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(nasm.info)Section 4.3.4

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4.3.4. Default Macro Parameters
-------------------------------

   NASM also allows you to define a multi-line macro with a _range_ of
allowable parameter counts. If you do this, you can specify defaults for
omitted parameters. So, for example:

     %macro  die 0-1 "Painful program death has occurred."
     
             writefile 2,%1
             mov     ax,0x4c01
             int     0x21
     
     %endmacro

   This macro (which makes use of the `writefile' macro defined in
*Note Section 4.3.3::) can be called with an explicit error message,
which it will display on the error output stream before exiting, or it
can be called with no parameters, in which case it will use the default
error message supplied in the macro definition.

   In general, you supply a minimum and maximum number of parameters
for a macro of this type; the minimum number of parameters are then
required in the macro call, and then you provide defaults for the
optional ones. So if a macro definition began with the line

     %macro foobar 1-3 eax,[ebx+2]

   then it could be called with between one and three parameters, and
`%1' would always be taken from the macro call. `%2', if not specified
by the macro call, would default to `eax', and `%3' if not specified
would default to `[ebx+2]'.

   You may omit parameter defaults from the macro definition, in which
case the parameter default is taken to be blank. This can be useful for
macros which can take a variable number of parameters, since the `%0'
token (see *Note Section 4.3.5::) allows you to determine how many
parameters were really passed to the macro call.

   This defaulting mechanism can be combined with the greedy-parameter
mechanism; so the `die' macro above could be made more powerful, and
more useful, by changing the first line of the definition to

     %macro die 0-1+ "Painful program death has occurred.",13,10

   The maximum parameter count can be infinite, denoted by `*'. In this
case, of course, it is impossible to provide a _full_ set of default
parameters. Examples of this usage are shown in *Note Section 4.3.6::.