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diff --git a/source/arduino/bitwisecompound.rst b/source/arduino/bitwisecompound.rst new file mode 100644 index 0000000..c3c808f --- /dev/null +++ b/source/arduino/bitwisecompound.rst @@ -0,0 +1,227 @@ +.. _arduino-bitwisecompound: + +compound bitwise AND (&=), compound bitwise OR (\|=) +==================================================== + +The compound bitwise operators perform their calculations at the +bit level of variables. They are often used to clear and set +specific bits of a variable. + + + +See the +`bitwise AND (&) <http://arduino.cc/en/Reference/BitwiseAnd>`_ and +`bitwise OR (\|) <http://arduino.cc/en/Reference/BitwiseAnd>`_ +operators for the details of their operation, and also the +`Bitmath Tutorial <http://www.arduino.cc/playground/Code/BitMath>`_ +for more information on bitwise operators. + + + +compound bitwise AND (&=) +========================= + +Description +----------- + +The compound bitwise AND operator (&=) is often used with a +variable and a constant to force particular bits in a variable to +the LOW state (to 0). This is often referred to in programming +guides as "clearing" or "resetting" bits. + + + +Syntax: +------- + +:: + + x &= y; // equivalent to x = x & y; + + + +Parameters +---------- + +x: a char, int or long variable +y: an integer constant or char, int, or long + + + +Example: +-------- + +First, a review of the Bitwise AND (&) operator + +:: + + 0 0 1 1 operand1 + 0 1 0 1 operand2 + ---------- + 0 0 0 1 (operand1 & operand2) - returned result + + + +Bits that are "bitwise ANDed" with 0 are cleared to 0 so, if myByte +is a byte variable, +``myByte & B00000000 = 0;`` + + + +Bits that are "bitwise ANDed" with 1 are unchanged so, +``myByte & B11111111 = myByte;`` + + + +Note: because we are dealing with bits in a bitwise operator - it +is convenient to use the binary formatter with +`constants. <http://arduino.cc/en/Reference/IntegerConstants>`_ The +numbers are still the same value in other representations, they are +just not as easy to understand. Also, B00000000 is shown for +clarity, but zero in any number format is zero (hmmm something +philosophical there?) + + + +Consequently - to clear (set to zero) bits 0 & 1 of a variable, +while leaving the rest of the variable unchanged, use the compound +bitwise AND operator (&=) with the constant B11111100 + +:: + + 1 0 1 0 1 0 1 0 variable + 1 1 1 1 1 1 0 0 mask + ---------------------- + 1 0 1 0 1 0 0 0 + + variable unchanged + bits cleared + + +Here is the same representation with the variable's bits replaced +with the symbol x + + + +:: + + x x x x x x x x variable + 1 1 1 1 1 1 0 0 mask + ---------------------- + x x x x x x 0 0 + + variable unchanged + bits cleared + + +So if: + +:: + + myByte = 10101010; + + myByte &= B1111100 == B10101000; + + + +compound bitwise OR (\|=) +========================= + +Description +----------- + +The compound bitwise OR operator (\|=) is often used with a +variable and a constant to "set" (set to 1) particular bits in a +variable. + + + +Syntax: +------- + +:: + + x |= y; // equivalent to x = x | y; + + + +Parameters +---------- + +x: a char, int or long variable +y: an integer constant or char, int, or long + + + +Example: +-------- + +First, a review of the Bitwise OR (\|) operator + +:: + + 0 0 1 1 operand1 + 0 1 0 1 operand2 + ---------- + 0 1 1 1 (operand1 | operand2) - returned result + +Bits that are "bitwise ORed" with 0 are unchanged, so if myByte is +a byte variable, +myByte \| B00000000 = myByte; + + + +Bits that are "bitwise ORed" with 1 are set to 1 so: +myByte & B11111111 = B11111111; + + + +Consequently - to set bits 0 & 1 of a variable, while leaving the +rest of the variable unchanged, use the compound bitwise AND +operator (&=) with the constant B00000011 + +:: + + 1 0 1 0 1 0 1 0 variable + 0 0 0 0 0 0 1 1 mask + ---------------------- + 1 0 1 0 1 0 1 1 + + variable unchanged + bits set + + +Here is the same representation with the variables bits replaced +with the symbol x + + + +:: + + x x x x x x x x variable + 0 0 0 0 0 0 1 1 mask + ---------------------- + x x x x x x 1 1 + + variable unchanged + bits set + +So if: + +:: + + myByte = B10101010; + + myByte |= B00000011 == B10101011; + + + +See Also + + +- `& <http://arduino.cc/en/Reference/BitwiseAnd>`_ (bitwise AND) +- `\| <http://arduino.cc/en/Reference/BitwiseAnd>`_ (bitwise OR) +- `&& <http://arduino.cc/en/Reference/Boolean>`_ (Boolean AND) +- `\|\| <http://arduino.cc/en/Reference/Boolean>`_ (Boolean OR) + + |