Fixed and Variable Word Length Storage:
The primary storage section 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 computer are said to be word addressable, and they employ a fixed word-length storage 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 characters then this computer will require one word (4 bytes) to store the word "CAT" and two words (8 bytes) to store the word "MORLEY".
In many computers, the primary storage section 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 storage 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 "MORLEY"
Both the fixed and variable word-length storage systems have their own merits and demerits. The fixed word-length storage 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 character. 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 machine with variable word-length primary storage because a character can be placed in every storage sell of this machine. However, word addressable computers possess faster calculating capability because they can add to data words in single operation.
Most of the today's business and scientific processing is handled by flexible computers which can employ either a fixed word-length storage organization. The set of instructions available with these computers allow them to be operated as either variable or fixed word-length computer.