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C# Cookbook by Jay Hilyard, Stephen Teilhet

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1.6. Obtaining the Most- or Least-Significant Bits of a Number

Problem

You have a 32-bit integer value that contains information in both its lower and upper 16 bits. You need a method to get the 16 most-significant bits and/or the 16 least-significant bits of this value.

Solution

To get the most-significant bits (MSB) of an integer value, perform a bitwise and between it and the value shown in the following method:

public static int GetMSB(int intValue)
{
    return (intValue & 0xFFFF0000);
}

To get the least-significant bits (LSB) of a value, use the following method:

public static int GetLSB(int intValue)
{
    return (intValue & 0x0000FFFF);
}

This technique can easily be modified to work with other sizes of integers (e.g., 8-bit, 16-bit, or 64-bit); this trick is shown in the Discussion section.

Discussion

In order to determine the values of the MSB of a number, use the following bitwise AND operation:

uint intValue = Int32.MaxValue;
uint MSB = intValue & 0xFFFF0000;

// MSB == 0xFFFF0000

This method simply ANDs the number to another number with all of the MSB set to 1. This method will zero out all of the LSB, leaving the MSB intact.

In order to determine the values of the LSB of a number, use the following bitwise AND operation:

uint intValue = Int32.MaxValue;
uint LSB = intValue & 0x0000FFFF;

// LSB == 0x0000FFFF

This method simply ANDs the number to another number with all of the LSB set to 1, which zeroes out all of the MSB, leaving the LSB intact.

The methods presented here accept only 32-bit integer values. To allow this method to accept other numeric data types, you can simply overload this method to accept any other data types that you require. For example, if you need to also acquire the least-significant byte or most-significant byte of a 16-bit integer, you could modify the GetMSB method as follows:

public static int GetMSB(short shortValue)
{
    return (shortValue & 0xFF00);
}

The GetLSB method is modified as shown here:

public static int GetLSB(short shortValue)
{
    return (shortValue & 0x00FF);
}

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