Volatile memory needs power to remember. Switch it off, even briefly, and the stored bits are lost. When power returns, the contents are unpredictable until something writes them again.
Why it happens depends on the cell:
- An SRAM cell is a latch, a loop of two inverters. Without power the inverters stop driving and the loop forgets.
- A DRAM cell is charge on a tiny capacitor, which leaks away within milliseconds once refresh stops.
- Registers and flip-flops are volatile for the same reason as SRAM.
Volatile memories are the fast, freely rewritable ones, so a computer does its work in them. But it can't start from them: at power-on they hold nothing useful. That's why every system pairs its RAM with some non volatile memory holding the start-up firmware.
Two separate questions describe a RAM cell, and it pays to keep them apart:
- Does it survive power-off? (volatile vs non-volatile)
- Does it need refresh while powered? (static vs dynamic)
SRAM answers "no" to both: it's volatile, but needs no refresh.