"how i can do comparsion between new and previous at the start of program..."

Easy, you read a value, then read again, and compare the two, then you can compare the values together.


ORG 00h
MOV R0,P1
NO NEW DATA:
MOV A,P1
CJNE A,r0,NEW DATA
jmp NO NEW DATA

NEW DATA:
mov R0,A ; store the new value
; do something with tha data here
..
jmp NO NEW DATA



It's probably better to use a memory location rather than R0, but you get the idea...

Chris.