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diff --git a/source/arduino/bitwisecompound.rst b/source/arduino/bitwisecompound.rst index c3c808f..d460dc8 100644 --- a/source/arduino/bitwisecompound.rst +++ b/source/arduino/bitwisecompound.rst @@ -1,227 +1,228 @@ +.. highlight:: cpp + .. _arduino-bitwisecompound: -compound bitwise AND (&=), compound bitwise OR (\|=) -==================================================== +Compound bitwise and (&=), or (\|=), XOR (^=) +============================================= 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 :ref:`bitwise math tutorial <arduino-bitwisemath>` for more +information on bitwise operators. +.. contents:: Contents + :local: -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. - - +.. _arduino-bitwisecompound-and: -Syntax: -------- +Compound bitwise AND (&=) +------------------------- -:: +The compound bitwise AND operator ``&=`` is often used with a variable +and a constant to force particular bits in a variable to be zero. This +is often referred to in programming guides as "clearing" or +"resetting" bits. In a program, writing the line ``x &= y;`` is +equivalent to writing ``x = x & y;``. That is, the value of ``x`` +after the line will be equal to its old value bitwise ANDed with the +value of ``y``:: - x &= y; // equivalent to x = x & y; + x &= y; // equivalent to x = x & y; +You can use any integer variable for ``x`` (i.e., any variable of type +``int``, ``long``, ``char``, ``byte``, etc.). You can use either an +integer variable or any :ref:`integer value +<arduino-constants-integers>` (like ``3`` or ``0x20``) for ``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 - -:: +Before doing an example of ``&=``, let's first review 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;`` + 0 0 0 1 (operand1 & operand2) = result +As shown above, bits that are "bitwise ANDed" with 0 become 0, while +bits that are "bitwise ANDed" with 1 are left unchanged. So, if ``b`` +is a ``byte`` variable, then ``b & B00000000`` equals zero, and ``b & +B11111111`` equals ``b``. +.. _arduino-bitwisecompound-binconst: -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?) +.. note:: The above uses :ref:`binary constants + <arduino-constants-integers-bin>`\ . The numbers are still the same + value in other representations, they just might not be as easy to + understand. + Normally, in C and C++ code, :ref:`hexadecimal + <arduino-constants-integers-hex>` or :ref:`octal + <arduino-constants-integers-oct>` are used when we're interested in + an integer's bits, rather than its value as a number. + While hexadecimal and octal literals might be harder to understand + at first, you should really take the time to learn them. They're + part of C, C++, and many other programming languages, while binary + constants are available only for compatibility with Arduino. -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 + Also, ``B00000000`` is shown for clarity, but zero in any number + format is zero. -:: +So, to clear (set to zero) bits 0 and 1 of a one-byte variable, while +leaving the rest of the variable's bits unchanged, use the compound +bitwise AND operator ``&=`` with the constant ``B11111100`` +(hexadecimal ``0xFC``\ ):: - 1 0 1 0 1 0 1 0 variable + 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 - + ^^^^^^^^^^^^^^^^ ^^^^ + unchanged 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: + ^^^^^^^^^^^^^^^^ ^^^^ + unchanged cleared -:: - myByte = 10101010; - - myByte &= B1111100 == B10101000; +So, using a byte variable ``b``\ , if we say:: + b = B10101010; // B10101010 == 0xAA + b &= B11111100; // B11111100 == 0xFC +then we will have :: -compound bitwise OR (\|=) -========================= + b == B10101000; // B10101000 == 0xA8 -Description ------------ +.. _arduino-bitwisecompound-or: -The compound bitwise OR operator (\|=) is often used with a -variable and a constant to "set" (set to 1) particular bits in a -variable. +Compound bitwise OR (\|=) +------------------------- +The compound bitwise OR operator ``|=`` is often used with a variable +and a constant to "set" (set to 1) particular bits in a variable. In +a program, writing the line ``x |= y;`` is equivalent to writing ``x = +x | y;``. That is, the value of ``x`` after the line will be equal to +its old value bitwise ORed with the value of ``y``:: + x |= y; // equivalent to x = x | y; -Syntax: -------- +You