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From: Brian Drummond <brian@shapes.demon.co.uk>
Newsgroups: comp.lang.ada
Subject: Re: Arbitrary Sandbox
Date: Thu, 8 Mar 2012 11:01:33 +0000 (UTC)
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Date: 2012-03-08T11:01:33+00:00
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On Wed, 07 Mar 2012 19:50:03 -0500, Robert A Duff wrote:


> 
> I agree that a 48-bit offset is big enough.  I don't agree that a 16-bit
> segment number is big enough.  OK, so maybe split 24/40 instead of
> 16/48?  Still not enough segments.  That's the problem -- however you
> split those 64 bits, you have not enough segments or not big enough
> segments, or both.

OK, divide the object space further. Reserving one bit of the object 
number as a tag to distinguish between object types, 2^30 small objects 
(32-bit offset) and 2^15 large (48-bit offset) objects is probably 
enough. 

Using more bits in the tag can distinguish more object types : perhaps 
allocated from different storage pools either for common sizes or 
different lifetimes (or remote objects). Reserve one tag value for 
"compact objects" (well, the architect was an astronomer!) where the type 
and value are encoded into the object number.

Ancient history:
http://en.wikipedia.org/wiki/Rekursiv
Object numbers there were just 40 bits; 64 gives much more room.

(I had not realised someone re-engineered it in 1992!)
alexandria.tue.nl/extra1/afstversl/E/374655.pdf

- Brian