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

der Mouse mouse at Rodents.Montreal.QC.CA
Mon Mar 14 10:46:54 CST 2005


>> 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?

> The fact that people today seem to believe that byte addressable is
> the only possible thing, along with a byte being 8 bits, is plain and
> simply because they haven't seen any other.

There even are relatively modern machines which don't fit it very well.
Some DSPs, for example, have 32 bits as their smallest directly
addressible unit.

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