What is the history of why bytes are eight bits?

Question

What were the historical forces at work, the tradeoffs to make, in deciding to use groups of eight bits as the fundamental unit?

There were machines, once upon a time, using other word sizes. But today, for non-eight-bitness, you must look to museum pieces, specialized chips for embedded applications, and DSPs. How did the byte evolve out of the chaos and creativity of the early days of computer design?

I can imagine that fewer bits would be ineffective for handling enough data to make computing feasible, while too many would have lead to expensive hardware. Were other influences in play? Why did these forces balance out to eight bits?

(BTW, if I could time travel, I'd go back to when the byte was declared to be 8 bits, and convince everyone to make it 12 bits, bribing them with some early 21st century trinkets.)

Answer 1

A lot of really early work was done with 5-bit baudot codes, but those quickly became quite limiting (only 32 possible characters, so basically only upper-case letters, and a few punctuation marks, but not enough "space" for digits).

From there, quite a few machines went to 6-bit characters. This was still pretty inadequate though -- if you wanted upper- and lower-case (English) letters and digits, that left only two more characters for punctuation, so most still had only one case of letters in a character set.

ASCII defined a 7-bit character set. That was "good enough" for a lot of uses for a long time, and has formed the basis of most newer character sets as well (ISO 646, ISO 8859, Unicode, ISO 10646, etc.)

Binary computers motivate designers to making sizes powers of two. Since the "standard" character set required 7 bits anyway, it wasn't much of a stretch to add one more bit to get a power of 2 (and by then, storage was becoming enough cheaper that "wasting" a bit for most characters was more acceptable as well).

Since then, character sets have moved to 16 and 32 bits, but most mainstream computers are largely based on the original IBM PC. Then again, enough of the market is sufficiently satisfied with 8-bit characters that even if the PC hadn't come to its current level of dominance, I'm not sure everybody would do everything with larger characters anyway.

I should also add that the market has changed quite a bit. In the current market, the character size is defined less by the hardware than the software. Windows, Java, etc., moved to 16-bit characters long ago.

Now, the hindrance in supporting 16- or 32-bit characters is only minimally from the difficulties inherent in 16- or 32-bit characters themselves, and largely from the difficulty of supporting i18n in general. In ASCII (for example) detecting whether a letter is upper or lower case, or converting between the two, is incredibly trivial. In full Unicode/ISO 10646, it's basically indescribably complex (to the point that the standards don't even try -- they give tables, not descriptions). Then you add in the fact that for some languages/character sets, even the basic idea of upper/lower case doesn't apply. Then you add in the fact that even displaying characters in some of those is much more complex still.

That's all sufficiently complex that the vast majority of software doesn't even try. The situation is slowly improving, but slowly is the operative word.

Answer 2

Seven bits for ASCII information, and one for error-detecting parity.

Answer 3

Take a look at Wikipedia page on 8-bit architecture. Although character sets could have been 5-, 6-, then 7-bit, underlying CPU/memory bus architecture always used powers of 2. Very first Microprocessor (around 1970s) had 4-bit bus, which means one instruction could move 4-bits of data between external memory and the CPU.

Then with release of 8080 processor, 8-bit architecture became popular and that's what gave the beginnings of x86 assembly instruction set which is used even to these days. If I had to guess, byte came from these early processors where mainstream public began accepting and playing with PCs and 8-bits was considered the standard size of a single unit of data.

Since then bus size has been doubling but it always remained a power of 2 (i.e. 16-, 32- and now 64-bits) Actually, I'm sure the internals of today's bus are much more complicated than simply 64 parallel wires, but current mainstream CPU architecture is 64-bits.

I would assume that by always doubling (instead of growing 50%) it was easier to make new hardware that coexists with existing applications and other legacy components. So for example when they went from 8-bits to 16, each instruction could now move 2 bytes instead of 1, so you save yourself one clock cycle but then end result is the same. However, if you went from 8 to 12-bit architecture, you'd end breaking up original data into halfs and managing that could become annoying. These are just guesses, I'm not really a hardware expert.

Answer 4

A byte has been variously (at least) 1, 4, 6, 7, 8, 9, 12, 18, 20 and possibly 36 bits, depending on what computer you are looking at. I am taking "byte" here to mean "smallest addressable unit of memory", rather than using any sort of text-centric interpretation. (For example, the Saturn CPU, a 64-bit CPU used in the popular HP48SX/GX calculator line addresses memory in nibbles -- 4-bits.)

The 20-bit bytes were extremely common in the "IAS machines", in the 50s. 6, 12, 18 (and maybe 36) were quite popular in a variety of architectures in the 60s, 70s and to some degree 80s.

In the end, having a nice correspondence between "powers of 2" and "bits in an addressable unit" seem to have won out.

Answer 5

First a bit of clarification: Octets (8-bit units) are not really a fundamental unit in modern computer architectures. At least not any more fundamental than other powers of two - 2, 4, 16, 32, 64, 128 etc. Octets were the fundamental unit for 8-bit processors (hence the name!), but modern architectures typically work with larger bit-sets internally. E.g. the x86_64 has 64 bit integer registers and 80 bit floating point registers. RAM is read and written in 64-bit chunks, and the processor just uses a bit of magic to make it look like you can address individual 8-bit bytes.

For older architectures, "byte" indicated the size of the data bus, and as the original question states, a lot of different bus sizes existed (4, 5, 6, 8, 12 etc.). But since 1993 a byte has been defined as 8 bits, in order to have a standardized SI unit for data sizes. Hence the meaning of "byte" has changed from being an architecture-dependent unit to an architecture-independent standardized unit.

So these days, bytes are the standard unit for addressing and quantifying data, but not really fundamental otherwise.

Octets unit became the de-facto standard for storage primarily due to concerns about storing text. For storing text you ideally want one byte to store one character. Two factors were important:

• Having units which are powers of two (2, 4, 8, 16, 32 etc.) is more convenient when designing digital systems.
• 8-bit is enough to store a single character in the ASCII character set (with room to spare for extending the character set to support say Cyrillic).

Of course 8-bits are not enough to support all scripts - something like Japanese requires at least 16 bits (and for what it is worth, Unicode is 21 bits), but at that point in time bits were expensive and most digital text were in the ASCII range anyway.

These days, text is typically stored in variable-width encodings like UTF-8, and with things like Unicode combining characters, the "one byte equals one character" have long been a thing of the past. Today byte is really just the standard for historical reasons.

Answer 6

A byte doesn't have to be 8 bits, but it appears that C and C++ define a byte as being at least 8 bits (although it could be more). This question on Stack Overflow mentions a few systems where 1 byte is not 8 bits.

Answer 7

According to my information the word byte itself was derived from the phrase by-eight which was eight (8) bits words. the convenience we find in 8 bits words is the conversion to hexadecimal values since the value 00000000 = 00 & 11111111 = FF (Dec 255 for unsigned and -127 for signed) it is easy to do all arithmetic operations on such structure including bit-wise operations.

I see bytes (8 bits words) as a natural evolution of word sizes from catastrophically small 3 bits up to ridiculously big 60 bits words