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Unformatted text preview: h with a small data shovel and the user of a machine having larger word length with
a large data shovel. Even though they both may be shoveling at comparable speeds, the
user with the smaller shovel will be slower because more shovelfuls are needed to move
the same amount of data.
Fixed and Variable Word-length Memory
The main memory of some computers is designed to store a fixed number of characters
(equal to its word-length in bytes) in each numbered address location. Such computers
are said to be word-addressable, and they employ a fixed-word-length memory approach.
In these computers, storage space is always allocated in multiples of word-length. So if a
word addressable computer has a fixed word-length of 4 bytes (4 characters) then this
computer will require one word (4 bytes) to store the word "CAT" and two words (8
bytes) to store the word "BOMBAY".
In many computers, the main memory is also designed in such a way that each numbered
address can only store a single character (A, B, 1,2, +, -, etc.). Computers designed in this
manner are said to be character-addressable and they employ a variable-word-length
memory approach. Thus in these machines, only 3 bytes will be required to store the
word "CAT" and only 6 bytes will be required to store the word "BOMBAY". Figure 7.4
summarizes the difference between the fixed-word-length and variable-word-length
Both the fixed and the variable word-length memory systems have their own merits and
demerits. The fixed-word-length memory approach is normally used in large scientific
computers for gaining speed of calculation. On the other hand, the variable-word-length
approach is used in small business computers for optimizing the use of storage space. For
example, let us consider a fixed-word-length machine with a word size of eight
characters. If most of the data words to be stored are of less than five characters then
more than half of the storage space will remain unused. This will not happen in case of a
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- Spring '14