VMS Help Symbol Assign, Parameters  *Conan The Librarian (sorry for the slow response - running on an old VAX)

  symbol-name

   Defines a 1- through 255-character alphanumeric string name for  the
   symbol.   The  symbol  name  must begin with an alphabetic character
   (uppercase and lowercase characters are equivalent), an  underscore,
   or  a  dollar sign.  After the first character, the name can contain
   any alphanumeric characters from  the  DEC  multinational  character
   set, underscores, or dollar signs.

   If you specify a single equal sign (=) in the assignment  statement,
   the  symbol name is placed in the local symbol table for the current
   command level.

   If you specify double equal signs (==) in the assignment  statement,
   the symbol name is placed in the global symbol table.

  expression

   Specifies  the  value  on  the  right-hand  side  of  an  assignment
   statement.   An  expression can consist of a character string value,
   an  integer  value,  a  symbol  name,  a  lexical  function,  or   a
   combination  of these entities.  The components of an expression are
   evaluated, and the result is assigned to a symbol.

   All literal character strings must be enclosed in  quotation  marks.
   If  you  specify  an  expression  containing  a  symbol, the command
   interpreter uses the symbol's value in evaluating the expression.

   The result of expression evaluation is either a character string  or
   an  integer  value.   The data type of a symbol is determined by the
   data type of the expression.   If  the  expression  evaluates  to  a
   string,  the  symbol  is assigned a string value.  If the expression
   evaluates to an integer, the symbol is assigned an integer value.

   For a summary of operators used in expressions, details  on  how  to
   specify  expressions,  and details on how expressions are evaluated,
   see the Open VMS Users Manual.

   DCL uses a buffer that is 1024 bytes  long  to  hold  an  assignment
   statement, and to evaluate the expression.  The length of the symbol
   name, the  expression,  and  the  expression's  calculations  cannot
   exceed 1024 bytes.

  [bit-position,size]

   Specifies that a binary overlay is to be  inserted  in  the  current
   32-bit  value  of  a symbol-name.  This type of assignment statement
   evaluates the current value of  the  symbol-name  and  replaces  the
   specified    number    of    bits    with    the   result   of   the
   replacement-expression.

   The bit-position is the location relative to  bit  0  at  which  the
   overlay  is  to  occur.  If the symbol that you are overlaying is an
   integer, then the bit position must be less than 32.  Also, the  sum
   of the bit position and the size must be less than or equal to 32.

   If the symbol you are overlaying is a string, then the bit  position
   must be less than 6152.  Because each character is represented using
   8 bits, you can begin an overlay at any character through the 768th.
   (The  768th  character  starts in bit position 6144.) The sum of the
   bit position and the size must be less than or equal to 6152.

   The size is the number of bits to be overlaid.   If  you  specify  a
   size that is greater than 32, then DCL reduces the size to 32.

   The square brackets are required notation;  no  spaces  are  allowed
   between  the  symbol  name and the left bracket.  Specify values for
   bit-position and size as integer expressions.

  replacement-expression

   Specifies the value that is used  to  overlay  the  symbol  you  are
   modifying.    Specify   the  replacement-expression  as  an  integer
   expression.

   If   the   symbol   you   are   modifying   is   an   integer,   the
   replacement-expression  defines  a  bit pattern which is overlaid on
   the value assigned to the symbol.  If the symbol you  are  modifying
   is  a  character  string,  the  result of the replacement-expression
   defines a bit pattern which is overlaid on the specified bits of the
   character string.  If the symbol you are modifying is undefined, the
   result of the replacement-expression is overlaid on a null string.