word bits?

[Colin]

Hi there, i'm having a few problems shifting a word about. i need to have the first 4 bits equal to 3 and the following 12bits to be an offset

e.g.

4bits = 3
12bits = 2000

i'm not sure if i am doing it correctly. doesnt seem to work.

diff = a word value such as 2000

output := (Diff and $FFF) or (3 shl 12);

any help will be appreciated