Help! External Sort/Merge Algorithm Needed

External sort/merge algoritm used for sorting large data files that
exceed
the main memory of the computer.
In the sorting stage, the records are read from the input(unsorted) file
&
sorted into several paritions which are written into the output file.
Each parition is
sorted in order.
There are 3 possible methods used in the sorting stage: internal
sorting,
replacment selection & natural selection algorithm.
Can anybody help me in the algorithms of the 3 methods ?

Email: chris...@mbox3.singnet.com.sg