MOS Technology 6509

The MOS Technology 6509, an enhanced version of the popular 6502 microprocessor, was capable of addressing up to 1 megabyte of RAM via bank switching. While numerous 6502-based processors could do bank switching, they did this via separate logic. The 6509 had this logic on-chip.
The 6509 had a reputation for being difficult to program, due to its bank switching scheme, and never fared well. It was used in the ill-fated Commodore CBM-II line of computers.