Signed multiplication

hi

Can anyone tell me how can i multiply two signed numbers in FPGA.
How the logic is really implemented..
ie., if i multiply two signed numbers are they multiplying the
positive number and the 2's complement (if the number is negative)
directly ..?
Or are they really changing the negative number to positive and do
normal multiplication and appends the sign accordingly..?
And is the positive number, and its 2's complement form same always...?