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Path: utzoo!utgpu!water!watmath!watdcsu!magore
From: magore@watdcsu.UUCP
Newsgroups: talk.bizarre,comp.misc
Subject: Re: What the world needs now [ is an exploding computer ]
Message-ID: <3411@watdcsu.UUCP>
Date: Mon, 25-May-87 12:42:49 EDT
Article-I.D.: watdcsu.3411
Posted: Mon May 25 12:42:49 1987
Date-Received: Tue, 26-May-87 02:50:35 EDT
References: <12067@topaz.rutgers.edu> <910@killer.UUCP> <170@auvax.UUCP>
Reply-To: magore@watdcsu.UUCP (Mike Gore, Institute Computer Research - ICR)
Organization: U. of Waterloo, Ontario
Lines: 34
Xref: utgpu talk.bizarre:1913 comp.misc:549


In article <910@killer.UUCP>, jfh@killer.UUCP (John Haugh) writes:
>> A friend of mine [...] told me [...] that one of the unused op-codes
>> in the M6800 had the nasty side effect of overloading the bus and causing
>> much grief on the PC board the chip was mounted on...
[munch]

In article <170@auvax.UUCP> rwa@auvax.UUCP (Ross Alexander) writes:
>The opcode exists, the mnemonic is HCF (Halt and Catch Fire) :-) and
>without my manuals I can't recall the hex.  It's a manufacturability
>instruction, for testing the chip after the wafers are cut apart, and
>before they mount the dies onto carriers.  It just counts 0000 to FFFF
>on the address buss continuously.

	I found that on the Intel 8087 if you do 2 floating point store
instructions WITHOUT wait, it will lock-up the local bus until reset!
Nothing, not NMI nor external DMA request will get through! You will
be forever caught in *_mid-instruction_* that will never finish.... oops :)
[ Note: this is *NOT* the interrupt lock-out problem Intel warns everyone
  about. A logic analyzer proves this... ] For reasons of design the 80287 
doesn't have such problems. 

	I have a unix machine with hardware MMU and 8086/8087. I have since 
dropped using the 8087. Now programs that crash can no longer crash the whole 
system.  Otherwise, one might guess that disabled interrupts in a user task 
might be a problem, but for this the MMU detects the condition and issues an
NMI which calls a system routine which patches the flags in the user task
... [ugh]

	I hope this just makes everyones day :-)

# Mike Gore 
# Institute for Computer Research. ( watmath!mgvax!root - at home )
# These ideas/concepts do not imply views held by the University of Waterloo.