Its easy to interface DS12887, simple map it to port 0 as external memory .. here is the pin connections...
Controller<=>DS12887
P0[7:0]<=>AD[7:0]
ALE<=>AS
RD<=>DS
WR<=>R/W
You can connect CS to permanent ground, and, as your microcontroller uses INTEL bus timing, don't forget to connect MOT pin to ground .. see picture below ..
Also, it is handy to use IRQ (interrupt) connection between RTC and microcontroller and read seconds (or all time and date memory locations) on interrupt, otherwise you will have to read seconds register all the time ..
Regards,
IanP