Backing Stores
Andrew J. T. Colin · 1980
The main store of a computer is the one which holds the instructions of the program being executed, and the individual items of data — numbers or characters — being manipulated. Main stores are always of the ‘random access’ variety: each cell can be reached equally quickly in a fraction of a microsecond. In 1979, the cost of main store was about 0.5p per bit, or 8p for a 16-bit word. This makes large quantities of main storage expensive. Even though the price of store may fall in absolute terms, it will remain high in comparison to the cost of the central processor. Computer designers are therefore compelled to use other, cheaper forms of store for data which does not need to be referred to at such short notice. These types are generally called ‘backing store’. There exist a number of storage technologies, of which we will discuss only a representative sample. The various techniques fall into one of two broad groupings, depending on the way the stored information is reached.