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CIE A-Level Computer Science Notes

13.3.3 Conversion between Binary and Denary

Understanding the conversion between binary and denary (decimal) systems for floating-point numbers is a foundational skill in A-Level Computer Science. This knowledge is vital for interpreting and manipulating numerical data in various computational contexts.

Understanding Binary and Denary Systems

The Binary System

  • Binary system: Utilises two symbols, 0 and 1, to represent values.
  • Base-2 system: Each position in a binary number represents a power of 2, with the rightmost digit representing 2^0, the next 2^1, and so on.

The Denary System

  • Denary system: Also known as the decimal system, it uses ten symbols (0-9).
  • Base-10 system: Each position in a denary number represents a power of 10.

Floating-Point Numbers in Binary and Denary

Structure of a Floating-Point Number

  • Consists of: Two main parts – a mantissa (or significand) and an exponent.
  • Mantissa: The significant digits of the number, functioning as a coefficient.
  • Exponent: Determines the power of the base (2 for binary, 10 for denary) to which the mantissa is multiplied.

Conversion Techniques

From Binary to Denary

Step-by-Step Conversion

  • 1. Identify the Components: Separate the binary number into its mantissa and exponent.
  • 2. Convert the Exponent: Change the binary exponent into its denary equivalent using standard binary-to-denary conversion.
  • 3. Convert the Mantissa: Interpret the mantissa as a binary fraction. Each digit after the binary point represents a negative power of 2.
  • 4. Apply the Exponent: Multiply the converted mantissa by 2 raised to the power of the converted exponent.

Detailed Example

  • Binary number: 1101.101 x 23
    • Mantissa: 1101.101
      • Conversion: 1x23 + 1x22 + 0x21 + 1x20 + 1x2-1 + 0x2-2 + 1x2-3 = 8 + 4 + 0 + 1 + 0.5 + 0 + 0.125 = 13.625
    • Exponent: 3
      • Conversion: 3 (since it's already in denary)
    • Result: 13.625 x 23 = 109.0 in denary

From Denary to Binary

Step-by-Step Conversion

  • 1. Separate Integer and Fractional Parts: Treat the integer and fractional parts separately for conversion.
  • 2. Convert the Integer Part: Divide the integer part by 2 repeatedly, recording the remainders, until the quotient is 0. The binary equivalent is the sequence of remainders in reverse order.
  • 3. Convert the Fractional Part: Multiply the fractional part by 2. The integer part of the result is the next binary digit. Repeat with the new fractional part until it becomes 0 or reaches a desired level of precision.
  • 4. Combine and Apply Exponent: Reassemble the binary mantissa. Determine the appropriate exponent to position the binary point correctly.

Detailed Example

  • Denary number: 109.0
    • Integer part: 109
      • Conversion: 109 ÷ 2 = 54 remainder 1, 54 ÷ 2 = 27 remainder 0, ... → 1101101 (binary)
    • Fractional part: 0 (no conversion needed)
    • Result: 1101101.0 x 2^0 in binary

Practical Considerations

Sign Representation

  • Positive numbers: Represented as is in both systems.
  • Negative numbers: In binary, often represented using the two’s complement method.

Precision and Rounding

  • Binary Limitations: Not all denary fractions can be represented exactly in binary, leading to approximation.
  • Rounding: Necessary in many conversions, especially for binary fractional parts, which can introduce minor errors.

Real-World Applications

  • Computing: Conversions are crucial in algorithms that process numerical data in different formats.
  • Data Analysis: Accurate conversion is key to maintaining the integrity of numerical data in analysis.

Tips for Effective Conversion

  • Regular Practice: Enhances familiarity and accuracy in conversions.
  • Verification: Always verify conversions, especially in critical computational tasks.
  • Use of Computational Tools: While calculators and software are helpful, understanding the manual process is essential for a deeper comprehension.

Challenges in Conversion

Handling Large Numbers

  • Overflow: Occurs when the number exceeds the representational capacity of the given number of bits.
  • Underflow: Happens with numbers too small to be represented, leading to a loss of precision or to being rounded down to zero.

Common Mistakes

  • Misinterpreting Binary Point: Unlike a fixed decimal point, the binary point's position varies based on the exponent, which can lead to errors if misunderstood.
  • Incorrect Rounding: Rounding errors, especially in the fractional part, can significantly impact the accuracy of subsequent computations.

Special Cases

  • Infinite Decimal Fractions: Some denary fractions, when converted to binary, result in an infinite series of digits. For instance, 0.1 in denary becomes a recurring binary fraction.

Importance of Normalisation

  • Normalisation: Adjusting the mantissa and exponent so that the mantissa starts with a non-zero digit (in binary, a 1). It ensures a standard form, making comparisons and arithmetic operations easier.

Exercises for Mastery

  • Practice Problems: Convert various binary and denary numbers, especially those with fractional parts, to solidify understanding.
  • Error Analysis: Evaluate the impact of rounding errors in different computational scenarios.


