Level 1 cache is implemented using Static RAM (SRAM) and until recently was traditionally 16KB in size. SRAM uses two transistors per bit and can hold data without external assistance, for as long as power is supplied to the circuit. The second transistor controls the output of the first: a circuit known as a flip-flop – so-called because it has two stable states which it can flip between. This is contrasted to dynamic RAM (DRAM), which must be refreshed many times per second in order to hold its data contents.