Q.Why is ROM called non volatile?

A microcontroller program lives in memory, and the different memory types behave in importantly different ways. The single most important distinction is volatility — whether stored data survives when the power is switched off.

ROM (Read Only Memory) is non-volatile: its contents are fixed and cannot be changed by the CPU during normal running, and they remain intact when power is removed. This is why a microcontroller keeps its fixed control program in ROM — the program is still there every time the device is switched on.

RAM (Random Access Memory) is read/write memory used for data that changes while a program runs, but it is volatile: everything stored in it is lost the moment power is removed. RAM comes in two forms — SRAM (Static RAM), which holds its data as long as power is applied, and DRAM (Dynamic RAM), which must be refreshed periodically. …