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From: oster@ucblapis.BERKELEY.EDU (David Phillip Oster)
Newsgroups: net.lang
Subject: Re: BCPL referenes ?
Message-ID: <425@ucbjade.BERKELEY.EDU>
Date: Tue, 11-Mar-86 21:43:38 EST
Article-I.D.: ucbjade.425
Posted: Tue Mar 11 21:43:38 1986
Date-Received: Fri, 14-Mar-86 03:47:21 EST
References: <1341@osu-eddie.UUCP> <254@vu-vlsi.UUCP>
Sender: network@ucbjade.BERKELEY.EDU
Reply-To: oster@ucblapis.UUCP (David Phillip Oster)
Organization: University of California, Berkeley
Lines: 29
Keywords: BCPL

It is interesting that BCPL is rearing its ugly head again.  According to
histories of the C language I have seen published in Dr. Dobbs (interviews
with the creator)  C was invented because, among other reasons, nobody
could figure out how to do a decent BCPL compiler for a byte addressable
machine.  You see:

The BCPL model of reality is that the world is a giant array, so an
integer is also an address.  BCPL assumes that if you increment an integer
you get the next slot in the world array, i.e. that your on a machine with
word addressing (like a dec-10.)  Since an value can be used as either an
integer or an address, the best the BCPL compiler on the PDP-11 could do
was shift the value one spot left (multiply by two) when it was actually
being used as an address, and shift it back again immediately after.

The designer of C hated all that shifting and unshifting.  In C, you say
whether a value is a pointer or an integer, and if a pointer, what it is a
pointer to.  Incrementing a pointer does NOT necessarily add one to the
value, it adds enough to make the pointer point at the next one of
whatever thet pointer points at.  BCPL on the other hand forces
incrementing by one.

That was how things stood back when C was invented.  Has BCPL changed?  Is
it still as inappropriate for byte addressable machines (like the Amiga
with its 68000)?

--- David Phillip Oster
-----------------------  ``What do you look like when you aren't visualizing?''
Arpa: oster@lapis.berkeley.edu
Uucp: ucbvax!ucblapis!oster