Binary Number Formats

The binary number system uses base 2 which includes only the digits 0 and 1.

In the United States among other countries, every three decimal digits is separated with a comma to make larger numbers easier to read. For example, 123,456,789 is much easier to read than 123456789. We adopt a similar convention for binary numbers. In order to make binary numbers more readable, we add a space every four digits starting from the least significant digit, on the left of the decimal point. For example, the binary value 1010111110110010 will be written as 1010 1111 1011 0010.

Using the above example 1010 1111 1011 0010:

• The rightmost sequence (0010) is in bit position zero, holding value 0. Bit zero is referred to as the least significant bit, or lsb
• Each bit to the left is given the next successive bit number. bit 1, bit 2 and so on...
• The left-most bit (1010) is typically called the most significant bit, or msb, holding value 1.

We typically write binary numbers as a sequence of bits. There are defined boundaries for the different sizes of a bit:

In any number base, we may add as many leading zeroes without ever changing its value, but in the binary number system, adding leading zeroes adjusts the bit to a desired size. For example, we can represent the number 5 as a:

Bit 101;

Nibble 0101;

Byte 0000 0101;

Word 0000 0000 0000 0101; or

Double Word 0000 0000 0000 0000 0000 0000 0000 0101

Number Base Conversion

Binary to Octal Conversion

It is easy to convert a binary value to an octal number.

First, break the binary number into 3-bit sections from the LSB to the MSB then convert the 3-bit binary number to its octal equivalent.

Let's breakdown the binary value 1010111110110010:

Octal to Binary Conversion

It is also easy to convert from an octal number to a binary value.

Convert the decimal number to its 3-bit binary equivalent, then combine the 3-bit sections.

Let's take the above octal number, 127662 and break it down:

This yields the binary number 001010111110110010 or 0000 1010 1111 1011 0010 in our more readable format.

Octal to Decimal Conversion

To convert from Octal to Decimal, multiply the value in each position by its Octal weight and add each value.

Again, let's take the Octal value, 127662

Decimal to Octal Conversion

To convert decimal to octal is slightly more difficult.

The typical method to convert from decimal to octal is repeated division by 8. For this method, divide the decimal number by 8, and write the remainder on the side as the least significant digit. This process is continued by dividing the quotient by 8 and writing the remainder until the quotient is 0. When performing the division, the remainders which will represent the octal equivalent of the decimal number are written beginning at the least significant digit (right) and each new digit is written to the next more significant digit (the left) of the previous digit. Consider the number 44978.

As you can see, we are back with the original number.

Hexadecimal to Binary

To convert a hexadecimal number into a binary number:

1. Breakdown the hexadecimal number into each individual Hexa digit

2. Substitute the corresponding binary value for each Hexa digit

3. Each binary value is written from the beginning of the MSB (left) to the LSB (right)

For example, to convert 0ABCDH into a binary value, simply convert each hexadecimal digit according to the table above.

0ABCDH = 0000 1010 1011 1100 1101

Binary to Hexadecimal

To convert a binary number into the hexadecimal format is almost as easy.

For example, given the binary number 1011001010, the first step would be to add two zeros in the MSB position so that it contains 12 bits. Then separate the binary value into groups of four bits, so the revised binary value reads 0010 1100 1010. Then we look up the binary values in the table above and substitute the appropriate hexadecimal digit:

Therefore 0010 1100 1010 = 2CA

Another example, 1010111110110010 converts into AFB2.

Hexadecimal to Decimal conversion

The weighted values for each position is as follows:

To convert from Hex to Decimal, multiply the value in each position by its weighted value then add. Using the table above and the previous example, 0AFB2H, we expect to obtain the decimal value 44978.

Decimal to Hex

To convert from decimal to hex is slightly more difficult. The typical method is repeated division by 16.

While we may also use repeated subtraction by the weighted position value, it is more difficult for large decimal numbers.

The Division Method

Divide the decimal number by 16, and write the remainder on the side as the least significant digit. This process is continued until the quotient is 0. When performing the division, the remainders which will represent the hex equivalent of the decimal number are written beginning at the least significant digit (right) and each new digit is written to the next more significant digit (the left) of the previous digit. Consider the number 44978.

