.dat file of large space as function argument?

I have a data file upto 1gb i want to process it as a function argument and store the operated result which might be greater in size , please help me

[per isakson]

What is a DAT file?

A DAT file is a generic data file created by a specific
application. It may contain data in binary or text format 
(text-based DAT files can be viewed in a text editor). DAT 
files are typically accessed only by the application that 
created them.

What is the exact format of your .dat file?