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

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

+/-3V to 0-4096 output is done like this

VisSim supports contiguous bit access on GPIO read or write, so just setup the digital out block to use the bits you want, and create a transform to map from +/-3 to 0 to 0xFFF. The key to the transform is to scale the +-3 input to 0-8 in a fx4.16 scaling, then using the VisSim fixed-point shift block to give a right adjusted 12-bit result. I have attached a simulation and an embedded diagram to illustrate this.