Path: utzoo!mnetor!uunet!seismo!sundc!pitstop!sun!quintus!pds
From: pds@quintus.UUCP (Peter Schachte)
Newsgroups: comp.arch
Subject: Re: Wirth's challenge (was Re: RISC
Message-ID: <489@cresswell.quintus.UUCP>
Date: 22 Dec 87 17:57:14 GMT
References: <6901@apple.UUCP> <28200075@ccvaxa> <12181@orchid.waterloo.edu>
Organization: Quintus Computer Systems, Mountain View, CA
Lines: 48
Summary: specialized chip features are ok

In article <12181@orchid.waterloo.edu>, atbowler@orchid.waterloo.edu (Alan T. Bowler [SDG]) writes:
> ....  Once you get over
> the fixation that your problem is so different that the hardware
> designer has to tailor an instruction just for you, you realize
> that what you want is something that does some basic functions
> fast, and let the programmer construct the other stuff.  The
> design problem is to choose the right basic operations. 

That's the issue, alright.  For symbolic languages, tagged pointer
operations are very important.  Typically, tagged dispatch and tagged
pointer following are done quite a lot, and cutting the number of
machine instructions to do these things can make quite a difference in
performance.  Take the example of following a tagged pointer.  If the
tag is kept in the high few bits of a 32 bit address, one must and the
pointer with a 32 bit constant.  Quite a lot of overhead, when a simple
addressing mode that ignored the high, say, 4 bits of the address would
do the trick perfectly.  I know this runs contrary to the RISC ideal.
But on a CISC, this is no more arcane than some of the other addressing
modes.  

Similarly for tagged dispatch.  If there were an instruction to take
the top, say, 2 bits of a register, shift them right a whole bunch of
places, add them to a given address, and jump to the address stored
there.  Sure, this could be done with a shift or rotate, an and, an add,
and an indirect jump.  But wouldn't you rather do one instruction than
4?

These are some of the operations that would make Lisp and Prolog run
faster.  I'm sure each language, and each class of languages, has it's
own favorite chip features.  The important questions are:  how much
will a given feature speed up a given task?  How much will it cost (in
terms of $, speed of other operations, etc.)?  And how important is
that task?  I imagine BCD probably wasn't worth it.  Perhaps the
features I've just asked for aren't worth it either.  Maybe it would be
better, on average, to have a scaled, post-incremented, memory indirect
addressing mode (0.5 :-), 0.5 :-().  Or a 48 bit one's complement
multiply instruction, or whatever.

The point machine designers should take into account is that more and
more, people are buying general-purpose hardware rather than the more
expensive specialized hardware.  Therefore, they should design their
machines taking symbolic languages, CAD, and other specialized tasks
into account.

-- 
-Peter Schachte
pds@quintus.uucp
...!sun!quintus!pds