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(nasm.info)Section 5.5

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5.5. `GLOBAL': Exporting Symbols to Other Modules
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   `GLOBAL' is the other end of `EXTERN': if one module declares a
symbol as `EXTERN' and refers to it, then in order to prevent linker
errors, some other module must actually _define_ the symbol and declare
it as `GLOBAL'. Some assemblers use the name `PUBLIC' for this purpose.

   The `GLOBAL' directive applying to a symbol must appear _before_ the
definition of the symbol.

   `GLOBAL' uses the same syntax as `EXTERN', except that it must refer
to symbols which _are_ defined in the same module as the `GLOBAL'
directive. For example:

     global _main
     _main:
             ; some code

   `GLOBAL', like `EXTERN', allows object formats to define private
extensions by means of a colon. The `elf' object format, for example,
lets you specify whether global data items are functions or data:

     global  hashlookup:function, hashtable:data

   Like `EXTERN', the primitive form of `GLOBAL' differs from the
user-level form only in that it can take only one argument at a time.