C $Header: /home/ubuntu/mnt/e9_copy/MITgcm/eesupp/src/exch_cycle_ebl.F,v 1.3 2001/09/21 03:54:34 cnh Exp $
C $Name: ecco-branch-mod2 $

CBOP
C     !ROUTINE: EXCH_CYCLE_EBL

C     !INTERFACE:
      SUBROUTINE EXCH_CYCLE_EBL( myThid )
      IMPLICIT NONE

C     !DESCRIPTION:
C     *==========================================================*
C     | SUBROUTINE EXCH_CYCLE_EBL                                 
C     | o Cycle exchange buffer level.                            
C     *==========================================================*
C     | Step the "exchange buffer level" to next value.           
C     | The buffer level is used to prevent consecutive messages  
C     | overwriting one another. Because we know that every tile  
C     | will participate in an exchange we can be sure that every 
C     | tile will cycle through the buffers with the same         
C     | sequence. Also we know that a tile cannot start a new     
C     | exchange before it has completed its end of the previous  
C     | exchange. Thus by cycling through three buffer levels we  
C     | can be sure that the buffer we write to will be free.     
C     |  That is by the time we are participating in the third    
C     | exchange transaction we know that both ends of the first  
C     | transaction must have completed.                          
C     | Note --                                                   
C     | 1. We allow more than three levels below although this    
C     | is not necessary.                                         
C     *==========================================================*

C     !USES:
C     == Global data ==
#include "SIZE.h"
#include "EEPARAMS.h"
#include "EESUPPORT.h"
#include "EXCH.h"

C     !INPUT PARAMETERS:
C     == Routine arguments ==
C     myThid :: Thread number of this instance of the routine
      INTEGER myThid

C     !LOCAL VARIABLES:
C     == Local variables ==
C     bi, bj :: Loop counters
      INTEGER bi, bj
CEOP

      DO bj=myByLo(myThid),myByHi(myThid)
       DO bi=myBxLo(myThid),myBxHi(myThid)
C--     Use next level exchange buffer in cycle.
        exchangeBufLevel(1,bi,bj) = exchangeBufLevel(1,bi,bj)+1
        IF ( exchangeBufLevel(1,bi,bj) .GT. NUMBER_OF_BUFFER_LEVELS )
     &   exchangeBufLevel(1,bi,bj) = 1
       ENDDO
      ENDDO

      RETURN
      END