For MSP430X devices in the large memory model, pointers and size_t are 20 bits in size. To handle this, GCC defines the __int20 type, which uses 20 bits of space in registers, and 32 bits of space in memory.

This type is built into the compiler, so it can be used like any other type in source code. Using unsigned __int20 designates the unsigned version of the type.

The __int20 type is not part of the ISO C specification, so using this type in any program compiled with flags that check for standards compliance may cause a warning or error to be emitted. A variation of the type that will not trigger these standards compliance messages is defined as __int20__ (using a trailing double underscore).