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From: mwm@eris.BERKELEY.EDU (Mike (My watch has windows) Meyer)
Newsgroups: comp.sys.amiga,comp.lang.misc
Subject: Re: Compilers?
Message-ID: <5073@jade.BERKELEY.EDU>
Date: Fri, 11-Sep-87 19:52:28 EDT
Article-I.D.: jade.5073
Posted: Fri Sep 11 19:52:28 1987
Date-Received: Sat, 12-Sep-87 20:38:44 EDT
References: <1708@amiga.amiga.UUCP> <581@sugar.UUCP> <935@unicus.UUCP> <66@oresoft.UUCP> <5042@jade.BERKELEY.EDU> <70@oresoft.UUCP>
Sender: usenet@jade.BERKELEY.EDU
Reply-To: mwm@eris.BERKELEY.EDU (Mike (My watch has windows) Meyer)
Organization: Missionaria Phonibalonica
Lines: 60
Keywords: compiler assembler dwim mating
Xref: mnetor comp.sys.amiga:8257 comp.lang.misc:656

In article <70@oresoft.UUCP> rick@oresoft.UUCP (Rick Lahrson) writes:
<I at least should have said "functional" instead of "semantic", I guess.
<Boy, am I embarrassed.  The remark that so irritated Mike, pulled out of
<context above, makes it look like I said a compiler should act like an
<assembler.

That's what I took it to mean. Since I *like* very-high-level
languages, I got upset about the suggestion that my favorite languages
features didn't belong in a compiler! Functional I still have problems
with, but that's not so upsetting. No hard feelings, I hope?

<>[Referring to BCPL and it's pointers in AmigaDOS.]
<><stupid, disgusting BPTR's.  (Now there's a job that a compiler SHOULD
<><be able to do -- generate code that relates to the target machine!)
<>
<>But you've got to convince the target machine to look like the machine
<>model the compiler wants to generate code for.
<
<Hmph.  I say the compiler should model the target hardware, and generate
<code for that.

As someone else pointed out, you have two different models: the
hardware model, and the model the language has. The compiler can't do
anything about them; it just has to live with them. For many languages
(Pascal, COBOL, FORTRAN, Icon, Snobol, LISP, etc.) the language model
just has "abstract" objects in it (integers, floats, strings, etc) and
they can be mapped to something reasonable on just about any machine.

On the other hand, languages that try to live close to the machine
(like C and BCPL) have less abstract types (integers, bytes) that have
certain properties. For instance, C insists that there's something
called a char, it holds exactly one char, and it's addressable. On
byte-addressable machines, this works great. But on word addressable
machines, you have to jump through hoops to fake byte pointers (or do
other ugly things).

On the other side, BCPL *insists* that the machine has words, and
words have three properties: 1) They are at least 16 bits long. 2)
they are long enough to hold a pointer to a word, and 3) adding 1 to
an address gives the address of the next word. On a word address
machine, this works just fine. But on a byte-addressed machine, you
have to jump through to make things work.

Mixing the two is a *bad* thing. On AmigaDOS, you get BPTRs. On
Unicos, I get ugly C byte pointers. The compilers in question can't be
blamed for either. Neither can the language designers. The people to
blame are the people who decided to mix the two.

<I think this is the first time I've been publicly "Oh, horse puckied".
<Kinda makes me scared to offer further postings.  1/2  8-)

Don't do that! Normally, your postings make lots of sense. I wanted to
make sure nobody mistook the misinterpretation for truth! 1/2 :-).

	<mike
--
And then up spoke his own dear wife,			Mike Meyer
Never heard to speak so free.				mwm@berkeley.edu
"I'd rather a kiss from dead Matty's lips,		ucbvax!mwm
Than you or your finery."				mwm@ucbjade.BITNET