Vim's yank

[Stevenisme]

Hi all,
I would like to know how to use the yank with register.
If I have two texts in defferent directory. ( a.txt in /home/me/Desktop b.txt in /home/)
I want to copy a part of a.txt to b.txt. however it is not a few lines( more than 100 )
I have heard that I can yank into a register but it seem that it cannot paste it in defferent directory. So may someone give me some idea to do copy and past?

[Venzent]

Open both files in "split-mode", go into Visual mode by pressing the "v", select the lines you wanne yank, press "y", press "ctrl-w" twice to switch to the second file, place cursor on right position and press "p".

[Stevenisme]

Yes, I recently have read the split-mode ... ... However ... totally forgot.....
Thx Venzent. I got the idea how to do so now.
But I would like to know how clear the content of the register?