Path: utzoo!attcan!uunet!ncc!alberta!access!edm!rroot From: rroot@edm.UUCP (uucp) Newsgroups: comp.arch Subject: Re: Is the Intel memory model safe from NO-ONE ?!? Message-ID: <3111@edm.UUCP> Date: 12 May 88 09:29:15 GMT References: <20618@think.UUCP> Organization: Unexsys Systems, Edmonton,AB. Lines: 28 From article <20618@think.UUCP>, by barmar@think.COM (Barry Margolin): ] In article <3095@edm.UUCP> rroot@edm.UUCP (uucp) writes: ]>From article <353@cf-cm.UUCP>, by mch@computing-maths.cardiff.ac.uk (Major Kano): >>> (As an aside, I've heard of 68000 routines doing all kinds of contortions to >>> check for/avoid overflow because the 68K traps on (eg., zerodivide) and traps >>> into SUPERVISOR mode (believe it or not). > Read the original message again, more carefully. He wasn't > complaining so much about the fact that divide by zero results in a > trap, but that it traps into SUPERVISOR mode, even though the program He seemed to be complaining about all the contortions that he heard that programs go thru to check for zero divide (and seemed to assume that it was also necessary for other overflow-type thins). I was basically defending the existance of -- and presumed logic behid -- the trap. The problem behind having the zerodivide interrupt trap into USER state is that it would mess up the whole world. RTI would then have to become a non-priveledged instruction, and user programs would have to set up for it whether they cared about recovering from zero divides or not (rather than tellin the OS when they did). It's not that difficult to emulate a vector into user state when you start in supervisor than it is to go the other way around without introducing some weird contortions on both the user and supervisor side of things. -- ------------- Stephen Samuel {ihnp4,ubc-vision,vax135}!alberta!edm!steve or userzxcv@uqv-mts.bitnet