This wasn't so hard. Run the built in BIOS diagnostic. And now the Start Menu (pure software) works, and the power button (Hardware but with a lotta software messing it up) works. a
Some website explained the way to get into the BIOS diagnostics was to hold down the ESC key while you hit the power on button. And this appears to work even while the power on button isn't working. According to a website, the BIOS diagnostics have been standard in HP laptops since 2009. Which means a lot of 'em have it. On my HP laptop, a 2014 model, the BIOS diagnostics do start up, but they don't give you any messages on the screen except for one, They ask if you want to skip the disk test.
And the diagnostics reset a bunch of internal variables, which revived both the power on button and the start menu. This shows a crappy design on the power button. Any decent power button ought to assert the reset line to the processor and the entire motherboard. When reset is released, all micro processors jump to the starting address, (top of memory on some, bottom of memory on others) and start executing code. The purpose of reset on micro processors is to regain control and start running the program from the top, no matter how messed up the software is. That ain't happening on HP laptops, some kinda hardware and software kluge is breaking control of the reset line, and the machine fails to start when the button is pressed. Running the BIOS diagnostics fixes the software part of this kluge.
Good work HP engineers. I wonder what else you have screwed up.
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