The GNU Fortran Compiler: Internal representation of LOGICAL variables

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### 5.2 Internal representation of LOGICAL variables

The Fortran standard does not specify how variables of `LOGICAL` type are represented, beyond requiring that `LOGICAL` variables of default kind have the same storage size as default `INTEGER` and `REAL` variables. The GNU Fortran internal representation is as follows.

A `LOGICAL(KIND=N)` variable is represented as an `INTEGER(KIND=N)` variable, however, with only two permissible values: `1` for `.TRUE.` and `0` for `.FALSE.`. Any other integer value results in undefined behavior.

See also Argument passing conventions and Interoperability with C.