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From: bayes@hpislx.HP.COM (Scott Bayes)
Newsgroups: comp.sys.mac.hardware
Subject: Re: Addressable memory of 68000 (was Re: New Macs)
Message-ID: <9700002@hpislx.HP.COM>
Date: 17 Jul 90 19:56:19 GMT
References: <26708@netnews.upenn.edu>
Organization: Measurement Systems Operation - Loveland, CO
Lines: 18

> This was a design decision made on the origional 128K Macintosh.  The
> 68000 had 24 bit address (16 M addresses), (Hmm, doesn't the processor
> also have 16 bit data paths, so that it could address 32 M 8-bit bytes?)

Nope.  It only has 23 address lines, and 2 auxiliary lines called BUDS
and BLDS (Byte Upper/Lower Data Strobe).  The 23 address lines decode
16M _words_, and BUDS provide upper or lower byte, or word access.

> so they divided the address map in half, with the lower 8 M for RAM, and
> the upper 8M for ROM, IO, etc.  
> 

> -- 
> Chris Wood     Bellcore     ...!bellcore!nvuxr!ccw
>                         or nvuxr!ccw@bellcore.bellcore.com

Scott Bayes
Hewlett-Packard Co