can use any integer variable for ``x`` (i.e., any variable of type +``int``, ``long``, ``char``, ``byte``, etc.). You can use either an +integer variable or any integer value (like ``3`` or ``0x20``) for +``y``. (This works the same way as :ref:`compound bitwise AND +<arduino-bitwisecompound-and>`\ , ``&=``). -:: +Before doing an example of ``|=``, let's first review the Bitwise OR +(``|``) operator:: - x |= y; // equivalent to x = x | y; + 0 0 1 1 operand1 + 0 1 0 1 operand2 + ---------- + 0 1 1 1 (operand1 | operand2) = result +Bits that are "bitwise ORed" with 0 are unchanged, while bits that are +"bitwise ORed" with 1 are set to 1. So if ``b`` is a ``byte`` +variable, then ``b | B00000000`` equals ``b``, and ``b & B11111111`` +equals ``B11111111`` (here we've used binary constants; see the +:ref:`note <arduino-bitwisecompound-binconst>` above). +So, to set bits 0 and 1 of a one-byte variable, while leaving the rest +of the variable unchanged, use the compound bitwise OR operator +(``|=``) with the constant ``B00000011`` (hexadecimal ``0x3``):: -Parameters ----------- + 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 + ^^^^^^^^^^^^^^^^ ^^^^ + unchanged set -x: a char, int or long variable -y: an integer constant or char, int, or long +Here is the same representation with the variable's 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 + ^^^^^^^^^^^^^^^^ ^^^^ + unchanged set +So, using a byte variable ``b``, if we say:: -Example: --------- + b = B10101010; // B10101010 == 0xAA + b |= B00000011; // B00000011 == 0x3 -First, a review of the Bitwise OR (\|) operator +then we will have :: -:: + b == B10101011; // B10101011 == 0xAB - 0 0 1 1 operand1 - 0 1 0 1 operand2 - ---------- - 0 1 1 1 (operand1 | operand2) - returned result +.. _arduino-bitwisecompound-xor: -Bits that are "bitwise ORed" with 0 are unchanged, so if myByte is -a byte variable, -myByte \| B00000000 = myByte; +Compound bitwise XOR (\^=) +-------------------------- +The compound bitwise XOR operator ``^=`` is used with a variable and a +constant to "toggle" (change 0 to 1, and 1 to 0) particular bits in a +variable. In a program, writing the line ``x ^= y;`` is equivalent to +writing ``x = x ^ y;``. That is, the value of ``x`` after the line +will be equal to its old value bitwise XORed with the value of ``y``:: + x ^= y; // equivalent to x = x ^ y; -Bits that are "bitwise ORed" with 1 are set to 1 so: -myByte & B11111111 = B11111111; +You can use any integer variable for ``x`` (i.e., any variable of type +``int``, ``long``, ``char``, ``byte``, etc.). You can use either an +integer variable or any integer value (like ``3`` or ``0x20``) for +``y``. (This works the same way as :ref:`&= +<arduino-bitwisecompound-and>` and :ref:`\|= +<arduino-bitwisecompound-or>`; in fact, these three operators all +work the same in this way). +Before doing an example of ``^=``, let's first review the Bitwise +XOR operator, ``^``:: + 0 0 1 1 operand1 + 0 1 0 1 operand2 + ---------- + 0 1 1 0 (operand1 ^ operand2) = result -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 +One way to look at bitwise XOR is that each bit in the result is a 1 +if the input bits are different, or 0 if they are the same. Another +way to think about it is that the result bit will be 1 when *exactly* +one (no more, no less) of the input bits is 1; otherwise, it will be +zero. This means that if you XOR a bit with 1, it will change (or +toggle) its value, while if you XOR a bit with 0, it stays the same. -:: +So, to toggle bits 0 and 1 of a one-byte variable, while leaving the +rest of the variable unchanged, use the compound bitwise XOR operator +``^=`` with the constant ``B00000011`` (hexadecimal ``0x3``\ ; see +:ref:`note <arduino-bitwisecompound-binconst>` above):: 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 - + ^^^^^^^^^^^^^^^^ ^^^^ + unchanged toggled -Here is the same representation with the variables bits replaced -with the symbol x +So, using a byte variable ``b``, if we say:: + b = B10101010; // B10101010 == 0xAA + b ^= B00000011; // B00000011 == 0x3 +then we will have :: -:: - - 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; - - + b == B10101001; // B10101001 == 0xA9 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) - - +- :ref:`Boolean operations <arduino-boolean>` (``&&``, ``||``) +- :ref:`Bitwise operators <arduino-bitwisemath>` (``&``, ``|``, ``^``, ``~``) |