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Fixed point numbers

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• The fixed point decimal number representation
  • The decimal point:

    Decimal point = a point places in a decimal number representation to indicate the location of the digit whose weight = 1

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  • Weights of the digits in a fixed point decimal number:

    The digit that immediately preceeds the decimal point has weight = 100 = 1 The weight of digits moving towards left increases by a factor of 10 The weight of digits moving towards right decreases by a factor of 10

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  • Example:

    Decimal number: 123.45 ^^^ ^^ ||| || ||| |+--- weight = 1/100 ||| +---- weight = 1/10 ||+------- weight = 1 |+-------- weight = 10 +--------- weight = 100

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• The fixed point binary number representation

  • The "binary decimal" point:

    Binary decimal point = a point places in a binary number representation to indicate the location of the digit whose weight = 1

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  • Weights of the digits in a fixed point decimal number:

    The digit that immediately preceeds the decimal point has weight = 20 = 1 The weight of digits moving towards left increases by a factor of 2 The weight of digits moving towards right decreases by a factor of 2

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  • Example:

    Binary number: 101.01 ^^^ ^^ ||| || ||| |+--- weight = 1/4 ||| +---- weight = 1/2 ||+------- weight = 1 |+-------- weight = 2 +--------- weight = 4 The value represented by 101.01 is: 1*(4) + 0*(2) + 1*(1) + 0*(1/2) + 1*(1/4) = 5 1/4 = 5.25 (decimal)

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• Converting: value <==> fixed point binary representation
  • In the next 2 sections, I will show you how to convert:

    fixed point binary representation ==> value that is represented Given a (fractional) value ==> find the fixed point binary representation

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• Convert: fixed point binary representation ==> value represented
  • Method:

    Compute the value represented by a fixed point binary represention by adding the weighted sum of the value of the digits in the representation

    Example:

    Given the following fixed point binary representation: 10111.1011 The value represented is computed as: 10111.1011 ||||| |||| ||||| |||+--- 1*(1/16) ||||| ||+---- 1*(1/8) ||||| |+----- 0*(1/4) ||||| +------ 1*(1/2) ||||| ||||+-------- 1*(1) |||+--------- 1*(2) ||+---------- 1*(4) |+----------- 0*(8) +------------ 1*(16) value represented = 16 + 4 + 2 + 1 + 1/2 + 1/8 + 1/16 = 23 11/16 = 23.6875

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• Convert: value ==> fixed point binary representation
  • Method

    Split the value into 2 parts: An integral part A fractional part Find the binary representation of the integral part by repeatedly divide the value by 2 (to obtain the powers of 2n) Find the binary representation of the fractional part by repeatedly multiply the value by 2 (to obtain the powers of 2-n)

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  • Example:

    Given the value 23.6875 Split the value into 2 parts: integral part = 23 fractional part = 0.6875 Convert the integral part 23 by repeated division: 23 /2 -------- 1 <---- remainder (= digit for the weight 20) 11 /2 -------- 1 <---- remainder (= digit for the weight 21) 5 /2 -------- 1 <---- remainder (= digit for the weight 22) 2 /2 -------- 0 <---- remainder (= digit for the weight 23) 1 /2 -------- 1 <---- remainder (= digit for the weight 24) 0 Binary representation for the value 23 = 10111 Convert the fractional part 0.6875 by repeated multiplication: 0.6875 x2 ------------ 1 <--- Overflow digit (= digit for the weight 2-1) 1.375 remove the overflow digit before continuing: 1.375 ==> 0.375 0.375 x2 ------------ 0 <--- Overflow digit (= digit for the weight 2-2) 0.75 remove the overflow digit before continuing: 0.75 ==> 0.75 0.75 x2 ------------ 1 <--- Overflow digit (= digit for the weight 2-3) 1.5 remove the overflow digit before continuing: 1.5 ==> 0.5 0.5 x2 ------------ 1 <--- Overflow digit (= digit for the weight 2-4) 1.0 remove the overflow digit before continuing: 1.0 ==> 0.0 Done (remainder of the value is 0 !) Binary representation for the value 0.6875 = 0.1011 Summary: Binary representation for the value 23 = 10111 Binary representation for the value 0.6875 = 0.1011 Therefore: Binary representation for the value 23.6875 = 10111.1011

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• Comment
  • Although we can design a way to store fixed point fractional numbers inside the computer, there is a better way to represent fractional values:

    Floating point numbers

    So we never use fixed point fractional numbers.

    This material is just used as an introduction to floating point numbers.

    We will study that next.