a) The bit is the most elementary data unit that corresponds to a binary digit in computer's memory. Generally the binary 1 is represented as high voltage and binary 0 is represented as low or negative voltage in a digital computer's registers and memory. However, this also depends on the nature of media. For example, on magnetic disks and tape reels, the bit is stored as traces of magnetism. The bit is also typically used to measure the rate at which data is transmitted over communication media (e.g. twisted copper wire, coaxial cable, air, etc). A kilobit per second is one thousand bits per second, a megabit per second is one million bits per second. As of 1990, we do not yet speak of gigabits per second since there are no medium over which that high a communication rate can be achieved.
b) The byte is a group of 8 bits. A byte may represent ASCII (American Standard Code for Information Interchange) characters or numbers in the computer. Byte is the basic unit for expressing storage capacities in computers. A kilobyte is a thousand bytes, a megabyte is a million bytes, and a gigabyte is one billion bytes.
c) The word is less well defined. Its definition is dependent upon the hardware architecture of the computer. In general, the term "word" is used to indicate the length of registers and addressable memory locations. If the computer has an 8-bit processor (CPU), such as INTEL 8080A, then the word is 8 bits. If the computer has a 16-bit processor, such as PDP-11, then the word is 16 bits. Finally, if the computer has a 32-bit processor, then the word is 32 bits. The term "word", other than indicating the smallest addressable units in a computer's memory, is not a very useful measure since it is not well defined. Perhaps the best definition is that a word is 16 bits (two bytes), a double word is 32 bits (four bytes).
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