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(nasm.info)Section 4.10.2

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4.10.2. `%!'`<env>': Read an environment variable.
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   The `%!<env>' directive makes it possible to read the value of an
environment variable at assembly time. This could, for example, be used
to store the contents of an environment variable into a string, which
could be used at some other point in your code.

   For example, suppose that you have an environment variable `FOO', and
you want the contents of `FOO' to be embedded in your program. You
could do that as follows:

     %define FOO    %!FOO
     %define quote   '
     
     tmpstr  db      quote FOO quote

   At the time of writing, this will generate an "unterminated string"
warning at the time of defining "quote", and it will add a space before
and after the string that is read in. I was unable to find a simple
workaround (although a workaround can be created using a multi-line
macro), so I believe that you will need to either learn how to create
more complex macros, or allow for the extra spaces if you make use of
this feature in that way.