44978 = 0AFB2

When you use hex numbers in an 8085 program, the Assembler usually requires the most significant hex digit to be 0 even if this number of digits exceed the size of the register. This is an Assembler requirement and your value will be assembled correctly.

Hex Addition

Add as if decimal numbers, except:

If individual digits are A through F, we must convert to its decimal equivalent (values from the table above) then add.

For example:

find the sum of D + C

0DH = 13 and 0CH = 12 so;

13 + 12 = 25 therefore;

D + C = 25 but;

If the sum is greater than or equal to 16, we replace the sum with the sum minus 16 and carry over 1.

For example:

25 - 16 = 9; then

D + C = 19; and finally

3) if the sum is between 10 and 15 (decimal), we replace the sum with the equivalent hexadecimal digit, for example:

More examples of binary and hex addition:

Positional number systems

Our decimal number system is known as a positional number system because the value of the number depends on the position of the digits. For example, the number 123 is different from 321 even though the same digits are used. Other ancient number systems used additional symbols to represent larger values, but in a positional number system the value of each digit is determined by its place in the full number. The lowest place value is the rightmost position, and each successive position to the left has a higher place value. The rightmost position represents the "ones" column, the next position represents the "tens" column, the next position represents "hundreds", etc. Therefore, the number 123 represents 1 hundred and 2 tens and 3 ones, whereas the number 321 represents 3 hundreds and 2 tens and 1 one.

The values of each position correspond to powers of the base of the number system. So for our decimal number system, which uses base 10, the place values correspond to powers of 10:

Converting from other number bases to decimal

Other number systems use different bases. The binary number system uses base 2, so the place values of the digits of a binary number correspond to powers of 2.

For example, the value of the binary number 10011 is determined by computing the place value of each of the binary digits of the number:

So the binary number 10011 represents the value

The same principle applies to any number base. For example, the number 2132 base 5 corresponds to

So the value of the number 2132 base 5 is

Converting from decimal to other number bases

In order to convert a decimal number into different number bases, we have to be able to express the number in terms of powers of the other base. To convert the decimal number 100 to base 4, we must figure out how to express 100 as the sum of powers of 4, for example:

Then we use the coefficients of the powers of 4 to form the number as represented in base 4:

Another method is to repeatedly divide the decimal number by the base in which it is to be converted until the quotient becomes zero. As the number is divided, the remainders - in reverse order - form the digits of the number in the other base. For example:

Convert the decimal number 82 to base 6:

The answer is formed by taking the remainders in reverse order: 2 1 4 base 6

Example conversions between number systems:

Negative numbers

Signed magnitude - Use one bit for sign, 7 bits for number.

Example: -1 (in an 8-bit system) could be 1000 0001 (base 2)

2's complement (most often used in computing)

Example: the 2's complement number representing -1 (in an 8-bit system) would be 1111 1111 (base 2)

Complement numbers

Consider odometer numbers on a bicycle: Consider odometer numbers on a bicycle:

Decimal examples Odometer numbers are useful for both addition and subtraction (complements) of signed numbers. - 2 + 998 + 3 + 003 + 1 1 001

In the second example, the correct result is just the +001; the overflow is ignored in fixed-length complement arithmetic. Do subtraction as addition by using complements, i.e. A - B = A + (-B) Important: It is easier to compute -B and add than to subtract directly. Example: - 005 + 995 + 003 + 997 + 008 1 992

Note that 995 and 997 were added in the normal fashion, the overflow was ignored, and the result was 992, which can be converted from the complement (or odometer) system back to -8, the correct answer.

signed   3-bit complement
    +3   003
    +2   002
    +1   001
     0   000
    -1   999
    -2   998
    -3   997
    -4   996
    -5   995
    -6   994
    -7   993
    -8   992

decimal 2's complement binary

  +127   01111111
  +126   01111110
  +125   01111101
   ...   ........
    +2   00000010
    +1   00000001
     0   00000000
    -1   11111111
    -2   11111110
   ...   ........
  -127   10000001
  -128   10000000

There are 256 numbers in an 8-bit fixed length 2's complement number system. Why are these numbers called 2's complement? M - N = M + (-N) where -N is the complement of N.

Example: decimal

   + 1   =     00000001
   - 1   =   + 11111111
     0   =   1 00000000

which does not equal 2^8, or 256; ignore the overflow, and the correct answer is zero. Now we need an easy way to do 2's complement operations.

