Vref is just a referance voltage… is i make Vref as 3 volt then my adc will calibarated b/w 0 to 3 volt…
you canuse any channel… just go to the links:
how to use adc 0808…
http://www.engineersgarage.com/microcontroller/8051projects/interface-LDR-ADC0808-interrupt-clock-AT89C51-circuit
might be helpful for you since you wanted top design asm code…
http://www.engineersgarage.com/forums/8051/c-code-assembly-code