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(nasm.info)Section 4.1.2

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4.1.2. Enhancing %define: `%xdefine'
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   To have a reference to an embedded single-line macro resolved at the
time that it is embedded, as opposed to when the calling macro is
expanded, you need a different mechanism to the one offered by
`%define'. The solution is to use `%xdefine', or it's case-insensitive
counterpart `%xidefine'.

   Suppose you have the following code:

     %define  isTrue  1
     %define  isFalse isTrue
     %define  isTrue  0
     
     val1:    db      isFalse
     
     %define  isTrue  1
     
     val2:    db      isFalse

   In this case, `val1' is equal to 0, and `val2' is equal to 1.  This
is because, when a single-line macro is defined using `%define', it is
expanded only when it is called. As `isFalse' expands to `isTrue', the
expansion will be the current value of `isTrue'.  The first time it is
called that is 0, and the second time it is 1.

   If you wanted `isFalse' to expand to the value assigned to the
embedded macro `isTrue' at the time that `isFalse' was defined, you
need to change the above code to use `%xdefine'.

     %xdefine isTrue  1
     %xdefine isFalse isTrue
     %xdefine isTrue  0
     
     val1:    db      isFalse
     
     %xdefine isTrue  1
     
     val2:    db      isFalse

   Now, each time that `isFalse' is called, it expands to 1, as that is
what the embedded macro `isTrue' expanded to at the time that `isFalse'
was defined.