It's just like Fortran, where everything including a constant is passed by reference, so if you assign to your arguments you might corrupt your program's only copy of 7 unless the linker put it in a read-only page.
For example, the first output of the following program is 4, but the second is still 7.
program main
integer m,n
m = 7
call corrupt(m)
write (*,*) m
n = 7
write (*,*) n
stop
end program main
subroutine corrupt (a)
integer a
a = 4
return
end call corrupt(7)
program main
integer m,n
call corrupt(7)
write (*,*), 7
stop
end program main
subroutine corrupt (a)
integer a
a = 4
return
end
The reason was that in one subroutine, a parameter value was stored in a local variable, then used for computation and restored at the end. Since constants was stored in read only memory when using g77 on Linux, but not on the f77 compiler on HPUX, the Linux port would crash, but not the original HPUX version. In that the code you had above would have worked.
I was going to mention Indiana but was more hoping that it wouldn't be mentioned at all.