*COMDECK IMSBB
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*1DC  BB - DOWNLINE BACK BUFFER 
* 
*     THE DOWNLINE BACK BUFFER IS A SHORT CUT FOR BYPASSING THE BUFFER
*     MANAGEMENT ROUTINES FOR GENERATING DOWNLINE BACKS.  THE DOWNLINE
*     BACK BUFFER IS A COMMON BLOCK WITH ROOM FOR 25 DOWNLINE BACK
*     NETWORK MESSAGES.  IT IS SET UP AT INITIALIZATION TIME TO HAVE
*     EACH 3 WORD ENTRY LINKED TO EACH OTHER IN A CHAIN.  THE COMMON
*     BLOCK CONTAINS A POINTER TO THE FIRST ENTRY IN THE CHAIN.  EACH 
*     ENTRY CONSISTS OF A CHAIN POINTER WORD FOLLOWED BY THE TWO WORD 
*     DOWNLINE NETWORK BACK MESSAGE.  WHEN NIP NEEDS A BACK TO BE SENT
*     DOWNLINE, NIP FIRST SEES IF THERE IS ONE AVAILABLE IN THIS BUFFER.
*     IF THERE IS, IT IS DELINKED FROM THE CHAIN, THE NETWORK BACK BLOCK
*     IS CREATED IN THE BUFFER, AND THE SECOND WORD OF THE ENTRY IS 
*     USED TO CREATE A REGULAR BUFFER ID WORD AND IT IS THEN
*     LINKED TO THE PIP OUTBOUND CHAIN.  IF THERE ARE NO FREE ENTRIES 
*     IN THE DOWNLINE BACK BUFFER, THEN THE BUFFER MANAGEMENT ROUTINE 
*     IS CALLED TO ALLOCATE A BUFFER FOR THE BACK.  WHEN BUFFERS FOR
*     DOWNLINE NETWORK MESSAGES ARE RELEASED, A CHECK IS MADE TO SEE IF 
*     THE BUFFER IS ONE OF THESE SPECIAL BUFFERS.  IF SO, THE BUFFER
*     BECOMES THE FIRST FREE BUFFER IN THE CHAIN AND THE PREVIOUS FIRST 
*     FREE BUFFER IN THE CHAIN IS POINTED TO BY THE NEW ONE.  IF THE
*     BUFFER IS NOT ONE OF THESE SPECIAL BUFFERS, THEN THE BUFFER 
*     MANAGEMENT ROUTINE IS CALLED TO RELEASE THE BUFFER. 
* 
* 
*     DOWNLINE BACK BUFFER ENTRY: 
* 
*                  1           2           3           4          5 
*      012345678901234567890123456789012345678901234567890123456789 
*     I-----------------------------I-----I------I----------------I 
*     I                                          I                I 
*     I                     0                    I    FPTR        I 
*     I                                          I                I 
*     I-----I-----------------I------------------I----------------I 
*     I     I                 I                                   I 
*     I 39  I        2        I                  0                I 
*     I     I                 I                                   I 
*     I-----I-I-------I-------I---I---I---------------------------I 
*     I       I       I       IP BI   I                           I 
*     I  DN   I   SN  I   CN  IR SI 3 I            0              I 
*     I       I       I       II NI   I                           I 
*     I-------I-------I-------I---I---I---------------------------I 
* 
*      W H E R E . . .
* 
*        FPTR  = ADDRESS OF NEXT FREE ENTRY IN THIS BUFFER
*              = 0 IF NO MORE ENTRIES 
*        DN    = DESTINATION NODE OF DOWNLINE NETWORK BACK BLOCK
*        SN    = SOURCE NODE OF DOWNLINE NETWORK BACK BLOCK 
*        CN    = CONNECTION NUMBER OF DOWNLINE NETWORK BACK BLOCK 
*        PRI   = PRIORITY FLAG OF CONNECTION
*        BSN   = BLOCK SEQUENCE NUMBER OF NETWORK BACK BLOCK
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