Normalising binary floating-point numbers is a process that ensures the representation of these numbers is consistent and efficient. The significance of normalisation lies in maximising precision and simplifying arithmetic operations. In a normalised number, the mantissa (or significand) is adjusted so that its most significant digit (the first digit to the left of the binary point) is non-zero (usually 1 in binary). This is done by adjusting the exponent accordingly. For instance, the binary number 0.101101 x 23 would be normalised to 1.01101 x 21. Normalisation ensures that the mantissa is within a certain range, allowing for the most efficient use of its digits. In floating-point arithmetic, normalised numbers are easier to compare and operate on since they follow a standard format. This consistency is crucial in computational applications where accuracy and efficiency are paramount. Normalisation also helps in detecting and handling special conditions like underflow and overflow, which are critical for maintaining numerical stability in computations.

Not all denary numbers can be accurately represented in binary format. This is mainly due to the fact that the binary system, being base-2, cannot precisely represent some fractions that are easily expressed in the base-10 (denary) system. For example, the denary fraction 0.1 cannot be represented exactly in binary; it becomes a recurring binary fraction, leading to a representation that is only an approximation of the actual value. This limitation has significant implications in computing, particularly in areas requiring high precision, such as scientific calculations, financial computations, and numerical analysis. Rounding errors and precision loss can accumulate in successive calculations, potentially leading to significant discrepancies. To mitigate these issues, computer scientists use various techniques such as extended precision formats, error bounds analysis, and alternative number systems like decimal floating-point representation in specific applications. Understanding these limitations and their effects is crucial for designing algorithms and systems that are robust and accurate.

Understanding binary floating-point representation is crucial in computer science for several reasons. Firstly, it's fundamental for computer hardware, as computers inherently operate using binary logic. Knowing how numbers are represented in binary allows computer scientists and engineers to design and optimize hardware and software systems efficiently. Secondly, in programming, many algorithms and data processing tasks involve numerical computations where floating-point arithmetic is used. A clear understanding of how numbers are represented and manipulated in binary form helps in debugging, optimizing performance, and ensuring accuracy in these computations. Moreover, in fields like computer graphics, scientific computing, and machine learning, precise and efficient handling of floating-point numbers is vital for producing accurate results and managing computational resources effectively. Lastly, an understanding of binary floating-point representation lays the foundation for grasping more complex concepts in computer science, such as encryption algorithms, data compression techniques, and error detection and correction methods.

In binary and denary systems, representing very large or small numbers involves different approaches. In the denary system, large numbers are typically represented in a straightforward manner, using an extended sequence of digits. However, representing extremely large numbers can be challenging due to limitations in physical storage and processing capabilities. In contrast, the binary system often employs a method known as "floating-point representation" for both large and small numbers. This method uses a fixed number of digits (bits) to represent the mantissa and exponent separately, allowing for a wide range of values with a relatively small number of bits. For example, in binary, a large number would have a small mantissa and a large exponent, while a small number would have a larger mantissa and a negative exponent. This method is particularly efficient for representing numbers that vary greatly in magnitude, but it can introduce rounding errors due to the finite number of bits used, leading to precision loss, especially in scientific and engineering calculations where accuracy is paramount.

Overflow and underflow are issues that arise in binary floating-point arithmetic when the numbers involved exceed the representational capacity of the system. Overflow occurs when a number is too large to be represented in the available number of bits, leading to a loss of significant data or an erroneous result. For example, if an arithmetic operation results in a number larger than what can be represented by the allocated bits for the exponent, overflow occurs. This often leads to the number being approximated to infinity or a large number, which can cause significant errors in calculations.

Underflow happens when a number is too small to be represented, typically occurring with numbers close to zero. In such cases, the number may be rounded down to zero or to a smaller number with less precision than required. Underflow can lead to a loss of accuracy, especially in iterative calculations where small errors accumulate over time.

Both overflow and underflow can have critical implications in computing, affecting the accuracy and reliability of numerical computations. To mitigate these issues, modern computing systems implement techniques like extended precision, error checking, and special handling of extreme values. Understanding and managing these issues are essential for designing robust and accurate computational algorithms and systems.

Practice Questions

Convert the denary number 75.375 into its binary floating-point equivalent. Show all steps in your conversion process.

To convert 75.375 from denary to binary, we first convert the integer part (75) and then the fractional part (0.375). For the integer part, 75 divided by 2 gives a quotient of 37 and a remainder of 1. Continuing this process, we get a binary integer part of 1001011. For the fractional part, we multiply 0.375 by 2, resulting in 0.75, so the first binary digit after the point is 0. Repeating this process, we get a binary fractional part of .011. Combining these, we get the binary representation of 75.375 as 1001011.011.

Describe the process of converting the binary floating-point number 1010.101 to a denary number. Include each step and the final denary value.

To convert the binary number 1010.101 to denary, we need to convert both the integer and fractional parts. The integer part, 1010, converts to 1x23 + 0x22 + 1x21 + 0x20, which equals 8 + 0 + 2 + 0, resulting in 10. For the fractional part, 0.101 is converted by multiplying each digit after the binary point by decreasing negative powers of 2, giving 1x2-1 + 0x2-2 + 1x2-3, which is 0.5 + 0 + 0.125, totalling 0.625. The final denary number is 10 + 0.625, equalling 10.625.

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Written by: Alfie
Cambridge University - BA Maths

A Cambridge alumnus, Alfie is a qualified teacher, and specialises creating educational materials for Computer Science for high school students.

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