GNU Modula-2 is a front end for the GNU Compiler Collection (GCC). The GNU Modula-2 compiler is compliant with the PIM2, PIM3, PIM4 and ISO dialects. Also implemented are a complete set of free ISO libraries and PIM libraries.
The four dialects commonly referred to are:
PIM2 'Programming in Modula-2' 2nd Edition, Springer Verlag, 1982, 1983 by Niklaus Wirth (PIM2).
PIM3 'Programming in Modula-2', 3rd Corrected Edition, Springer Verlag, 1985 (PIM3)
PIM4 'Programming in Modula-2', 4th Edition, Springer Verlag, 1988 (PIM4)
ISO the ISO Modula-2 language as defined in 'ISO/IEC Information technology - programming languages - part 1: Modula-2 Language, ISO/IEC 10514-1 (1996)'