An SDA symbol can represent several value types. It can represent
    a constant, a data address, a procedure or function descriptor
    address, or a routine address. Constants are usually offsets of
    a particular field in a data structure; however, they can also
    represent constant values such as the BUG$_xxx symbols.

    Symbols are composed of up to 31 letters and numbers, and can
    include the dollar sign ($) and underscore (_) characters. When
    you invoke SDA, it reads in the global symbols from the symbols
    table section of SYS$BASE_IMAGE.EXE, and from REQSYSDEF.STB, a
    required subset of the symbols in the file SYSDEF.STB. You can
    add other symbols to SDA's symbol table by using the DEFINE and
    READ commands.

    All address symbols identify memory locations. SDA generally
    does not distinguish among different types of address symbols.
    However, for a symbol identified as the name of a procedure
    descriptor, SDA takes an additional step of creating an
    associated symbol to name the code entry point address of the
    procedure. It forms the code entry point symbol name by appending
    _C to the name of the procedure descriptor.

    Also, SDA substitutes the code entry point symbol name for the
    procedure descriptor symbol when you enter the following command:

    SDA> EXAMINE/INSTRUCTION procedure-descriptor

    For example, enter the following command:

    SDA> EXAMINE/INSTRUCTION SCH$QAST

    SDA displays the following information:

    SCH$QAST_C:     SUBQ    SP,#X40,SP

    Now enter the EXAMINE command but do not specify the /INSTRUCTION
    qualifier, as follows:

    SDA> EXAMINE SCH$QAST

    SDA displays the following information:

    SCH$QAST:  0000002C.00003009   ".0..,..."

    This display shows the contents of the first two longwords of the
    procedure descriptor.

    Note that there are no routine address symbols on Alpha systems,
    except for those in MACRO-64 assembly language modules.
    Therefore, SDA creates a routine address symbol for every
    procedure descriptor it has in its symbol table. The new symbol
    name is the same as for the procedure descriptor except that it
    has an _C appended to the end of the name.