Page 184 - Bebop to The Boolean Boogie An Unconventional Guide to Electronics Fundamentals, Components, and Processes

Memory dCs 165

RAM and ROM Architectures
The smallest unit of memory, called a cell, can be used to store a single bit
of data: that is, a logic 0 or a logic 1. A number of cells physically grouped
together are classed as a word, and all the cells in a word are typically written
to, or read from, at the same time. The core of a memory device is made up of a
number of words arranged as an array (Figure 15-2).

bit 0

bit 3

Figure 15-2. Memory cells,
wordst and arrays

The width (w) of a memory is the number of bits used to form a word, where

the bits are usually numbered from 0 to (w - Similarly, the depth (d) of a
memory is the number of words used to form the array, where the words are
usually numbered from 0 to (d - I). The following examples assume a memory
array that is four bits wide and eight words deep-real devices can be much
wider and deeper.

For the purposes of this discussion, it is convenient to visualize a ROM as
containing an array of hard-coded cells. In reality, the physical implementation
of a RQM is similar to that of a PROM, which is discussed in more detail in

In the case of Dynamic RAMS (DRAMS),‘ each cell is formed from a tran-
sistor-capacitor pair. The term “dynamic” is applied because a capacitor loses its
charge over time and each cell must be periodically recharged to retain its data.

5 Note that there is no official definition as to the width of a word: this is always system-dependent.
6 In conversation, DRAM is pronounced as “D-RAM.” That is, spelling out the “D” and following
it with “RAM” to rhyme with “ham.”