Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Posting-Version: version B 2.10.2 9/18/84; site rlgvax.UUCP Path: utzoo!watmath!clyde!burl!ulysses!allegra!bellcore!decvax!genrad!teddy!panda!talcott!harvard!seismo!rlgvax!guy From: guy@rlgvax.UUCP (Guy Harris) Newsgroups: net.micro.mac Subject: Re: high byte of address question Message-ID: <371@rlgvax.UUCP> Date: Fri, 18-Jan-85 00:21:01 EST Article-I.D.: rlgvax.371 Posted: Fri Jan 18 00:21:01 1985 Date-Received: Mon, 21-Jan-85 02:34:26 EST References: <224@ucbopal.CC.Berkeley.ARPA> Organization: CCI Office Systems Group, Reston, VA Lines: 17 > I've noticed that the high byte of an address (the 68000 only uses the bottom > 24 bits) is used alot in the Mac for special purposes. Well, let's hope that no future Mac software ever desires >16MB of address space (which will probably be true for quite a while) - a certain big blue computer company started doing this ~20 years ago on a certain line of their computers; now that they're running out of address space, they had to introduce a mode bit into their machines, and a whole new version of the OS to handle it, and the trade tabloids are *still* talking about the hassles of converting to MVS/XA. The 68012 and 68020 don't have mode bits, although if the upper 8 bits don't do anything other than leave the chip on appropriate pins, this can be handled in off-chip logic (i.e., run those lines to /dev/null). Guy Harris {seismo,ihnp4,allegra}!rlgvax!guy