Re: Whether something is RISC or not (Re: PDP-8 theology, not Concertina II Progress)

Lawrence D'Oliveiro <ldo@nz.invalid> writes:

The 68020 certainly had that. It also added bit-field instructions, on top
of the single-bit instructions of the original 68000.
>
And in typical big-endian fashion, they added yet another inconsistency to
the way the bits were numbered ...

How typical is that?

Certainly the Power(PC) numbers its bits in big-endian fashion.  But
does PowerPC have instructions where that matters?  If so, I expect
that's not so great with the switch to little-endian in Linux-PPC
(IIRC AIX is still big-endian).

I expect that the s390x uses the same bit numbering as Power.  Does it
have instructions where that matters?

The 88000 has instructions where that matters and has little-endian
bit-ordering (like the 68000, so Motorola is stubborn in its mistakes;
OTOH, with Apple as a prospective customer, they probbly did not want
to require software changes, even if those changes would make the code
shorter).  It supports both byte orders, but AFAIK all 88K systems are
big-endian.

MIPS-I has no instructions where the bit numbering plays a role.  It
was available in big- and little-endian systems.

SPARC uses little-endian bit ordering, but AFAICS has no instructions
where that plays a role.  AFAIK there is no little-endian SPARC
system.

So, yes, using little-endian bit ordering with big-endian byte
ordering is frequent, but OTOH instructions that actually use bit
numbers exist only in few instruction sets.

- anton
--
'Anyone trying for "industrial quality" ISA should avoid undefined behavior.'
  Mitch Alsup, <c17fcd89-f024-40e7-a594-88a85ac10d20o@googlegroups.com>