How do I use GPIO to write to 12-bit DAC?

Submitted by Viswanathan on Mon, 06/29/2009 - 20:36 

I need to send out 2 different signals of 12 bits each. Just for testing I have connected both outs to the same signal for testing.
I dont know how to setup uni-polar or bi-polar.
My signal will be from say -3 to +3. I expect -3 to give 0x0000 and +3 to be 0xFFF