Path: utzoo!utgpu!attcan!uunet!seismo!sundc!pitstop!sun!decwrl!labrea!rutgers!mailrus!utah-gr!uplherc!esunix!bpendlet
From: bpendlet@esunix.UUCP (Bob Pendleton)
Newsgroups: comp.arch
Subject: Re: Software Distribution
Message-ID: <965@esunix.UUCP>
Date: 1 Sep 88 14:24:48 GMT
References: <22778@amdcad.AMD.COM>
Organization: Evans & Sutherland, Salt Lake City, Utah
Lines: 49

From article <22778@amdcad.AMD.COM>, by rpw3@amdcad.AMD.COM (Rob Warnock):

> The thought occurs that if one designed a virtual "machine" that was
> specifically easy to emulate -- given these modern techniques -- that
> this *might* be a suitable form for "portable" object programs (as
> contrasted with some "universal intermediate form"). At least it bears
> some thought.

There really is no difference between an easy to emulate virtual
machine and an universal intermediate form. Quads map directly to a 3
address machine, reverse polish maps to a stack machine, and trees can
also be directly executed. (EVAL in microcode anyone? Yes, I know its
been done.) If the intension is to translate it for the target machine
anyway, why not use a form with as much information left in as
possible?

The problems come from incompatable data formats, addressing modes,
and high level operations. Most modern computers use 8 bit bytes and
support 16 and 32 bit twos complement integers so there isn't much of
a problem there. Floating point, packed decimal, and fixed point might
cause some problems.

Not all modern computers are byte addressable, so virtual machine code
that assumes byte adressablitiy isn't going to run well on a word
addressed machine. Also, if the virtual machine code lays out data in
structures and arrays you can take a serious performance hit if the
virtual machine doesn't allign data to match the addressing
granularity of your machine.

If the virtual machine is specified at too low a level it will be very
difficult to take advantage of any special instructions the target
processor may have. Even something as simple as a block copy
instruction may not be usable if the virtual machine code has loops to
do copies.

So, if you have an universal intermediate language, you can write an
emulator for it and execute it with a minimal amount of preprocessing,
or you can convert it native machine code, or you can build a machine
that directly executes it.


> Rob Warnock
> Systems Architecture Consultant

-- 
Bob Pendleton @ Evans & Sutherland
UUCP Address:  {decvax,ucbvax,allegra}!decwrl!esunix!bpendlet
Alternate:     utah-cs!esunix!bpendlet
        I am solely responsible for what I say.