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(nasm.info)Section 4.3.8

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4.3.8. Condition Codes as Macro Parameters
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   NASM can give special treatment to a macro parameter which contains a
condition code. For a start, you can refer to the macro parameter `%1'
by means of the alternative syntax `%+1', which informs NASM that this
macro parameter is supposed to contain a condition code, and will cause
the preprocessor to report an error message if the macro is called with
a parameter which is _not_ a valid condition code.

   Far more usefully, though, you can refer to the macro parameter by
means of `%-1', which NASM will expand as the _inverse_ condition code.
So the `retz' macro defined in *Note Section 4.3.2:: can be replaced by
a general conditional-return macro like this:

     %macro  retc 1
     
             j%-1    %%skip
             ret
       %%skip:
     
     %endmacro

   This macro can now be invoked using calls like `retc ne', which will
cause the conditional-jump instruction in the macro expansion to come
out as `JE', or `retc po' which will make the jump a `JPE'.

   The `%+1' macro-parameter reference is quite happy to interpret the
arguments `CXZ' and `ECXZ' as valid condition codes; however, `%-1'
will report an error if passed either of these, because no inverse
condition code exists.