers, a 'Byte' is eight bits. Let's not try to confuse enquirers and generalities with the more obscure exceptions, particularly as the word 'Byte' comes from the expression "By Eight". I've worked in the semiconductor/electronics industry for over thirty years and not once known 'Byte' used to express anything more than eight bits.

Answer 3

It seems all the answers assume high level languages and mainly C/C++.

But the question is tagged "assembly" and in all assemblers I know (for 8bit, 16bit, 32bit and 64bit CPUs), the definitions are much more clear:

byte  = 8 bits 
word  = 2 bytes
dword = 4 bytes = 2Words (dword means "double word")
qword = 8 bytes = 2Dwords = 4Words ("quadruple word")

Answer 4

The terms of BYTE and WORD are relative to the size of the processor that is being referred to. The most common processors are/were 8 bit, 16 bit, 32 bit or 64 bit. These are the WORD lengths of the processor. Actually half of a WORD is a BYTE, whatever the numerical length is. Ready for this, half of a BYTE is a NIBBLE.

Answer 5

Why not say 8 bits?

Because not all machines have 8-bit bytes. Since you tagged this C, look up CHAR_BIT in limits.h.

Answer 6

Byte: Today, a byte is almost always 8 bit. However, that wasn't always the case and there's no "standard" or something that dictates this. Since 8 bits is a convenient number to work with it became the de facto standard.

Word: The natural size with which a processor is handling data (the register size). The most common word sizes encountered today are 8, 16, 32 and 64 bits, but other sizes are possible. For examples, there were a few 36 bit machines, or even 12 bit machines.

The byte is the smallest addressable unit for a CPU. If you want to set/clear single bits, you first need to fetch the corresponding byte from memory, mess with the bits and then write the byte back to memory.

The word by contrast is biggest chunk of bits with which a processor can do processing (like addition and subtraction) at a time. That definition is a bit fuzzy, as some processor might have different word sizes for different tasks (integer vs. floating point processing for example). The word size is what the majority of operations work with.

There are also a few processors who have a different pointer size: for example, the 8086 is a 16-bit processor which means its registers are 16 bit wide. But its pointers (addresses) are 20 bit wide and were calculated by combining two 16 bit registers in a certain way.