Arithmetic types in Arduino
The Arduino "language" is actually C/C++. GNU GCC and tool chain are used. The Arduino IDE uses a preprocessor to make it easier for beginners by removing some of the C/C++ requirements such as forward declaration of functions.

The GNU AVR GCC data types and representation, and the answer to your question can be found on the project Wiki page; https://gcc.gnu.org/wiki/avr-gcc

For C/C++ there are build-in data types but also a set of standards. One of the standard header files with data types is stdint.h. http://www.nongnu.org/avr-libc/user-manual/group__avr__stdint.html

Arduino.h and additional header files within the Arduino core libary contains the extensions or redefinitions made. The complete reference list; https://www.arduino.cc/en/Reference/HomePage

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