Hardware Reference
In-Depth Information
to disk storage. Without a memory from which the processors can read and write
information, there would be no stored-program digital computers.
The basic unit of memory is the binary digit, called a
bit
. A bit may contain a
0 or a 1. It is the simplest possible unit. (A device capable of storing only zeros
could hardly form the basis of a memory system; at least two values are needed.)
People often say that computers use binary arithmetic because it is ''efficient.''
What they mean (although they rarely realize it) is that digital information can be
stored by distinguishing between different values of some continuous physical
quantity, such as voltage or current. The more values that must be distinguished,
the less separation between adjacent values, and the less reliable the memory. The
binary number system requires only two values to be distinguished. Consequently,
it is the most reliable method for encoding digital information.
If you are not
familiar with binary numbers, see Appendix A.
Some computers, such as the large IBM mainframes, are advertised as having
decimal as well as binary arithmetic. This trick is accomplished by using 4 bits to
store one decimal digit using a code called
BCD
(
Binary Coded Decimal
). Four
bits provide 16 combinations, used for the 10 digits 0 through 9, with six combina-
tions not used. The number 1944 is shown below encoded in decimal and in pure
binary, using 16 bits in each example:
decimal: 0001 1001 0100 0100
binary: 0000011110011000
Sixteen bits in the decimal format can store the numbers from 0 to 9999, giving
only 10,000 combinations, whereas a 16-bit pure binary number can store 65,536
different combinations. For this reason, people say that binary is more efficient.
Consider, however, what would happen if some brilliant young electrical engi-
neer invented a highly reliable electronic device that could directly store the digits
0 to 9 by dividing the region from 0 to 10 volts into 10 intervals. Four of these de-
vices could store any decimal number from 0 to 9999. Four such devices would
provide 10,000 combinations. They could also be used to store binary numbers, by
only using 0 and 1, in which case, four of them could store only 16 combinations.
With such devices, the decimal system would obviously be more efficient.
Memories consist of a number of
cells
(or
locations
), each of which can store a
piece of information. Each cell has a number, called its
address
, by which pro-
grams can refer to it. If a memory has
n
cells, they will have addresses 0 to
n
1.
All cells in a memory contain the same number of bits. If a cell consists of
k
bits,
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