Random Access Memory
RAM is considered "random access" because you can access any memory cell directly if you know the row and column that intersect at that cell. The two main forms of modern RAM are static RAM (SRAM) and dynamic RAM (DRAM). Most modern operating systems employ a method of extending RAM capacity, known as "virtual memory".
Early computers used relays, or delay lines for "main" memory functions. Ultrasonic delay lines could only reproduce data in the order it was written. Drum memory could be expanded at low cost but retrieval of non-sequential memory items required knowledge of the physical layout of the drum to optimize speed. Latches built out of vacuum tube triodes, and later, out of discretetransistors, were used for smaller and faster memories such as random-access register banks and registers. Such registers were relatively large, power-hungry and too costly to use for large amounts of data; generally only a few hundred or few thousand bits of such memory could be provided.