Path: utzoo!attcan!uunet!husc6!uwvax!astroatc!johnw
From: johnw@astroatc.UUCP (John F. Wardale)
Newsgroups: comp.arch
Subject: Re: RISC vs CISC on Low-End Processors
Keywords: RISC, real-time
Message-ID: <1035@astroatc.UUCP>
Date: 26 May 88 00:51:35 GMT
References: <1521@pt.cs.cmu.edu> <1532@pt.cs.cmu.edu> <476@pcrat.UUCP> <9561@sol.ARPA> <1658@pt.cs.cmu.edu>
Reply-To: johnw@astroatc.UUCP (John F. Wardale)
Organization: Astronautics Technology Cntr, Madison, WI
Lines: 40

In article <1658@pt.cs.cmu.edu> koopman@a.gp.cs.cmu.edu (Philip Koopman) writes:
>How about stack architectures?  They seem to meet the criteria you
>set forth.  Does anyone have arguments for or against them?

People claim stack machines can give you fast execution and dense code.
I have two arguments against this:  (From "The Case Against Stack-Oriented 
Instruction Sets" G. Myers @ IBM (SIGCA-August 77) and other stuff.)

1: Code Size
	Several studies yield overwelming evidence that almost all code
	takes on of these three forms:
	1: a=b    2: a=a+b    3: a=b+c    (+ is an operation)

	Stack based code gains NOTHING in these cases.  Mem-to-mem code wins!
2: Speed (pipelining / schecualing)
	consider a = (b+c) * (e+f)  (push,push,+,push,push,+,*,pop)
	Since all the operands and results share 2 or 3 entries on the
	stack, its very hard (impossible?) to have any parallelism.
	(The third push must wait for the + to replace two operands 
	 with one result.)  

	With a Register-Based machine (RISC if you like) you have/use
	lots of registers to avoid these confiltcs (hazards).  In other
	words register (RISC) machines encourage scheduling; Stack 
	machines prevent it. (Schecualed REG code could be:
	4 load, +, +, *, store [two addes in parallel]

Scientific advances in SW and HW have provided us better ways of doing
thins,  These ways just happen to NOT be very compatable with stack
architectures.  (The Model T [car] was good in it's day, but they're
not used much any more.)

Comments welcome.  If you want to flame, flame yourself....my asbestoes
underware wore out last month.

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

To err is human, to really foul up world news requires the net!