To compare two 16-bit values, use SUBB instruction. This method can be expanded to as many bytes as you wish (32-bits, 64 bits etc.).

Here is an example. We'll check if DPTR is greater than 083ch.
SETB C
MOV A,DPL
SUBB A,#03CH
MOV A,DPH
SUBB A,#08H
JC NOT_GREATER ;jump if not greater


If Carry bit is set, DPTR is not greater than 083ch.

Here is an example how to check, if DPTR equals 12f5h:

MOV A,DPL
CJNE A,#03CH,NOT_EQUAL ;jump if not equal
MOV A,DPH
CJNE A,#08H,NOT_EQUAL ;jump if not equal

Now we will check if DPTR is less than 16-bit value ramptr (HI-byte first)

CLR C
MOV A,DPL
SUBB A,ramptr+01H
MOV A,DPH
SUBB A,ramptr
JNC NOT_LESS ;jump if not less