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(nasm.info)Section 2.1.13

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2.1.13. The `-u' Option: Undefine a Macro
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   The `-u' option undefines a macro that would otherwise have been pre-
defined, either automatically or by a `-p' or `-d' option specified
earlier on the command lines.

   For example, the following command line:

     nasm myfile.asm -dFOO=100 -uFOO

   would result in `FOO' _not_ being a predefined macro in the program.
This is useful to override options specified at a different point in a
Makefile.

   For Makefile compatibility with many C compilers, this option can
also be specified as `-U'.