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Path: utzoo!linus!decvax!bellcore!petrus!dan
From: dan@petrus.UUCP
Newsgroups: net.arch
Subject: Re: Strange architecture proposal
Message-ID: <289@petrus.UUCP>
Date: Tue, 6-Aug-85 09:09:30 EST
Article-I.D.: petrus.289
Posted: Tue Aug  6 09:09:30 1985
Date-Received: Wed, 20-Feb-85 07:41:46 EST
References: <968@calgary.UUCP>
Distribution: net
Organization: Bell Communications Research, Inc
Lines: 43

> 
> Any comments on the following idea?
> 
> 
> Take a machine with bytes, words, longs, etc. - all of sizes which are
> a power of two of some basic unit - and which requires such data to be
> aligned, with the address of a unit of size 2**N being required to have
> N low-order zero bits.
> 
	...
> 
> Here's the idea: Get rid of the MOVEB, MOVEW, MOVEL, etc. instructions
> and replace them with a single MOVE instruction, with the type of 
> unit to be moved determined by the memory address given. Addresses
> have a new format, as for instance:
> 
>       0 0 1 A8 A7 A6 A5 A4 A3 A2
> 
> This example specifies a long operand (four bytes) with address
> 
>       A8 A7 A6 A5 A4 A3 A2 0 0
> 
> In general, the size of an operand is 2**N, where N is the number of
> leading zeros in its address. The "actual" address is the bits after
> the highest-order one bit, padded with zeros to make it come out the
> the right size. In this scheme, the address
> 

This is a neat idea.  Perhaps it would be more convenient to put the
size encoding bits on the right.  The automatic "stride adjustment"
feature goes away, but addresses no longer have to be shifted before
use, the effects of arithmetic overflow during address computation
are less painful, and direct comparison of pointers to objects of
different sizes makes a little more sense.  Think how much easier it
would be to explain this address representation to a freshman computer
programming class.  An extra machine instruction could be provided for
pointer arithmetic with automatic "stride adjustment".

I think the main problem with encoding size in addresses is that it
would be necessary to override encoded size too often (I guess I don't
really know this -- hacking a C compiler to gather statistics is too
much work).  What kind of machine language would a C compiler generate
to access structure members?