MOS 6510 processors
Introduction: 1975 (?)
The MOS Technology 6510 is a microprocessor designed by MOS Technology, Inc., and is a direct successor of the very successful 6502.
The primary change from the 6502 was the addition of an 8-bit general purpose I/O port (only six I/O pins were available in the most common version). In addition, the address bus could be made tristate.
The 6510 was only widely used in the Commodore 64 home computer (and in significantly smaller numbers in the C64's portable version, the SX-64). In both the C64 and SX-64 the extra pins of the processor were used to control the computer's memory map, and in the C64 also for controlling the electric motor of the Datassette tape recorder. It was possible, by writing the correct bit pattern to the processor at address $01, to completely expose the full 64KB of RAM in the C64, leaving no ROM or I/O hardware exposed.
Source: Wikipedia, the free encyclopedia.