Byte sizes

Johnny Billquist bqt at Update.UU.SE
Mon Mar 14 16:17:05 CST 2005


On Mon, 14 Mar 2005 der Mouse <mouse at Rodents.Montreal.QC.CA> wrote:

> >> A byte is the smallest INDIVIDUALLY addressable unit of data on a
> >> system.
>
> Well, it sounds reasonable, but probably isn't the most useful, as it
> would mean that, for example, the PDP-8 had 12-bit bytes.
>
> > The PDP-10 is an excellent example of when this isn't true.
> > The smallest addressable unit is a word, which is 36 bits.
> > A byte is, as noted, anything between 0 and 36 bits.  Bytes are
> > stored in a word, as many as can be fitted.  To access bytes on a
> > PDP-10, you have a byte pointer, which consist of a word address, and
> > a bit pointer, and byte size.
>
> That sounds a whole lot like a hardware-supported way of addressing an
> object of an arbitrary size in bits.  And that would mean that bytes of
> any size *are* individually addressible.
>
> Or have I misunderstood?

Well, yes and no.
Please note that there are special instructions to deal with bytes in the
PDP-10. These instructions can deal with objects of arbitrary size (well,
0 to 36 bits anyway). However, most instructions do not deal with bytes,
and thus will not play with the bit field stuff, and only deal with a
simple plain address, which address a 36 bit word. Among these
instructions are such things as a normal MOVE instruction. So these byte
pointers are only handled by a few special instructions, that deal with
bytes. For all other purposes, the PDP-10 only deal with words, which
always are 36 bits.

	Johnny

Johnny Billquist                  || "I'm on a bus
                                  ||  on a psychedelic trip
email: bqt at update.uu.se           ||  Reading murder books
pdp is alive!                     ||  tryin' to stay hip" - B. Idol



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