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From: johnw@astroatc.UUCP (John F. Wardale)
Newsgroups: comp.arch
Subject: Re: register windows Really Stack machines/stack-caches
Message-ID: <543@astroatc.UUCP>
Date: Tue, 3-Nov-87 00:13:06 EST
Article-I.D.: astroatc.543
Posted: Tue Nov  3 00:13:06 1987
Date-Received: Sat, 7-Nov-87 00:28:28 EST
Reply-To: johnw@astroatc.UUCP (John F. Wardale)
Organization: Astronautics Technology Cntr, Madison, WI
Lines: 38
Summary: Stack machines turn out to be bad

There has been discussion about using a stack, a stack
architecture (I assume) and a stack-cache to get many of the gains
of register windows cheaply.

Unless I'm totally missing the boat, this requires using a stack
architecture.  These architectures were thought to be great at
increasing code density by eliminating all those repetative
operandes....just keep them on a stack.

Analyses of real code shows that most expressions are of one of
these 3 forms:  (Sorry, I forgot the numbers...)
a=0;
a++;
a=b+c;
All of which tend to take more code on a stack architecture than
on a vax, or RISC beast (to be fair, count bits...)

The other *BIG* loss for a stack machine is that using the stack
for temporaries, tends to *CREATE* hazards (in the stack)

A = B*C + D*E
B;C;*;D;E;*;+;store A  (operations pushes implied)
1 2 1 2 3 2 1   0      (stack depth)

Here the 2nd * must wait for the the first to complete.
With a pipelined multiplier on a register based machine, the *'s
can proceed in parallel.

Sorry if this is clear as mud...basically, Stack machines have
lost popularity because we've discovered better ways to build fast
computers, or more specifically, its hard to build really fast 
stack-architecture machines.

-- 
					John Wardale
... {seismo | harvard | ihnp4} ! {uwvax | cs.wisc.edu} ! astroatc!johnw

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