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From: ok@goanna.cs.rmit.oz.au (Richard A. O'Keefe)
Newsgroups: comp.arch
Subject: Re: 64 bits--why stop there?
Message-ID: <3656@goanna.cs.rmit.oz.au>
Date: 2 Sep 90 04:52:57 GMT
References: <6106@vanuata.cs.glasgow.ac.uk> <2437@crdos1.crd.ge.COM> <1990Aug31.174957.9612@cimage.com>
Organization: Comp Sci, RMIT, Melbourne, Australia
Lines: 35

In article <1990Aug31.174957.9612@cimage.com>, paulh@cimage.com (Paul Haas/1000000) writes:
> Adding bit pointers to C compilers would be easy on machines which
> already have multiple incompatible pointer types.  Bit-pointers would
> be just one more type.  On nicer machines, you have to teach the
> compiler about multiple pointer types

Not at all.  Nobody was proposing a new kind of pointer.  The suggestion
was to have *one* kind of address *only*, namely a bit address.  Only the
size of addressible units was to change.

> I used to use PR1ME 50 series processors from about 1979 to about
> 1985.  They had many pointer formats, including a bit pointer.  It was
> not supported by any of their compilers except as a character pointer.

Forget the *compilers*.  The point was that bit pointers weren't
supported by the *hardware*.  The 3-word pointer format had a 4-bit
"bit number within word" field, but there were no instructions to
fetch or store bits, and there were no instructions to do arithmetic
on these pointers in bit units (you _could_ fetch and store characters
and adjust by characters).

Then there were fun things like this:  you could only fetch and store
characters through these pointers when they were in certain special
registers, and one of those registers overlapped the double-precision
floating-point register.  (I wondered for a long time how they managed
to fit 67 bits of pointer into a 64-bit register until it dawned on me
that they dropped the 3 bit-within-byte bits.)

The main point of character pointers in the 50 series was to support
COBOL editing.

Didn't the Burroughs 1700/1800 machines support bit-level addressing?

-- 
You can lie with statistics ... but not to a statistician.