First get a transparent latch. Connect ALE to the Latch Enable. Connect the Port 0 Data Lines to the latch inputs. connect the latch outputs to address lines A0 through A7 on the 28C256. Ground the Output Enable pin.
Connect the Port 2 Address lines from the processor to A8-A14 on the 28C256. If this is the only memory chip in the system you can ground its chip select. PSEN-bar to the 28C256 Output Enable.
Connect the ROM data lines D0-D7 back to port 0.
I assume you can handle the remainder of the pins.
Dont forget to ground the EA-bar pin on the processor.
Don't forget to write 0xFF to the P0 and P2 data latches to enable the alternate function.
Code? Any instructions you put in the 28C256 will be executed by the processor. If you want to read the contents of the the ROM then use the "movc" instruction.