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From: richardt@orstcs.UUCP (richardt)
Newsgroups: net.arch
Subject: Re: Stack architectures - why not?
Message-ID: <12200021@orstcs.UUCP>
Date: Sun, 15-Sep-85 21:16:00 EDT
Article-I.D.: orstcs.12200021
Posted: Sun Sep 15 21:16:00 1985
Date-Received: Sat, 21-Sep-85 05:30:36 EDT
References: <172@myriasa.UUCP>
Organization: Oregon State University - Corvallis, OR
Lines: 40
Nf-ID: #R:myriasa:-17200:orstcs:12200021:000:2240
Nf-From: orstcs!richardt    Sep 15 17:16:00 1985

Stack architectures are beautiful -- to a point.  My major gripes with them
are the following:

1) A very heavy reliance on RPN.  Although routines can be written to get 
around this, and some people actually like RPN, this is still a dubious 
feature at best.  Algebraic and other forms of higher math don't work well
in RPN, and (although I don't know about you) RPN makes logic notation far
more confusing than it needs to be.  Since I use logic functions in a lot of
situations, this is a hassle.  Even the If...Then structure is made more
complex than necessary because of forcing it into RPN.

2) Stack architectures and variables don't mix well.  Although it's not 
impossible, dealing with stack frames to find variables and work with them,
and using stack references for absolute variables is yet another hassle.

3) Although this is somewhat Forth-specific, relying on the Jump rather than
the Goto Subroutine.  Because the stack is being used for parameters, you
can either (1) deal with unpleasant stack frames; (2) never use subroutines,
with the inherent increase in execution time while addresses are being looked
up and the overhead for faking a subroutine call; (3) use subroutines only
for the very low level routines, and then you get stuck with all the 
disadvantages of (1) and (2).

4) It is very difficult to use multiple dynamic data spaces on a stack machine.
Again, it's not impossible, but stack machines are designed against it.

Note:  These opinions are based on writing a Forth for the 68000, where you can
have all the stacks/heaps that you want; writing a Forth for the 6502, whcih
has an unpleasant and unreliable indirect jump, thereby forcing self-modifiying
code (which has it's place, but still); writing a Forth for the 6809, which
has a two stacks (eliminating (3) above); and 6 years of programming 
experience on most of the microprocessors running around and several larger
machines.  I'm sorry, I'd far prefer a register architecture which works to
either a memory architecture or a stack machine.

				orstcs!richardt
"What shall we do with a stack machine,
	Stack Machine, Stack Machine,
Er-ly in the morning?"   <-- I consider Stack machines to be on about the level
					of a drunken sailor.


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