Multiple Components, Single Executable  

Components processors could overlap. 

Sequential calls of different components. 

============================================================
! pop.f   
!     subroutine  pop(pop_world)
!     integer  pop_world   ! input communicator containing 
                           ! all processors involved in pop.

! If "ocean" processors overlap with "atmosphere" processors, 
! above two subroutines will execute one after another (PCM mode).

! If there is no over-lap in processors, the two subroutines will
! execute simultaneously on different processor subsets (GEM mode).

===========================================================
(A tentative interface for master.F) 

call MPH_setup_SE(
     'atmosphere',     ! 'atmosphere' is present
     'ocean',          !  'ocean' is present
     'coupler'         ! 'coupler'  is present
      )        ! You can add more components here. 


if(PE_in_component('ocean', comm))     call pop(comm)
if(PE_in_component('atmosphere',comm)) call ccm(comm)

! comm is an output in function PE_in_component().

=============================================================
! More complicated ways can be adopted. For example, suppose POP 
! always run on procs 0-31, while CCM and coupler are invoked 
! one after another on procs 32-63, 

if(PE_in_component('ocean', comm))     call pop( comm)
do ii=1, nsteps
   if(PE_in_component('atmosphere',comm)) call ccm(comm)
   if(PE_in_component('coupler',comm)) call coupler(comm)
end do
! There could have implicit syncronizations 
! between POP and CCM, or between POP and coupler.

END

=========================================================
Processor.map  file:  list component names (tags) 
		and processor ranges:

         PROCESSOR_MAP
         BEGIN
         atmosphere  0  15
         ocean       16 30
         coupler     31 31
         land        32 35
         ice         36 39
         END

Processor ranges could over-lap. For example, on 
		64 processors

         atmosphere  0    23
         land        0    23   ! over-lap with ccm
         coupler     24   29
         biosphere   30   31
         ocean       32   63
         ice         32   63    ! over-lap with pop

It is users' responsibility to know who is overlapping 
with who else, and invoke components appropriately.
One can always use the logical function 
PE_in_component("ocean", ocean_comm)
to check if "ocean" covers this processor,  and if yes, 
the "ocean" communicator is ocean_comm.

=========================================================
Keywords "atmosphere", "ocean", "coupler", etc are tags 
or identifiers.
They are provided  for complete flexibility.
One maybe equally well use "CCM", "CCM_LIN_ROOD", "POP_LANL", 
"POP_NCAR", etc. The only requirement is they are used 
consistently between processor.map file, and  MPH_setup_SE(), 
MPH_comm_join(), MPH_global_id().