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Extending the address space of MIT Cscheme

In order to effectively use CScheme on modern multiprocessors there is a real
need to extend its 24 bit address space.  In the past I have heard two
suggestions for accomplishing this task. 
1)  Interprest the 24 bits as a word address (object address) rather than a
byte address, effectively increasing the address space by a factor of 4.
(Unfortunately, this requires repeated shifting of addresses on most
2)  Steal a type code bit and make it part of the address field.
Neither of these solutions is really satisfactory since even in combination
they do not allow one to address even 4M of memory on each of 64 processors.
Does anyone have any other suggestions?  I have been considering doubling the
size of Scheme objects to 8 bytes, the first 4 for the type code and the second
4 for the address.  This would obviously be quite wasteful of memory, but at
least it would allow me to use the full 32 bit address space of my processors.
How hard do people think it would be to modify CSchemme for such an object
representation?  Someone must have a better solution.  This sounds real grim.
                                        Morry Katz