The 6509 was a version of MOS Technology's 6502 but which could address up to 1MB of RAM.

Whilst BASIC programs could still only make use of the first 64K of this, machine code programs could access all of it via bank switching, where different 64K sections of memory could be switched in and out on demand.

Whilst other processors (including the 6502 and even the 6507) could do bank switching via external hardware, the 6509 had the capability built in to the CPU.