Hi Krasik,

There is the short description of the port4 access and an example. Look at www.winbond.com/tw/sheet/w78e52b.pdf for more details.

Another bit-addressable port P4 is also available and only 4 bits (P4<3:0>) can be used. This port
address is located at 0D8H with the same function as that of port P1, except the P4.3 and P4.2 are
alternative function pins. It can be used as general I/O pins or external interrupt input sources (INT2 ,
INT3 ).
Example:
P4 REG 0D8H
MOV P4, #0AH ; Output data "A" through P4.0-P4.3.
MOV A, P4 ; Read P4 status to Accumulator.
SETB P4.0 ; Set bit P4.0
CLR P4.1 ; Clear bit P4.1