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

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名媛妹妹
名媛妹妹 2021-02-14 07:01

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

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  • 2021-02-14 07:41

    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;
    
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  • 2021-02-14 07:46

    Use bitwise AND (&) to clear out the old bits, shift the new bits to the correct position and bitwise OR (|) them together:

    value = (value & 0xF) | (newFirstFour << 4);
    

    Here's what happens:

                           value        : abcdefgh
                           newFirstFour : 0000xyzw
    
                                   0xF  : 00001111
                           value & 0xF  : 0000efgh
                      newFirstFour << 4 : xyzw0000
    (value & 0xF) | (newFirstFour << 4) : xyzwefgh
    
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  • 2021-02-14 07:46

    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.

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  • 2021-02-14 07:46
    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
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  • 2021-02-14 07:52

    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.

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  • 2021-02-14 07:56

    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);
    }
    
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