home *** CD-ROM | disk | FTP | other *** search
- Newsgroups: comp.sys.mac.hardware
- Path: sparky!uunet!gatech!news.byu.edu!eff!ckd
- From: ckd@eff.org (Christopher Davis)
- Subject: Re: 040 caching
- In-Reply-To: gmw1@cunixa.cc.columbia.edu's message of Wed, 22 Jul 1992 02:53:00 GMT
- Message-ID: <CKD.92Jul22040059@loiosh.eff.org>
- Sender: usenet@eff.org (NNTP News Poster)
- Nntp-Posting-Host: loiosh.eff.org
- Organization: Electronic Frontier Foundation Tech Central
- References: <1992Jul22.025300.10797@news.columbia.edu>
- Date: Wed, 22 Jul 1992 08:03:09 GMT
- Lines: 38
-
- Gabe> == Gabe M Wiener <gmw1@cunixa.cc.columbia.edu>
-
- Gabe> Can someone explain exactly what is going on when a program
- Gabe> "trips over" the 040 cache? Does the 040 cache have any relation
- Gabe> to the external cache that can be added in the PDS slot? Why
- Gabe> does this problem not manifest itself on IIci's with cache cards?
-
- Basically, imagine that you have a program, with instructions A B C D
- and E. E happens to be a "loop back to A" instruction of some sort. B
- is an "increment the following location" instruction.
-
- So the machine runs A B D D E A B E A B F D E and so on. All is happy
- (except that the programmer wasn't following Apple's rules). Then we
- get a chip with an onboard cache, which allows tight loops to run
- completely inside the chip. Now, when B modifies the next location, it
- modifies the location in the DATA cache, not the INSTRUCTION cache... so
- you get A B C D E A B C D E etc.
-
- An external cache (like a IIci cache card) isn't split instruction &
- data like the '040 cache, and doesn't hit the problem. That doesn't
- mean self-modifying code is smart :)
-
- Gabe> Apparently there is some sort of Apple CDEV that can be used to
- Gabe> turn the 040 cache on and off. It does not seem to be on the
- Gabe> 7.0.1 distribution. What is this little CDEV called? I'll
- Gabe> probably prowl around ftp.apple.com and see if it's there
-
- This should be on your Tidbits disk; it's called Cache Control, and has
- an icon of a "swooshing" 040.
-
- Gabe> (btw, does any of this have anything to do with the serial port switch?)
-
- Nope. That's a different potential incompatibility altogether. :-)
- --
- Christopher Davis * ckd@eff.org * System Administrator, EFF * +1 617 864 0665
- ``The First Amendment is often inconvenient. But that is besides the
- point. Inconvenience does not absolve the government of its obligation
- to tolerate speech.'' --Justice Anthony Kennedy, in 91-155
-