Byte sizes (was Re: 2.8M 3.5' floppy (was: three and a quarter inch floppy?)

[Eric Smith]

Ethan writes about bytes on the PDP-10:
> On the -10,
> one stores 6 6-bit characters in a machine word, and one speaks of
> bytes that are 6 bits.

Not necessarily.  On the PDP-10, the byte size could be anywhere from
zero (really!) to 36 bits.  Seven bits was the most commonly used for
general ASCII text, with one leftover bit per word.  In fact, this
was so common that the KS10 CPU has special dedicated hardware to make
the 7-bit byte case more efficient than the other supported byte sizes.

Six bits were used for "SIXBIT", the subset of ASCII without the control
characters and lower case.  SIXBIT was used for filenames and extensions,
and other fairly limited purposes, but not for text processing.

But many other byte sizes were routinely used on the PDP-10.

Zero bit bytes were not commonly used, but had the advantage that you
could store an infinite number of bytes in a single word.  :-)

> AFAIK, 'octet' was invented in networking circles
> to prevent any ambiguity about which size byte one was discussing.

Yes.

Eric