Re: New Vax - future directions :-)

[Johnny Billquist <bqt%update.uu.se@localhost>]

On 2021-07-06 16:22, Mouse wrote:
> > I sortof agree with Jason here.  In a sense, it's a bit more strange
> > to start affecting neighbouring registers.
>
> Yes, I sort-of agree with him too.  But I don't entirely agree with
> him; I was arguing the opposing point of view because it _was_ the
> opposing point of view.  Not because I want to be difficult, but
> because I think that we'll get the best result if we examine all the
> angles first.

Fair enough. In the end, I am happy with the way the VAX did it, so this 
is sortof a nit-picking thing. :-)

> > If you wanted to go extreme, you would have brought up CALL, which
> > goes crazy on registers.  But it's all very clearly documented and
> > intentional.
>
> Yes, but so is CLRQ.

Arguable much less so.
While the various registers involved is clearly documented for each of 
the other instructions mentioned, the implicit effect on another 
register in the Q instructions are only explained in a different place 
that covers the general behavior of quad values in registers.

> > It's really just the instructions that deals with quads
>
> ...or octawords; there's no ASHO, but there are CLRO and MOVO (and
> MOVAO and PUSHAO, which appear to be less upsetting).

There is no CLRO or MOVO. I checked. :-)
But had there been, it's the same story, yes.

MOVAO and PUSHAO does not affect any implicit registers, since there 
we're just dealing with an address, which always fits into one register.

> > But support for quads in VAX is limited, as observed.  But I can see
> > that it is needed. The CLRQ and MOVQ could arguably have been
> > skipped.  But the EMUL is pretty much needed, so the problem could
> > never go away.
>
> But EMUL could have been
>
>                opcode mulr.rl, muld.rl, add.rl, prodl.wl, prodh.wl
>
> instead, with a similar change to EDIV.

True. I would have considered it more ugly than the quad in a register 
using Rn/Rn+1, but it would have been a possibility.

But that then also leads to the implied concatenation of registers when 
we talk about D-floats, G-floats and H-floats...
Should all instructions there then also explicitly list all registers to 
be used for all the parts?

It would become very unwieldy. :-)

> > The question is, though, if EMUL should be changed to work on
> > octawords in a VAX64 instead.
>
> Yes, I think it should be: either drop it or make it a 64×64->128
> multiply.  (I think EMULU would be more useful, but that's independent
> of the 32->64 bit shift.)

I agree. :-)

> > EMUL is not needed for 64 bit multiplications then,
>
> No, but do you think the desire for fullword×fullword->doubleword
> multiply will go away?  I don't.  It's just a guess, but I would guess
> that most of the EMUL instructions executed are, conceptually, not just
> doing 32×32->64 but rather being part of >64-bit arithmetic.  (That's
> one reason I would like EMULU; it's more useful for that.)

Yes.

> > I do hope Jason sees the problem, and comes to the same conclusion
> > that something like EMUL will need to hit two registers always.
>
> > And obviously, EDIV also needs two registers for the dividend.
>
> My impression - though that's all it is - is that Jason would have been
> happier to see EMUL/EDIV have two explicit operands for the two halves
> of the 64-bit operand, as I sketched above.  I speculate - and, again,
> that's all it is - that the reason DEC didn't do that is PDP-11
> heritage giving them an inclination towards pasting two registers
> together.

The PDP-11 does it slightly differently, but it usually ends up the 
same. And as I am so used to it, I don't have any real problems.
It took me a little while to find the correct place in the processor 
handbook that explained it, but after that I was happy.

But I do agree it is a bit of a problem in that it's an implied register 
reference that is slightly different, and which clearly should change 
some if we were to extend the size of the registers.

  Johnny

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