Default values are values that are defined in the macro
    definition. They are used when no value for a formal argument
    is specified in the macro call.

    Default values are specified in the .MACRO directive as follows:

    formal-argument-name = default-value

    An example of a macro definition specifying default values
    follows:

    .MACRO  STORE   ARG1=12,ARG2=0,ARG3=1000
    .LONG   ARG1
    .WORD   ARG3
    .BYTE   ARG2
    .ENDM   STORE

    The following three examples show possible calls and expansions
    of the macro defined previously:

    STORE                   ; No arguments supplied
    .LONG   12
    .WORD   1000
    .BYTE   0

    STORE   ,5,X            ; Last two arguments supplied
    .LONG   12
    .WORD   X
    .BYTE   5

    STORE   1               ; First argument supplied
    .LONG   1
    .WORD   1000
    .BYTE   0