<P> The first practical form of random - access memory was the Williams tube starting in 1947 . It stored data as electrically charged spots on the face of a cathode ray tube . Since the electron beam of the CRT could read and write the spots on the tube in any order, memory was random access . The capacity of the Williams tube was a few hundred to around a thousand bits, but it was much smaller, faster, and more power - efficient than using individual vacuum tube latches . Developed at the University of Manchester in England, the Williams tube provided the medium on which the first electronically stored program was implemented in the Manchester Baby computer, which first successfully ran a program on 21 June 1948 . In fact, rather than the Williams tube memory being designed for the Baby, the Baby was a testbed to demonstrate the reliability of the memory . </P> <P> Magnetic - core memory was invented in 1947 and developed up until the mid-1970s . It became a widespread form of random - access memory, relying on an array of magnetized rings . By changing the sense of each ring's magnetization, data could be stored with one bit stored per ring . Since every ring had a combination of address wires to select and read or write it, access to any memory location in any sequence was possible . </P> <P> Magnetic core memory was the standard form of memory system until displaced by solid - state memory in integrated circuits, starting in the early 1970s . Dynamic random - access memory (DRAM) allowed replacement of a 4 or 6 - transistor latch circuit by a single transistor for each memory bit, greatly increasing memory density at the cost of volatility . Data was stored in the tiny capacitance of each transistor, and had to be periodically refreshed every few milliseconds before the charge could leak away . The Toshiba Toscal BC - 1411 electronic calculator, which was introduced in 1965, used a form of DRAM built from discrete components . DRAM was then developed by Robert H. Dennard in 1968 . </P> <P> Prior to the development of integrated read - only memory (ROM) circuits, permanent (or read - only) random - access memory was often constructed using diode matrices driven by address decoders, or specially wound core rope memory planes . </P>

In most desktop computers there is more rom than ram