Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!mailrus!tut.cis.ohio-state.edu!purdue!haven!vrdxhq!daitc!bms-at!stuart
From: stuart@bms-at.UUCP (Stuart Gathman)
Newsgroups: comp.arch
Subject: RISC and emulated languages
Message-ID: <158@bms-at.UUCP>
Date: 26 Apr 89 17:05:04 GMT
Organization: Business Management Systems, Inc., Fairfax, VA
Lines: 28

There is one interesting feature of RISC architecture that I haven't seen
mentioned much: the fact that an emulated/interpreted program will often
run faster than a compiled one.

I first noticed this in benchmark results for the ARM where BASIC ran the
benchmarks faster than C!

The conditions necessary for this to take place are:

1)	the system uses slow main memory with a high speed cache.
2)	the emulated code is significantly smaller than equivalent
	direct code.
3)	the emulator mostly fits in the cache.
4)	the emulator overhead is low enough not to swamp the benefits
	of 2.

The most effective emulators are "semi-compiled", i.e. variable references
and branch targets are resolved prior to execution.  Table searches will
kill condition 4.  RM-cobol would be a typical example of an emulator meeting
these conditions.  (The interpreter is large, but the core routines are
small.)

This approach turns a RISC machine into a CISC machine with user defined
microcode.  Unlike loadable microcodes of yesteryear, the cache is swapped
by line on a demand basis.
-- 
Stuart D. Gathman	<stuart@bms-at.uucp>
			<..!{vrdxhq|daitc}!bms-at!stuart>