Memories

Overview

Memories play a special role among circuit components in the sense that theydisplay a highly regular structure and are therefore particularly suitable for miniaturization and extreme integration. They most spectacularly demonstrate the advances of semiconductor technology: Whereas in 1975 chips with 1K bits represented the stateof the art in 1995 chips with a capacity of 16M bits are available. This is a factor of 214 in 20years, or a factor of 2 about every 18 months.

We distinguish between read/write and read-only memories. As was explained in Chapter 2, read-only memories (ROM) are purely combinational circuits. An essential property of these memories is that the access time does not depend on the cell selected (addressed).This implied that any cell can be addressed “at random”, yielding the same access delay. This property has led to the term “random access memory” (RAM). It stands in contrast to memories where access proceeds through a sequence of “cells”, and access time therefore strongly depends on the address value. Such sequential access is typical for all memories with moving parts, such as magnetic tapes and disks. RAMs are electronic memories, also called semiconductor memories, and are on average about 1000 times faster than magnetic memories with moving parts. Here we shall discuss electronic memories only. Among read/ write memories one distinguishes further between static (SRAM) and dynamic (DRAM) memories.