HP 3000 Manuals

HP FORTRAN 77/iX Programmer's Guide

Using Comments 

When your program has code that is specific to the MPE/iX operating
system, place a C or * in the first column of each line of
system-specific code to identify the lines as comment lines.  When not
compiling on MPE/iX, the code remains in the program without being
executed.  When the program is compiled on MPE/iX, remove the C or *.

MPE/iX 5.0 Documentation