Memory

Computers rely on different types of memory to store data (this means: values and addresses). Read-Only Memory (ROM) can only be read. Random-Access Memory (RAM) can be read and written to. Other types of memory include EEPROMs, Flash RAM, disks, CD-ROMs, tapes and so on. Within the scope of these notes, we shall only be concerned with RAM type memory. This is very fast (in modern PCs RAM chips are refreshed at frequencies that are around one tenth as that of the CPU), comparatively expensive, and loses information if the power is switched off. The model of memory that best fits these C++ notes is that of a linear array, as depicted in Fig. 1.

Figure 1: A linear array model of memory: the $i$ -th box has address $a_i$ and contains value $v_i$ .

RAM can be seen as a long (finite) list of boxes; the $i$ -th box in the list is addressed by a value $a_i$ and contains a value $v_i$ . The size of an address is usually the same as the processor bus (normally 32 or 64 bit). The size of the value, on the other hand, can be anything depending on the type of value being stored.