Primary storage, presently known as main memory, is the largest memory directly accessible to the CPU in the prevalent Von Neumann model and stores both data and instructions (program code). The CPU continuously reads instructions stored there and executes them. Also called Random Access Memory (RAM), to indicate that load/store instructions can access data at any location at the same cost, it is usually implemented using DRAM chips, which are connected to the CPU and other peripherals (disk drive, network) via a bus.
The earliest computers used tubes, then transistors and since the 1970s in integrated circuits. RAM chips generally store a bit of data in either the state of a flip-flop, as in SRAM (static RAM), or as a charge in a capacitor (or transistor gate), as in DRAM (dynamic RAM). Some types have circuitry to detect and/or correct random faults called memory errors in the stored data, using parity bits...