CCS: changing resloution of ADC on F28335

Tool/software: Code Composer Studio

Hi I want to know how an I change the resolution of my ADC for example I want to change 12 bit to 8 bit , iam using f28335 microcontroller and iam using the following code on ADC interrupt 

interrupt void  adc_isr(void)
{
       PVvoltage = AdcRegs.ADCRESULT0 >>4; }