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  Syntax:   OPTION M510
 
            OPTION NOM510
 
  See also: /Zm
 
  Description:
 
     The M510 option sets all options to maximize compatibility with
     MASM 5.10. NOM510 is the default. OPTION M510 enables the
     following:
 
     ◄OLDMACROS►
     ◄OLDSTRUCTS►
     ◄DOTNAME►
     ◄CASEMAP:ALL►
     ◄EXPR16►
     ◄SETIF2:TRUE►
 
     The M510 option enables OPTION SCOPED if a language type is specified
     in a .MODEL statement. If the .MODEL directive does not specify
     a language type, OPTION NOSCOPED is used.
 
     Using OPTION M510 also changes the default for OFFSET to GROUP if a
     .MODEL directive is in effect and to SEGMENT otherwise.
 
     With OPTION M510, keywords and instructions that are not available
     in the current CPU mode (such as ENTER under .8086) are not treated as
     keywords. The USE32, FLAT, FAR32, and NEAR32 segment types and the
     80386/486 registers are not keywords with processor selection
     directives less than .386.
 
     In MASM 5.1, a large constant value that can fit only in the processor's
     default word (four bytes for .386 and .486, two bytes otherwise) is
     assigned a size attribute of the default word size. The value of the
     constant affects the number of bytes changed by the instruction.
 
     Without OPTION M510, the assembler never assigns a size automatically.
     You must state it explicitly with the PTR operator, as shown in the
     following example:
 
          ; Without OPTION M510
                  mov     [bx], WORD PTR 0100h
 
     MASM 5.1 allows a code label definition in a data definition statement
     if that statement does not also define a data label. MASM 6.1 also
     allows such definitions if OPTION M510 is enabled; otherwise it is
     illegal:
 
          ; Legal only with OPTION M510
                 MyCodeLabel:    DW      0
 
     By default, MASM 6.1 changes the way that expressions are evaluated.
     In MASM 5.1,
 
          var-2[bx]
 
     is parsed as
 
          (var-2)[bx]
 
     Without OPTION M510, you must rewrite the statement, since the assembler
     parses it as
 
          var-(2[bx])
 
     which generates an error.
 
     In compatibility mode, use the PTR operator to type-cast a constant to a
     structure type. This is most often done in data initializers to affect
     the CodeView information of the data label. Without OPTION M510, the
     assembler generates an error.
 
     MYSTRC  STRUC
            f1    DB      0
          MYSTRC  ENDS
          MyPtr   DW      MYSTRC PTR 0    ; Illegal without OPTION M510
 
     MASM 6.1 recursively expands text macros used as values, whereas
     MASM 5.1 simply replaces the text macro with its value. The following
     example illustrates the difference:
 
; With OPTION M510
                  tm1     EQU     <contains tm2>
                  tm2     EQU     <value>
                  tm3     CATSTR  tm1             ; == <contains tm2>
 
          ; Without OPTION M510
                  tm3     CATSTR  tm1             ; == <contains value>
                                    -♦-