re The definition of a Byte
re The definition of a Byte
As usualwith me, this will be longer than most can tolorate, but It is not so simplea question.
IBM coined the term Byte when they dessigned the System 360 series in the early 1960's. A byte was the basic increment of memory, and could be considered as a quarter-word, which was 32 bits. It also defined the length of a character in the IBM Standard EBCDiC code (Extended Binary Coded DDecimal (for) Information Interchange, or something close to that) which defined 256 characters. It deliberately ignored ASCII (American Standard Code for Informatioin Interchange) which had not yet been selected as the standard for computers purchased by hte US Government. ASCII was only a seven bit code and only defined 128 characters. Notice that both ASCII and EBCDIC ignore the use of an extra bit for parity and the necessity of tacking on a start bit and one or two stop bits for asynchronus communications applications.
In my opinion, the definition of a Byte has nothing much to do with a word oength, unless the word dlengthis 32 bits. Sixteen bits is a half word, but a byte is rarely described as a quarter word. And, one canthink of a Byte being a character length, except that sometimes it doesent, like when an application iswritten that communicates ssome items of data as BCD, 1/2 byte Knibbles! Over the years there have been a half dozen or moe "character codes used , but any resembelence or appearant similarity to the Byte is purely coincidental.
As for DEC, they have used only three alphaneumeric dodes for their basic I/O uses. In the 12-bit and 18-bit computers such as the PDP-1, -4,-7, 09, -15 (18-bit machines) and the PDP-8 series the PDP-5 and a couple other special purpose versions of the PDP-8, (the 12-bit machines) all used 6-bit alphanumeric I/O codes whichdefined 64 characters. I think one can include the PDP-6 here too, bacause it was a 36-bit machine. All of the later machines which include the PDP-11 and all of the VAX machines and I think the PDP-10 too) were all 16-bit or 32-bit machines and used the ASCII codesfor I/O to printers, console printers, and other similar devices
To summarze: A Byte is 8 bits long. It does not relate to any word-length other that as a factor of the physical length of a word or a fraction of a word or a multiple of a word. If it is not a factor of the word length it is not defineable as a Byte.
Ray