C# Language: Changing the First Four Bits in a Byte

Question

In order to utilize a byte to its fullest potential, I\'m attempting to store two unique values into a byte: one in the first four bits and another in the second four bits. How

Answer 1

I know you asked specifically about clearing out the first four bits, which has been answered several times, but I wanted to point out that if you have two values <= decimal 15, you can combine them into 8 bits simply with this:

    public int setBits(int upperFour, int lowerFour)
    {
        return upperFour << 4 | lowerFour;            
    }

The result will be xxxxyyyy where

xxxx = upperFour
yyyy = lowerFour

And that is what you seem to be trying to do.

Answer 2

public int SplatBit(int Reg, int Val, int ValLen, int Pos)
{
    int mask = ((1 << ValLen) - 1) << Pos;
    int newv = Val << Pos;
    int res = (Reg & ~mask) | newv;
    return res;            
}

Example:

• Reg = 135
• Val = 9 (ValLen = 4, because 9 = 1001)

• Pos = 2

• 135 = 10000111

• 9 = 1001
• 9 << Pos = 100100
• Result = 10100111

Answer 3

I'm not really sure what your method there is supposed to do, but here are some methods for you:

void setHigh(ref byte b, byte val) {
    b = (b & 0xf) | (val << 4);
}

byte high(byte b) {
    return (b & 0xf0) >> 4;
}

void setLow(ref byte b, byte val) {
    b = (b & 0xf0) | val;
}

byte low(byte b) {
    return b & 0xf;
}

Should be self-explanatory.

Answer 4

A quick look would indicate that a bitwise and can be achieved using the & operator. So to remove the first four bytes you should be able to do:

byte value1=255; //11111111
byte value2=15; //00001111

return value1&value2;

Answer 5

When I have to do bit-twiddling like this, I make a readonly struct to do it for me. A four-bit integer is called nybble, of course:

struct TwoNybbles
{
    private readonly byte b;
    public byte High { get { return (byte)(b >> 4); } }
    public byte Low { get { return (byte)(b & 0x0F); } {
    public TwoNybbles(byte high, byte low)
    {
        this.b = (byte)((high << 4) | (low & 0x0F));
    }

And then add implicit conversions between TwoNybbles and byte. Now you can just treat any byte as having a High and Low byte without putting all that ugly bit twiddling in your mainline code.

Answer 6

You first mask out you the high four bytes using value & 0xF. Then you shift the new bits to the high four bits using newFirstFour << 4 and finally you combine them together using binary or.

public void changeHighFourBits(byte newHighFour)
{
    value=(byte)( (value & 0x0F) | (newFirstFour << 4));
}

public void changeLowFourBits(byte newLowFour)
{
    value=(byte)( (value & 0xF0) | newLowFour);
}