No system clears the memory on reset (in hardware). If it happens it's done in firmware.
Reset is required for CPU and chipset, the RAM doesn't care.
There are systems out there that may lose a few bits due to a reset (when the RAM controller's refresh stops for too long) but I wouldn't call that 'erasing'.
To topic: if you connect a switch to the keyboard connector's pins 3 & 6 you'll have your reset button.
