The most common numbering system we use is the decimal number system. in this system, numbers are expressed as digits from 0 to 9. Each number can be interpreted according to a system of place values, we proceed from right to left. The digit in the extreme right represents the number of 1s, the next digit the number of 10s, the next digit the number of 100s, and so forth. For example, the number 2375 stands for:
If we carefully investigate this numbering representation, it can be seen in another way:
In the binary number system, numbers are represented by either 0 or 1 digits. Just as the decimal number system is based on powers of 10, the binary number system is based on powers of two. Let us take a binary number like: 10001111 starting from the right most digit which is corresponds to the number of 1s the next digit to the number of 2s, the next digit to number of 4s and the next to 8s and so forth. The binary number 10001111 can be written as:
As a result the binary number 10001111 corresponds to the decimal number 143. The binary system of numbers are extremely important in the operations of computers as memory can be presented as on or off to correspond to binary digit of 0 or 1 this one binary digit is also called a bit. As we shall see, 8 bit and 16 bit binary numbers play a special role in internal working of computers. The number of 8 bit also called byte represents the number 0 through 255. Similarly the number of 16 bit binary number (2 byte) represents the decimal numbers 0 through 65535.
As we have seen, every binary number has a decimal equivalent, however, the reverse is also true, every decimal number has a binary equivalent. For example, consider the decimal number 41, in order to find the equivalent binary number we have to apply the following steps. In each step we divides the number by 2 and write down the remainder.
The hexadecimal number system is closely connected with the binary number system and is much easier to work with in many applications, one hexadecimal digit is corresponding to 4 binary digits, and so 8 bit binary numbers can be represented by two hexadecimal digits. There are 16 possible hexadecimal digits:
Let us take a hexadecimal number say A2BF and imply the translation to decimal number system, as we done before we receive:
The octal numbering system is based on 8 numeric digits, namely from 0 to 7, and as seen before it represents powers of 8. For example the octal number 723 represent the decimal value of 467.
Binary number 1110011010100111 Divide into groups of three 1 110 011 010 100 111 Convert each group into octal 1 6 3 2 4 7 Final octal number 163247
Octal number 2 7 0 3 5 Convert each digit into binary 10 111 000 011 101 Final binary number 10111000011101
In some applications it is often required to move all the bits of a certain binary number to the left or to the right. Such operations are called a left shift or a right shift. For example, consider this 8-bit (byte) binary number: 10110111, which is equivalent to 183 in decimal representation. If we apply a left shift, we obtain the binary number 01101110 = 110 decimal notes that a zero was added at the right most digit and the original left most bit has been pushed off the end. Similarly, a right shift applied to the binary number 10110111 yields 01011011 = 91 decimal then a second right shift can be applied to yield 00101101 = 45. The shift operation can be summarized as in the next tables.
A computational left shift can be done by multiplying the decimal value by 2, this can lead to the left most bit to be pushed to the 9th place. In order to eliminate the 9th bit, we perform the MOD operation (division remainder) on the decimal number by the number 256, this operation will return only the 8 right most bits.
A right shift on the other hand can be done by integer division of the decimal value by 2, that is
In the above examples we assumed that right and left shifts were operated on 8 bit (byte) binary number, but shift operation can be performed also on 16 bit (2 byte or a word) or 32 bit or on any number of bits that we wish, in this case the same rules of shifting applies, for example the 16-bit binary number 11100110 10111011 can be left shifted to obtain 11001101 01110110.
If two corresponding bits of A and B have a one, then the result is a one, if either A or B has a zero, the result is a zero. For example, consider the following 16-bit binary numbers:
Switching control Suppose we have an 8-bit number like 11110011, this array of bits can simulate 8 switches that each of them can be in on or off position. Let’s think what happens if we need to turn bit 6 (from right or switch number 6) to off. This process is done by the AND operation, in order to find the second number needed for the AND operation, we will choose the binary number 11011111. All bits are 1 except the one we want to turn off.
Note that all the bits remained untouched, except bit number 6 that has been turned off. The same result would be obtained by the operation on the equivalent decimal numbers 243 AND 223 = 211 By applying the AND operation we can only turn switches off in order to turn a bit on we need to apply the OR operation as described in the next paragraph. We can turn off multiple switches in a single operation, this will be done by choosing zeroes in the second number in the places we want to turn them off, like 10001101 in this case we will turn off 4 bits (all the 0 values).
The OR operation performed on two identical length binary numbers is obtained by comparing A and B bit by bit. For a given bit position, if either A or B have a one, then the corresponding bit of A OR B is one. If both A and B have a zero, the corresponding bit of A OR B is a zero. For example, consider the two 8-bit binary numbers
By performing the OR operation we can turn on a single switch, this is done as follows, Lets take the switch array 11010111 and try to turn on switch number 6 (from right). The second binary number is chosen to be 00100000, all bits are zero except the one we want to turn on.
Note that switch number 6 is now turned on and all other switches remained untouched.
The operation A XOR B is called the exclusive OR, it is performed on two identical length binary numbers by comparing A and B bit by bit. For a given bit position, if A and B have different bits values, then the corresponding bit of A and B is a one. If A and B have the same bits value, the corresponding bit A XOR B is a zero. For example, consider the 8-bit binary number A = 11001001 and B = 10001101 then,
If A and B are integers given in decimal form, we may also apply the operation XOR the same way like 201 XOR 141 = 68. An interesting use of The XOR operation is in the field of encoding text expressions.
The NOT operation reverses all the bits of a given number. For example, consider the binary number 00111001. We may compute this number by changing every 0 to 1 and every 1 to 0, see the next lines.
The NOT operation can be performed on decimal numbers as well. For example, NOT 57 = 198. It is strictly important to define previously the length of the binary number, as different lengths has different NOT values. The above calculations were based on 8-bit length, if the same values were calculated on 16 bit length, the results would be as follows,
One interesting use of the XOR operation is for encoding purposes, this encoding is based on the fact that by performing the XOR operation twice on a certain character, we receive back the original character. The value for the encoding can be selected to be any value as our imagination goes. For example examine the letter "s", this letter has the equivalent Ascii value of 115 or in binary presentation A = 01110011. For the encoding code we choose the value 66 or in binary presentation B = 01000010, now we apply the XOR operation to receive.
So, we encoded the letter "s" by the Ascii code of 49 which is the digit "1". Now applying the encoding value once again to restore the original letter yields.
As we expected the original letter has been restored. The only rule we must remember is to apply the same encoding value twice. By applying this technique on each letter, we can encode the entire document. The above example uses the same encoding value to all the characters. To further improve the encoding process, we can apply a trigonometric function or a varying encoding value. For example, we can define an array of arbitrary values like 3, 44, 77, 295, 3393, 858, 44, 338 and so on, then apply this encoding values, each of them on a progressing character, the only thing we must remember is to apply the same encoding values to restore the original letter.
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