Thanks very much, that is usable information.

From a power-on situation, I can understand that uninitialized SRAM contains random data.
The HW reset pulls the CHIP_PU pin down but Vdd and Vdd_RTC remains at 3.3V. With the power-up pin low, the ESP internals get powered down, I now see from the specs.

The few milliseconds of CHIP_PU=LOW are ...

Hi,

I like to store a value in memory that is retained after a reset. The value is used to distinguish between a power-on reset and a (manual) hardware reset. The succession of multiple resets is taken by the application to enter a special configuration mode.
Something like https://github.com ...