The 89C420 2kB internal ROM can load external RAM (0h – FFFFh).
It solve your NV-SRAM usage and loading.
For other type memory it will be probably more difficult by my opinion,
because the ROM loader can load internal flash memory only.

If you haven't "DS89C420 User's Guide", look at :
http://dbserv.maxim-ic.com/quick_vie...k=2963

Vaclav