again:
mov p1,#00h
mov p1,#ffh
jmp again

Do you have a VERY VERY fast osciloscope? Or do you have a time delay inbetween the two instructions.
something like:

again:
mov p1,#00h
mov R0,#0FFh
djnz R0,$;stay here while counting down
mov p1,#0FFh
mov R0,#0FFh
djnz R0,$;stay here while counting down
jmp again

Also note the 0 in front of mov p1,#0FFh
Always use a zero if a hex value starts
with a letter!