RCVCMD →     RPL:     ACB field→  ACB
                      Note: ECB and EXIT are mutually exclusive.
                      If ECB is used, only one is allowed.
                      ECB field → external ECB
                      ECB field = internal ECB
                      EXIT field → RPL exit routine
                      BRANCH field = YES|NO
                      AREA field → input area for message
                      AREALEN field = length of input area
                      OPTCD field = (SYN|ASY,TRUNC,Q|NQ)

 
     Registers 0 and 15 = return codes
 
     RPL:     RTNCD field = recovery action return code
              FDB2 field = specific error return code
              REQ field = request code (40)
              AAREALN field = maximum length of reply
               allowed (if requested)
              RECLEN field = length of input message