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A comment on A wild bug: 1970s Intel 8271 disc chip ate my data. Parent comment

Re:A wild bug: 1970s Intel 8271 disc chip ate my data by nynyny7 on Wednesday June 24, 2020 @04:32AM (#23625552)
At least the drive I accidentally tried the same had a mechanical stop. Although banging the head against it 170 times or so wasn't exactly beneficial for its alignment.

Even today, one is better off not to write undocumented values to registers. True story: I had to investigate a SW bug report concerning a modern, fairly popular microcontroller. (I won't name which.) Sometimes data in RAM changed without our code writing to it. Turns out that our startup code accidentally wrote to a 'reserved' bit in a register, activating some kind of internal RAM test mode. This was confirmed by the µC's manufacturer.

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Re:A wild bug: 1970s Intel 8271 disc chip ate my data by rbanffy on Wednesday June 24, 2020 @06:35AM (#23626332)
> Although banging the head against it

This was how an Apple II made sure you were in track zero - by banging the head against the stop. This is why it makes that typical noise when booting up.

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Re:A wild bug: 1970s Intel 8271 disc chip ate my data by 0xTJ on Wednesday June 24, 2020 @07:47AM (#23626772)
I really wish it was common for manufacturers to fully document features like that. Even if they "DON'T USE THIS, IT WON'T DE WHAT YOU WANT, WE'RE NOT RESPONSIBLE FOR ANYTHING YOU DO!", it might open interesting opportunities. Even stuff like the Z180 test pin or the 68010 internal processor state that's pushed on address and bus exceptions, it would be interesting to know what those things do.
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