We want to place an external ROM on our dev board so that if our code exceeds flash limit we can revert to old fashioned crash & burn techniques without having to lay out a new dev board. So the question is: with EA tied low program code appears to start at 000000h. Can I simply place a contiguous 512k byte ROM there or do I have to map out the SFR locations etc?
Can I simply place a contiguous 512k byte ROM there Yes. You will not be able to access the external ROM through addresses 0x8000-0xFFFF because reads/writes to these addresses will access on-chip RAM, SFR and so on. Of course, you can access those memory locations in external ROM through other (mirrored) address areas, say 0x88000-0x8FFFF. - mike