At a minimum, the user-written module must contain:

    o  #include statements for DESCRIP.H and SDA_ROUTINES.H

    o  The global variable SDA$EXTEND_VERSION, initialized as
       follows:

               int sda$extend_version = SDA_FLAGS$K_VERSION;

    o  The routine SDA$EXTEND (prototype follows)

    Optionally, the user-written module may also contain the
    statement:

            #define __NEW_STARLET

    You should use this option because it provides type checking of
    function arguments and gives consistency in casing and naming
    conventions.

    The entry point in the user-written module, SDA$EXTEND, is
    called as a routine with three arguments and no return value.
    The declaration is as follows:

            void sda$extend (
                    int *transfer_table,
                    struct dsc$descriptor_s *cmd_line,
                    SDA_FLAGS sda_flags)

    The arguments in this code example have the following meanings:

    Table 3-1 SDA$EXTEND Arguments

    Line of
    Code        Meaning

    transfer_   Address of the vector table in the base image. The
    table       user-written routine SDA$EXTEND must copy this to
                SDA$VECTOR_TABLE (declared in SDA_ROUTINES.H) before
                any SDA routines can be called.
    cmd_line    Address of the descriptor of the command line as
                entered by the user, less the name of the extension.
                So, if you enter "SDA> MBX" or "SDA> DO MBX", the
                command line is a zero length string. If you enter
                the command "SDA> MBX 80102030", the command line is
                " 80102030" (the separating space is not stripped).
    sda_flags   Definition for the following four bits in this
                structure:
                Bit                Meaning

                sda_flags.sda_     Indicates SDA has been activated
                flags$v_override   with the ANALYZE/CRASH_
                                   DUMP/OVERRIDE command
                sda_flags.sda_     Indicates SDA has been activated
                flags$v_current    with the ANALYZE/SYSTEM command or
                                   was invoked from the kept debugger
                                   during an SCD session
                sda_flags.sda_     Indicates that SDA was invoked
                flags$v_target     from the kept debugger during
                                   an SCD or SDD session or when
                                   analyzing a process dump
                sda_flags.sda_     Indicates SDA was activated with
                flags$v_process    the ANALYZE/CRASH_DUMP command to
                                   analyze a process dump
                sda_flags.sda_     Indicates that SDA is analyzing an
                flags$v_ia64       Integrity server system or dump
                None of the above  Indicates SDA was activated with
                bits set           the ANALYZE/CRASH_DUMP command to
                                   analyze an Alpha system dump
                Other bits         Reserved to VSI:may be nonzero

    The first executable statement of the routine must be to copy
    TRANSFER_TABLE to SDA$VECTOR_TABLE (which is declared in SDA_
    ROUTINES.H):

            sda$vector_table = transfer_table;

    If this is not done, you cannot call any of the routines
    described below. Any attempts to call the routines receive a
    status return of SDA$_VECNOTINIT. (For routines defined not to
    return a status, this value can be found only by examining the
    return value directly.)

    The next statement should be one to establish a condition
    handler, as it is often difficult to track down errors in
    extensions such as access violations because the extension is
    activated dynamically with LIB$FIND_IMAGE_SYMBOL. A default
    condition handler, SDA$COND_HANDLER, is provided that outputs
    the following information in the event of an error:

    o  The error condition

    o  The VMS version

    o  A list of activated images, with start and end virtual
       addresses

    o  The signal array and register dump

    o  The current call frame chain

    You can establish this condition handler as follows:

            lib$establish (sda$cond_handler);

                                   NOTE

       The error condition, signal array, and register dump are
       output directly to SYS$OUTPUT and/or SYS$ERROR, and are not
       affected by the use of the SDA commands SET OUTPUT and SET
       LOG.

    Thus, a minimal extension would be:

            #define __NEW_STARLET 1
            #include <descrip.h>
            #include <sda_routines.h>

            int sda$extend_version = SDA_FLAGS$K_VERSION;

            void sda$extend (int *transfer_table,
                             struct dsc$descriptor_s *cmd_line,
                             SDA_FLAGS sda_flags)
              {
              sda$vector_table = transfer_table;
              lib$establish (sda$cond_handler);

              sda$print ("hello, world");
              return;
              }