Example: What is the representation of -27 (base 10) in 2's complement 8-bit notation?

 2^8 - 1     11111111

- 27 - 00011011

             11100100   corresponds to flipping all the bits;
                        also known as 1's complement
   add 1   + 00000001
  result     11100101   -27 in 2's complement representation

This approach is necessary because: 1. Fixed-length nature of numbers stored in a computer 2. More computationally efficient to implement a fast adder than an adder and a subtractor ________________________________________ Examples of 2's complement number operations Addition and subtraction in 2's complement form • Addition: To compute N1 + N2, add N1 to N2 • Subtraction: To compute N1 - N2, add N1 to -N2 Examples: decimal binary hex

   11   =     00001011   =     000B
 + 21   =   + 00010101   =   + 0015
   32   =     00100000   =     0020

   21   =     00010101   =     0015
 - 11   =   + 11110101   =   + FFF5
   10   =     00001010   =     000A

   11   =     00001011   =     000B
 - 21   =   + 11101011   =   + FFEB
 - 10   =     11110110   =     FFF6

 - 11   =     11110101   =     FFF5
 - 21   =   + 11101011   =   + FFEB
 - 32   =   1 11100000   =     FFE0

How we got the 2's complements of -11 and -21:

11     00001011

~11 - 11110100 1's complement + 1 + 00000001 add 1 -11 11110101 2's complement representation

Algorithm 1. Store N. 2. Obtain ~N, the 1's complement of N, by replacing (bit by bit) every 0 with a 1 and vice versa in the number's binary representation. 3. Add 1 and ignore any carry after the eighth bit. Note: This algorithm works in hex by replacing each digit x with its hex complement, i.e. 15 - x. Example: The hex equivalent of 11 is $000B; its hex complement is then $FFF4, where each digit was computed as $F - x. Adding 1 to $FFF4 gives the result $FFF5 for the 2's complement of 11. (Using a dollar sign ($) before a number indicates that it is base 16.) Example: In binary:

N     0000 0110 0100 0111

~N - 1111 1001 1011 1000 1's complement +1 + 0000 0000 0000 0001 add 1 -N 1111 1001 1011 1001 2's complement representation

In hex:

N     0647

~N - F9B8 1's complement +1 + 0001 add 1 -N F9B9 2's complement representation

A calculator will always give you the 2's complement directly. The Most Significant Bit (MSB) is the binary digit corresponding to the largest power of 2 and is always 1 for a negative 2's complement number. As a result it is often called the sign bit. In 2's complement, -0 = +0. Technically, 0 is the complement of itself.

N     00000000

~N - 11111111 1's complement +1 + 00000001 add 1 -N 00000000 2's complement representation

________________________________________ Problems with fixed-length arithmetic Overflow and underflow For a 16-bit fixed-length number system,

Adding signed numbers can easily exceed these limits. first digit

      0011   $3000
      0110   $6000
      1001   $9000

This result, from adding two positive numbers in the number system, results in $9000, which is larger than $7FFF, the largest allowed positive number. In fact, $9000 equals 1001 0000 0000 0000, which is a negative number. The sign of the result is different from that of the operands. Rule: If there is a sign change in adding two positive 2's complement numbers, then overflow has occurred. We can generalize these rules to signed addition and subtraction.

Sign extension How about if numbers are of different length? decimal 2's complement binary

         3-bit 4-bit  8-bit
    +3   011   0011   00000011
    +2   010   0010   00000010
    +1   001   0001   00000001
     0   000   0000   00000000
    -1   111   1111   11111111
    -2   110   1110   11111110
    -3   101   1101   11111101
    -4   100   1100   11111100

To extend a 2's complement number to a greater number of binary digits, you just continue the sign bit to the left. Examples: Extend $9B to 16 bits. $9B = 1001 1011 = 1111 1111 1001 1011 = $FF9B

Extend $5F to 16 bits. $5F = 0101 1111 = 0000 0000 0101 1111 = $005F

Adding two 2's complement numbers of different length 43A0 43A0

 9B     FF9B   need to sign extend; you can't just add zeros.

???? 1 433B

Note that the 8-bit $9B and the 16-bit $FF9B both represent -101 in their respective number systems.