anson@elrond.CalComp.COM (Ed Anson) (03/22/88)
This may be old news to some, but it just bit me and might bite someone
else: There is a version of the Maxbug debugger which causes system crashes
if more than 1 Mb of RAM is installed. My copy shows a creation date of
Mon, May 13, 1985 3:38 PM. A more recent version appears to have solved
the problem.
About two weeks ago, I started experiencing frequent system crashes. Usually,
the cursor froze, but bomb boxes occurred and the display occasionally freaked
out. These failures were mostly repeatable, but varied with the context. I
suspected memory or logic failures, and took my Mac to the shop. After all,
I have AppleCare :-(
The tech at the shop ran every diagnostic he had, and even spilled Coke on
the screen. It wouldn't fail for him. I brought in my hard disk and made it
fail immediately. The same hard disk worked on another Mac. Then he removed
my Dove memory upgrade, and everything worked. He marked the upgrade "NG"
and billed me $125. I made him put the memories in a protective bag, and took
them home with me.
I then had several lengthy talks with the Tech Support people at Dove. They
told me several things that had caused other people's upgrades to fail even
with good memories. I tried their suggestions, but to no avail. Then I got
them to tell me how the memories mapped into the address space. I wrote a
quick and dirty memory diagnostic, and found a range of addresses where a
write would cause the system crash. I then moved the memories around,
expecting to see the bad locations move. They didn't.
I won't go into any more detail. The bottom line is: After two weeks and
$125, I discovered that the version of Maxbug I was using was stepping on
the memory. It apparently assumes that 1 Mb is the biggest any Mac will
ever be. :-) It installs itself right under the screen memory. Unfortunately,
when a second meg is added, the screen memory moves but Maxbug doesn't.
I received my copy with the Lightspeed C distribution a coupla years ago. I
have seen a fixed version included with the more recent distributions, but
even though I am upgraded to version 2.15 I still have the old version of
the debugger. I got fooled, since I "knew" the debugger worked. After all,
it'd been around for two years. I have it turned off most of the time,
and it took awhile to realize that the crashes started happening the first
time I turned it on after upgrading the memory.
MORAL (lessons I learned the hard way):
1. Always remove third-party stuff before going in for Apple service.
2. Don't always assume that something caused the problem just because
removing it makes the problem appear to go away.
3. Sometimes the debugger is the bug.
4. At least it wasn't a virus.
--
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Ed Anson, Calcomp Display Products Division, Hudson NH 03051
(603) 885-8712, anson@elrond.CalComp.COM