Byte sizes (was Re: 2.8M 3.5' floppy (was: three and a quarter inch floppy?)
bv at norbionics.com
Sun Mar 13 18:58:54 CST 2005
On Sun, 13 Mar 2005 20:36:58 +0100, Eric Smith <eric at brouhaha.com> wrote:
> 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.
A byte is the smallest INDIVIDUALLY addressable unit of data on a system.
It was usual to speak of a character when a part of a whole word was
extracted. I do not remember any details of the Univac 1107 (I only
programmed in FORTRAN and Simula on it), but I think the normal character
was six bits. I am fairly certain that was not a byte, it was extracted
from an 18-bit halfword or a 36-bit word. A byte is something you can load
and store to an individual address, and the hardware takes care of the
rest. A character needs some software or firmware mapping in order to read
or write just one of them.
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