The use of semi-conductor memory elements (bistables) has been made possible by the use of large scale integrated circuits (LSI) which provide reliability, ease of application and good storage capacity per unit volume. A disadvantage of such memories is that a power supply is needed to hold the information stored. Batteries may be provided to prevent loss of memory in case of power failure. This is not necessary for memories made up of ferrite cores.
Typically, 15 elements are needed for one word, therefore for a 2,000 word RAM we would need 30,000 elements arranged in a three dimensional matrix. Figure 12 shows a 9-word store, each word consisting of 3-bits.
If one wishes to read the word at address W5, the memory address register sends, by way of the X and Y drive and decode circuits, a 1-bit along wires X2 and Y2. The contents of W5 are then read sequentially from least significant bit to most significant bit by the sense wire, which is connected to each of the 27 elements.
As part of a general-purpose computer ROMs may be used to store information which is unchanged over most, if not all, of the operational life of the equipment. If all the hardware of a computer is wired in using ROMs then we no longer have a general-purpose computer and hence have lost the inherent flexibility. However, saving in circuitry results, and some degree of flexibility can be regained by having interchangeable ROMs mounted on printed circuit boards (PCB) which can be removed from and fitted to the computer.