The add-symbol-file command comment in the manual is confusing

[Stephen Smith <ischis2 at home dot com>]

The add-symbol-file symbol file command has the following prose in the manual:

     add-symbol-file filename address [ -readnow ] [ -mapped ]
     The add-symbol-file command reads additional symbol table information from the file filename. You would use this
command when filename has been dynamically
     loaded (by some other means) into the program that is running. address should be the memory address at which the
file has been loaded; GDB cannot figure this out for itself.
     You can specify address as an expression. The symbol table of the file filename is added to the symbol table
originally read with the symbol-file command. You can use
     the add-symbol-file command any number of times; the new symbol data thus read keeps adding to the old. To discard
all old symbol data instead, use the
     symbol-file command. add-symbol-file does not repeat if you press RET after using it. You can use the `-mapped' and
`-readnow' options just as with the
     symbol-file command, to change how GDB manages the symbol table information for filename.

Several questions
1)  What is the difference between useing and not using the -mapped command?   Maybe I should ask first what is the use
of the mapped switch?

2)  The quote says that "You can specify address as an expression".  How would you do that?  And just a curriosity, how
would you figure out the address to use?