Assuming that you are talking about the
ADuC812 from Analog Devices:


MOV DACCON,#02DH ; DAC0 and DAC1 On
;in 12 bit Mode
;(Range select 0-> 5V)
;SYNC OFF, CLR DAC1 AND DAC0
MOV DAC0H,#0FH ;
MOV DAC0L,#0FFH ; Write to DAC0