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                              MPL 2.1
                              -------
  
  
      MPL 2.1 differs from MPL 2.0 in the following ways:
  
 1.  Unsigned binary multiplication and division are allowed.
  
     Example:
  
       DCL B2 BIN(2), B1A INIT(100), B1B INIT(100)
       B2 = B1A*B1B
       /* B2 becomes 10000 */
  
 2.  Integer, decimal, hexadecimal, string, and address constants may be
     given symbolic names called "compile-time constants".  The process is
     called "declaring a compile-time constant" and is done as follows:
  
          DCL name CONST(constant)
       or DCL name CONSTANT(constant)
  
     Wherever a name so declared appears in the program after its
     declaration, the effect is exactly the same as if the associated
     constant had appeared in its place.
  
     Compile-time constants may not be declared within structures.
  
     Example:
  
       DCL B1, D5 DEC(5), ZERO CONST(0)
       B1 = ZERO     /* assigns BIN(1) 0 */
       D5 = ZERO     /* assigns DEC(5) 0 */
  
 3.  Variables may be declared to be in a labelled common block by
     labelling their DCL statement with the name of the block.  For
     example, to declare SAM to be a BIN(2) variable in a BSCT labelled
     common block BBL1, the statement
  
       BBL1:  DCL BSCT SAM BIN(2)
  
     may be used.
  
     Variables declared in labelled common blocks may not have the
     EXTERNAL or CSCT attributes.
  
 4.  Local procedures may be defined.  (A local procedure is one which
     may be called only from procedures within the same compilation.)  A
     procedure is defined to be local by nesting it within another
     procedure.  For example, in
  
       P:PROC OPTIONS(MAIN)
         DCL I
         .
         .
         .
         I = 1
         Q:PROC
           .
           .
           .
           RETURN
           END
         I = 2
         .
         .
         .
         END
  
     Q is a local procedure.
  
     Control flows around local procedures.  This means, for example, that
     the next statement to be executed after "I = 1" above will be "I = 2".
  
 5.  The following synonyms are provided:
  
       = for EQ
       # or <> for NE
       <= for LE
       >= for GE
       EXT for EXTERNAL
       OPTS for OPTIONS
  
 6.  Variables declared in BSCT with the EXTERNAL attribute will be
     addressed with single-byte addressing.
  
 7.  Declaring a structure with the EXTERNAL or GLOBAL attributes results
     in only the (level-1) name of the structure being known globally.  The
     level-n components of the structure (n > 1) are known only within the
     compilation.
  
     Example:
  
                  (separate compilations)
       DCL 1 S GLOBAL,           DCL 1 S EXTERNAL,
             2 B1A,                    2 B2 BIN(2)
             2 B1B
  
 8.  When a variable is declared with the "DEFINED variable" attribute, its
     size (the number of bytes required to store its value) may be less
     than or equal to the size of the variable to which it is DEFINED.
  
     Example:
  
       DCL 1 S,
             2 BIT4A BIT(4),
             2 BIT4B BIT(4),
             2 F              /* S is 2 bytes long */
       DCL SB DEF S           /* BIN(1) version of the "BIT" byte */
  
 9.  Names longer than six characters are truncated to six characters
     before being used in the label fields of statements in the AI file.
  
 10. Output format changes:
  
     a.  Source listings include a page heading and an error count.
  
     b.  Embedded assembly language statements, like all others, are
         included as comments in the AI file if the S option is specified.
  
     c.  The first record in the AI file has the form
  
           * MADE BY MPL revisionnumber
  
 11. Error message changes:
  
     511:  Unsigned variable declared with an INITIAL attribute containing
           a constant preceded by a minus sign.
  
     536:  More than 3 parameters specified in CALL p<...>, GIVING <...>,
           p:PROC<...>, or RETURN<...>.
  
     537:  a.  Decimal constant specified in CALL p(...).
  
           b.  Two-byte hexadecimal constant used in CHAR(1) context.
  
           c.  Integer or decimal constant with too many integer-part
               digits used in DEC(m, n) or SIGNED DEC(m, n) context.
  
           d.  Integer or decimal constant with too many digits used in
               CHAR(m) context.
  
           e.  Integer or decimal constant used in CHAR(m) context, m > 30.
  
           f.  Character-string constant with too many characters for its
               context used (was error 511).
  
     568:  Compile-time constant declared within structure.
  
     569:  One variable has been DEFINED to another. The size of the first
           exceeds the size of the second.
  
     570:  "DEFINED variable" attributes are nested.
  
     594:  Variable declared in labelled common block has EXTERNAL or CSCT
           attributes.
  
     596:  (No longer issued.)