#1,093 – How Positive and Negative Zero Values Are Stored

.NET can represent both positive and negative zero floating point values.  They are stored as 32-bit floating point values as follows:

  • +0 : sign bit 0, mantissa 0 (23 bits), exponent 0 (8 bits)
  • -0: sign bit 1, mantissa 0 (23 bits), exponent 0 (8 bits)

A 32-bit positive zero is therefore stored as 00000000 (hex) and negative zero as 80000000 (hex).  We can verify this by looking at these values in memory from within Visual Studio.

Assuming the following code:

            float zero = 0.0f;
            float negZero = -0.0f;

Positive zero:

1093-001

Negative zero:

1093-002

 

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#1,092 – Positive and Negative Zero

Because .NET uses the IEEE 754 standard to represent floating point numbers, it allows representing both positive and negative zero values.  (+0.0 and -0.0).

Mathematically, +0.0 is equal to -0.0 and an equality check in C# will return a true result.  However, although the values are considered equal, either value can be represented in C# and they are stored differently in memory.

            float zero = 0.0f;
            float negZero = -0.0f;

            bool theyAreEqual = zero == negZero;   // true

            // 00-00-00-00
            Console.WriteLine(BitConverter.ToString(BitConverter.GetBytes(zero)));

            // 00-00-00-80
            Console.WriteLine(BitConverter.ToString(BitConverter.GetBytes(negZero)));

            float sum1 = zero + 1.0f;
            float sum2 = negZero + 1.0f;
            bool sumsEqual = sum1 == sum2;    // true

1092-001

 

You can think of  a floating point representation of zero as being either zero or a very small positive number that rounds to zero when stored as a floating point.  If the value was a tiny bit above zero before rounding, it’s stored as +0.0.  If it was a bit below zero before rounding, it’s stored as -0.0.

#1,091 – Subnormal Floating Point Numbers

32-bit binary floating point numbers are normally stored in a normalized form, that is:

1091-001

where d is the fractional part of the mantissa, consisting of 23 binary digits and e is the exponent, represented by 8 bits.

In this form, the minimum allowed value for e is -126, which is stored in the 8-bit exponent as a value of 1.  Because the leading 1 is implicit, this means that the  minimum positive floating point value is:

1091-002

We could use the 8-bit value of 0 in the exponent to represent an exponent of -127, but that would only gain us a single power of two, or one more value that we could store.

Instead, a value of of 0 stored in the 8-bit exponent is a signal to drop the leading 1 in the mantissa.  This allows storing a set of much smaller numbers, known as subnormal numbers, of the form:

1091-003

We can now use all 23 digits in the mantissa, allowing us to store numbers as low as 2^-149.

#1,090 – Using Visual Studio to Verify How Floating Point Numbers Are Stored

Recall that floating point numbers are stored in memory by storing the sign bit, exponent and mantissa.

We showed that the decimal value of 7.25, stored as a 32-bit floating point value, is stored as the binary value 0x40E80000.

1089-001

We can verify this in Visual Studio by assigning a float to contain the value 7.25 and then looking at that value in memory.

1090-001

Notice that the bytes appear to be backwards, relative to their order as written above.  This is because Intel is a little-endian platform (bytes at “little” end of 32-bit word are stored first).

#1,089 – How 32-Bit Floating Point Numbers Are Stored in .NET, part II

(part I)

We store the sign, exponent, and mantissa of a binary floating point number in memory as follows.

The sign is stored using a single bit, 0 for positive, 1 for negative.

The exponent is stored using 8 bits.  The exponent can be positive or negative, but we store it as a positive value by adding an offset (bias) of 127.  We add 127 to the desired exponent and then store the resulting value.  Exponents in the range of -126 to 127 are therefore stored as values in the range 1 to 254.  (Stored exponent values of 0 and 255 have special meaning).

Because we normalize the binary floating point number, it has the form 1.xxx.  Since it always includes the leading 1, we don’t store it, but use 23 bits to store up to 23 digits after the decimal point.

For example:

1089-001

 

 

 

#1,088 – How 32-Bit Floating Point Numbers Are Stored in .NET, part I

Floating point numbers in .NET (on Intel-based PCs) are stored using the IEEE 754 standard, which defines how to store both 32-bit (float) and 64-bit (double) floating points.

Floating point numbers are stored in memory by storing the value as a binary floating point value represented using scientific notation (binary).

For example, to store a decimal value of 7.25:

1088-001

The exponent is expressed as binary, so it has a value of 2.

We can now store this floating point number in memory by storing three things:

  • The sign of the number (positive)
  • The mantissa (1.1101)
  • The exponent (10)

On Intel-based PCs, 32-bit floating point numbers are stored as follows:

  • 1 bit to store the sign (0 for positive numbers, 1 for negative numbers)
  • 8 bits to store the exponent
  • 23 bits to store the mantissa

More coming in part II