As others have already said, this is not something to do in 'C'.

'C' compilers implement multiple-word arithmetic in assembler, and call these as required - you would need to do the same.

Take a look at the following threads, in which Michael Karas recently gave a description - with example code - of multiple byte arithmetic:
http://www.8052.com/forum/read.phtml?id=77267
http://www.8052.com/forum/read.phtml?id=77273
You should be able to work it from there.

Also try clicking the 'Code Library' button on the left-hand side of your screen.