In native code (gnt , dll , exe , so) Micro Focus COBOL will optimize subscript access. This optimization is controlled via the BOUNDOPT compiler directive. The optimizations can impact code migrated from other platforms.

Consider this case where a table is passed (by the caller) in Linkage to the callee. The caller code is:-

       working-storage section.
       01  ws-data.
           03 ws-data1         pic x(80).
           03 ws-data2         pic x(80).
           03 ws-data3         pic x(80).
           03 ws-data4         pic x(80).
           03 ws-data5         pic x(80).
           03 ws-data6         pic x(80).
           03 ws-data7         pic x(80).
           03 ws-data8         pic x(80).
           03 ws-data9         pic x(80).
           03 ws-data10        pic x(80).
           03 ws-data11        pic x(80).
           03 ws-data12        pic x(80).
           03 ws-data13        pic x(80).
           03 ws-data14        pic x(80).
           03 ws-data15        pic x(80).
           03 ws-data16        pic x(80).
           03 ws-data17        pic x(80).
           03 ws-data18        pic x(80).
           03 ws-data19        pic x(80).
           03 ws-data20        pic x(80).

       01  ws-table.
           03  ws-table-element    pic x(80) occurs 100.



       procedure division.
       start-it section.

           initialize ws-data

           initialize ws-table

           call "callee" using ws-table
           if ws-data20 not = spaces
               display "Error"
           end-if

           goback.

 The callee code is:-

      $set nobound boundopt
       working-storage section.

       01  ws-sub              pic 9(4).
       linkage section.
       01  lnk-table.
           03  lnk-table-element    pic x(80) occurs 2.



       procedure division using lnk-table.
       start-it section.

           perform varying ws-sub from 1 by 1 until ws-sub > 100
               move all "A" to lnk-table-element(ws-sub)
           end-perform

           goback.

When control is returned to the caller variable ws-data20 contains all "A"s. Although a table with 100 items was passed the table is defined in the callee as occurs 2. While this is poor practice this code can produce the desired results on some platforms.

The corruption occurs when table subscript 10 is used. The native compiler optimizes access to the array and only uses the most significant byte of the subscript due to the occurs 2.  This causes an array update at element 0. This corrupts the ws-data20 variable.

To get the desired results in this case the NOBOUNDOPT directive needs to be used. When this is used ws-data20 is unaffected and all 100 elements of the array are populated.