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From: tmb@talcott.UUCP (Thomas M. Breuel)
Newsgroups: net.micro.68k
Subject: Re: Re: 32 vs. 24 Bit Addresses
Message-ID: <319@talcott.UUCP>
Date: Sun, 3-Mar-85 18:45:33 EST
Article-I.D.: talcott.319
Posted: Sun Mar  3 18:45:33 1985
Date-Received: Tue, 5-Mar-85 01:46:17 EST
References: <342@oakhill.UUCP> <429@bonnie.UUCP>
Organization: Harvard University
Lines: 18

> Of course, hardware upward compatibility doesn't guarantee software
> compatibility.  A number of 32-bit processors have been crippled by 
> operating systems writers who, when they designed their software,
> saw only physical hardware with 16mb or less and said "we only need
> 24 bits".  Typically the upper byte is used for the number of arguments
> or argument words.

Well, the argument is not "we only need 24 bits". The argument goes
more like "if we use 32 bit pointers, then we need at least an extra
word for type/number of argument information, and at least an extra
memory access per operation. This means that our software will use at
least 33% more memory and time than other people's software".

If you are careful, btw, you can write your programs (in, say, 'C')
in such a manner that implementation details like this don't make the
code unportable.

						Thomas.