<P> Primary storage (also known as main memory or internal memory), often referred to simply as memory, is the only one directly accessible to the CPU . The CPU continuously reads instructions stored there and executes them as required . Any data actively operated on is also stored there in uniform manner . </P> <P> Historically, early computers used delay lines, Williams tubes, or rotating magnetic drums as primary storage . By 1954, those unreliable methods were mostly replaced by magnetic core memory . Core memory remained dominant until the 1970s, when advances in integrated circuit technology allowed semiconductor memory to become economically competitive . </P> <P> This led to modern random - access memory (RAM). It is small - sized, light, but quite expensive at the same time . (The particular types of RAM used for primary storage are also volatile, i.e. they lose the information when not powered). </P> <P> As shown in the diagram, traditionally there are two more sub-layers of the primary storage, besides main large - capacity RAM: </P>

What part of the computer hardware only retains data while the computer is on