Amstrad HCF instruction?

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NorthWay
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Amstrad HCF instruction?

Post by NorthWay » Thu Nov 30, 2017 1:34 pm

I know of _1_ Halt and Catch Fire instruction: Set a PET to not move the monitor scanning beam and watch the monitor burn a hole in the middle.

But then I read this https://forums.theregister.co.uk/forum/ ... gh_sierra/ where this stood:
"You're not one of those who'd go into Dixons and type the magic sequence of peeks and pokes into an Amstrad CPC464 (or whichever one it was, someone out there will know) that would render it a smoking ruin in a few minutes time?
(It was possible to get two peripherals driving the data bus at the same time - a logic design error. Get one of them trying to write all 0's, and the other all 1's, and you'd made a short circuit from software. Some chip would then get hot enough to let the magic smoke out...)."

Does anyone know anything more about this? For real? What chip let out its magic blue smoke?

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Matt_B
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Re: Amstrad HCF instruction?

Post by Matt_B » Fri Dec 01, 2017 7:32 am

There would be a couple of nasty things you could do that might damage the CPC hardware. One would be to set the Horizontal Sync for the monitor to an out of range value, which might cause permanent damage if the machine wasn't attended to for a while, or you could rapidly oscillate the cassette motor relay which would produce a loud hum and possibly wear it out after a while.

Note that the "catch fire" part of HCF is used figuratively. I don't think there's ever been a machine you could literally set on fire as a result of a single CPU instruction. Rather, it's more to refer to things that'd necessitate powering the machine down to re-start it, rather than being able to send a reset signal or interrupt.

NorthWay
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Re: Amstrad HCF instruction?

Post by NorthWay » Fri Dec 01, 2017 9:18 am

I always thought of HCF as a kinda mythical instruction, a joke story told to newbies in the trade (I believe electricians have plenty of those).
And therefore actually meant to be a damaging thing, not just a halt.

I can see the monitor burning out if it doesn't have its own clocking or safety checks, but the story in the thread was specific about an internal chip which made it sound like this should be well known.

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Matt_B
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Re: Amstrad HCF instruction?

Post by Matt_B » Fri Dec 01, 2017 9:56 am

NorthWay wrote:
Fri Dec 01, 2017 9:18 am
I always thought of HCF as a kinda mythical instruction, a joke story told to newbies in the trade (I believe electricians have plenty of those).
And therefore actually meant to be a damaging thing, not just a halt.

I can see the monitor burning out if it doesn't have its own clocking or safety checks, but the story in the thread was specific about an internal chip which made it sound like this should be well known.
It's not a story I've ever heard, so I'd think that he's probably just repeating an urban myth. It's not like he sounds that sure of which computer he's talking about, after all.

As for HCF instructions in general, all the examples I can find are of it in the figurative sense, the most widely known being the Motorola 6800 with its undocumented DD instruction which sends it into an unrecoverable loop. Turn it off an back on again though, and it's fine.

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