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diff --git a/docs/source/arduino/bitshift.rst b/docs/source/arduino/bitshift.rst deleted file mode 100644 index 00c466f..0000000 --- a/docs/source/arduino/bitshift.rst +++ /dev/null @@ -1,148 +0,0 @@ -.. highlight:: cpp - -.. _arduino-bitshift: - -Bit shift left (``<<``), bit shift right (``>>``) -================================================= - -Description ------------ - -(Adapted from `The Bit Math Tutorial -<http://www.arduino.cc/playground/Code/BitMath>`_ in `The Arduino -Playground <http://www.arduino.cc/playground/Main/HomePage>`_\ ) - - -There are two bit shift operators in C++: the left shift operator -``<<`` and the right shift operator ``>>``. These operators cause the -bits in the left operand to be shifted left or right by the number of -positions specified by the right operand. - -More information on bitwise math can be obtained in the Wikipedia -article on `bitwise operations -<http://en.wikipedia.org/wiki/Bitwise_operation>`_\ , especially the -section on shifts in `C, C++, and Java -<http://en.wikipedia.org/wiki/Bitwise_operation#Shifts_in_C.2C_C.2B.2B.2C_C.23_and_Java>`_\ . - - -Syntax ------- - -``some_int << number_of_bits`` - -``some_int >> number_of_bits`` - - -Parameters ----------- - -* **some_int** An integer value or variable. - -* **number_of_bits** integer whose value is at most ``8 * - sizeof(variable)`` (so ``number_of_bits`` can be at most 32 for - ``int`` values, at most ``8`` for ``char`` values, etc.; the various - integer sizes are summarized :ref:`in this table - <arduino-arithmetic-int-sizes>`\ ). - - - -Example: --------- - -Here are some examples of bit shifting, with the binary representation of the number in comments:: - - int a = 5; // binary: 101 - int b = a << 3; // binary: 101000, or 40 in decimal - int c = b >> 3; // binary: 101, or back to 5 like we started with - - -When you left shift a value x by y bits (x << y), the leftmost y bits -in x are lost, literally shifted out of existence. We'll do this -example with ``char`` values (which are integers in the range 0-255, -and take up 8 bits of memory):: - - char a = 5; // binary (all 8 bits): 00000101 - char b = a << 7; // binary: 10000000 - the first 1 in 101 was discarded - - -If you are certain that none of the ones in a value are being shifted -into oblivion, a simple way to think of the left-shift operator is -that it multiplies the left operand by 2 raised to the right operand -power (in math notation, ``x << y`` equals x * 2\ :sup:`y`\ , as long -as none of the bits of x get shifted out). For example, to generate -powers of 2, the following expressions can be employed:: - - 1 << 0 == 1 - 1 << 1 == 2 - 1 << 2 == 4 - 1 << 3 == 8 - ... - 1 << 8 == 256 - 1 << 9 == 512 - 1 << 10 == 1024 - ... - -.. _arduino-bitshift-signbit-gotcha: - -When you shift x right by y bits (``x >> y``), and the highest bit in -x is a 1, the behavior depends on the exact data type of x. If x is of -type ``int``, the highest bit is special, and determines whether x is -negative or not; the details are too complicated to explain here, but -they are thoroughly explained in the Wikipedia article on `two's -complement arithmetic -<http://en.wikipedia.org/wiki/Two%27s_complement>`_\ , which the -system most computers use to store integers. In that case, the sign -bit is copied into lower bits, for esoteric historical reasons:: - - int x = -16; // binary (all 32 bits): 11111111111111111111111111110000 - int y = x >> 3; // binary: 11111111111111111111111111111110 - - - -This behavior, called "sign extension", is often not what you -want. You probably wish zeros to be shifted in from the left. It -turns out that the right shift rules are different for ``unsigned -int`` values, so you can use a type cast to suppress ones being copied -from the left:: - - int x = -16; // binary: 11111111111111111111111111110000 - int y = (unsigned int)x >> 3; // binary: 00011111111111111111111111111110 - - - -If you are careful to avoid sign extension, you can use the -right-shift operator, ``>>``, as a way to divide by powers of 2. For -example:: - - int x = 1000; - int y = x >> 3; // integer division of 1000 by 8, causing y = 125. - - -Arduino Compatibility ---------------------- - -Since it's part of the C++ language, bit shifting on the Maple is -compatible with the Arduino; however, you should keep in mind that the -Maple has bigger integer types (as in, more bits) than the Arduino. - -Since the STM32 is a 32-bit processor, the ``int`` type takes up 32 -bits instead of 16, like on Arduino's 16-bit microcontroller. This -means that you can shift left, like ``x << y``, with bigger values of -``y`` on the Maple before ones in ``x`` start to get shifted out. - -To calculate the number of bits of an integer type on the Maple, -multiply its size in bytes (see :ref:`this table -<arduino-arithmetic-int-sizes>` for these) by 8, since there are 8 -bits in 1 byte. For example, a ``short`` takes up 2 bytes of memory, -or 2 * 8 = 16 bits. - -See Also --------- - -- :ref:`arduino-bit` -- :ref:`arduino-bitread` -- :ref:`arduino-bitwrite` -- :ref:`arduino-bitclear` - - -.. include:: cc-attribution.txt
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