Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Posting-Version: version B 2.10.1 6/24/83; site redwood.UUCP Path: utzoo!watmath!clyde!burl!ulysses!mhuxr!ihnp4!zehntel!hplabs!hpda!fortune!rhino!redwood!rpw3 From: rpw3@redwood.UUCP (Rob Warnock) Newsgroups: net.micro.mac Subject: Re: high byte of address question Message-ID: <130@redwood.UUCP> Date: Thu, 17-Jan-85 03:24:24 EST Article-I.D.: redwood.130 Posted: Thu Jan 17 03:24:24 1985 Date-Received: Mon, 21-Jan-85 03:36:30 EST References: <224@ucbopal.CC.Berkeley.ARPA> Organization: [Consultant], Foster City, CA Lines: 31 +--------------- | I've been playing around with sumacc C programming and had to do a little | programming in assembly language to handle some special cases. 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. However, even when | I do a Load Effective Address (lea) command, the high byte is sometimes | non-zero and I have to clear it out. Does anyone know if the hardware in | the Mac uses that high byte for something? | Edward Moy | edmoy@ucbopal | ucbvax!ucbopal!edmoy +---------------NO, NO! SAY IT ISN'T SO! If they have done this, it's going to be a major disaster when they go to a 68020, where those bits are significant. Don't we learn ANYTHING from our mistakes in this industry? Wasn't it enough that all those 360 programs had to be re-written for 370s???????? I don't expect the front line troops to remember (they're too young in many cases), but doesn't Management ever come to design reviews? Using "spare" bits for cutesy tags is simply too dangerous to consider (unless the usage is tightly confined to a few SMALL modules you plan on replacing in the next generation). ((...and even then, clever users will discover them, use them, and be greatly upset when you take them away...)) Rob Warnock Systems Architecture Consultant UUCP: {ihnp4,ucbvax!dual}!fortune!redwood!rpw3 DDD: (415)572-2607 USPS: 510 Trinidad Lane, Foster City, CA 94404