Re: how can a bit be off in memory?

Jean-David Beyer wrote:


Even that might not be enough. I had a PDP-11/45 with LOTS of peripheral
hardware. We had a contract with DEC for maintenance, and one day, while
doing PM, a techie dropped a screw driver and blew out a few IO cards on the
Unibus. They repaced them, of course. But then the machine got flakey. Once
every few days it would crash. We had lots of test programs (on punched
paper tape in those days) and I ran them and discovered nothing. So I called
in the techies and they ran the test programs and said it must be user
software at fault. It felt like memory to me, but they insisted I write a 10
instruction program to show it. Eventually I lost it and ran each test
program for several hours instead of a minute or so. The double precision
floating add/subtract program would fail after a few hours. So they replaced
the FPU and that did not help at all. They replaced the memory, and that did
not help either. They finally send down some techies from Maynard (or
wherever their engineering department was) and found an interface module
with a flakey address decoder card. That screw driver made the address
decoder unreliable and when an address meant for another device went down
the bus, this card mistakenly recognized it as its own. We lost something
like 2 weeks with that one.

Wonderful story..

Without the ability to put real hardware testing on stuff, this kind of fault is really hard to track down too.
.