Instruction Set Architecture and Design - Fundamentals of Computer Organization and Architecture - page 16

organized in the form of groups of fixed number, say n, of cells that can be dealt
with as an atomic entity. An entity consisting of 8 bits is called a byte. In many
systems, the entity consisting of n bits that can be stored and retrieved in and out
of the memory using one basic memory operation is called a word (the smallest
addressable entity). Typical size of a word ranges from 16 to 64 bits. It is, however,
customary to express the size of the memory in terms of bytes. For example,
the size of a typical memory of a personal computer is 256 Mbytes, that is,
256 2 28 bytes.

In order to be able to move a word in and out of the memory, a distinct address
has to be assigned to each word. This address will be used to determine the location
in the memory in which a given word is to be stored. This is called a memory write
operation. Similarly, the address will be used to determine the memory location
from which a word is to be retrieved from the memory. This is called a memory
read operation.
The number of bits, l, needed to distinctly address M words in a memory is given
by l ¼

2 20

¼

log 2 M. For example, if the size of the memory is 64 M (read as 64 mega-
words), then the number of bits in the address is log 2 (64 ¼

26 bits. Alternatively, if the number of bits in the address is l, then the maximum
memory size (in terms of the number of words that can be addressed using these
l bits) is M ¼

2 20 ) ¼ log 2 (2 26 )

2 l . Figure 2.1 illustrates the concept of memory words and word
address as explained above.
As mentioned above, there are two basic memory operations. These are the
memory write and memory read operations. During a memory write operation a
word is stored into a memory location whose address is specified. During a
memory read operation a word is read from a memory location whose address is
specified. Typically, memory read and memory write operations are performed by
the central processing unit (CPU).

Figure 2.1 Illustration of the main memory addressing