It is clear that there is some boot sector on my PC which contains the loader with menu, which allows me to choose the OS to load. But, I have noticed that PC does not display me this menu after Windows-restart or Linux-suspend. How does PC, which has lost all RAM decides to bypass the primary boot and load something different?
From what I read about the POST:
In the case of a hard reboot, the northbridge will direct this code fetch (request) to the BIOS located on the system flash memory. For a warm boot, the BIOS will be located in the proper place in RAM and the northbridge will direct the reset vector call to the RAM.
I see that my guess that RAM is lost on restart is wrong.