On 28/02/2021 19:29, bob prohaska wrote:
> I can understand a
> USB hub not working right, but it's much harder to understand
> how it could work right for pointing and wrong for scrolling.

Blind guess based on experience. Faulty hardware is often pattern
sensitive...

...example if you have hardware that commonly sets a bit high, no data
in which that bit is already high will notice...

I had a really nasty bug to trace years ago - program transfer from
floppy to hard drive was being corrupted. But only when a video capture
card was installed...turned out that a unique combination of address and
I/O (this on an *86 hardware platform with I/O instructions) coupled
with a terrible piece of hardware design - massive propagation delays in
the decoder - meant that if you happened to have the lower order bytes
of the address on te bus correspond to the video card, and then changed
them and raised an IO request for data from the floppy,  the card would
still think it was 'selected' and would grab the IO bus. And stuck a
couple of FFHs on the data stream.

We wrote to the manufacturer of the card showing them  detailed timing
calculations as to why it was a POS. They replied that no one else had
complained and it was nothing wrong

People think hardware works, or it doesn't., In reality it can go very
flaky - low voltage, marginal timing, temperature stability issues.

Code works or it doesn't yes - apart from issues with asynchronous
interrupts etc. Take off a coder hat and put on an analogue hardware hat
and  its perfectly possible to see why stuff can work most of the time
but crash some of the time.



--
"And if the blind lead the blind, both shall fall into the ditch".

Gospel of St. Mathew 15:14

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