[analog31]

It's easy to overlook for those who program mostly on higher level systems. There was a recent HN thread that I think pointed to this article on what happens before main() in C programs:

https://embeddedartistry.com/blog/2019/04/08/a-general-overv...

On microcontrollers, there are often some preliminaries that are programmed into nonvolatile settings of the hardware, such as the type of clock oscillator. On a microprocessor like the Z80, your circuit was supposed to ensure that things like the clock oscillator and power supplies were stable before releasing the RESET pin.