It is totaly possible to implement as many code memory you want in the 8051, using some available port pins, or implementing a 74HCT573 8 bits latch (for a max of 16MB) or a 74HCT75 4 bits latch (for a max of 1MB eprom).
preview:
http://www.ustr.net/electronics/512k.shtml
2/19/2003
