MOS Technology 6510
The MOS Technology 6510 is an 8-bit microprocessor designed by MOS Technology. It is a modified form of the very successful 6502. The 6510 is widely used in the Commodore 64 home computer and its variants. It is also used in the Seagate ST-251 MFM hard disk.
The primary change from the 6502 is the addition of an 8-bit general purpose I/O port, although only 6 I/O pins are available in the most common version of the 6510. In addition, the address bus can be made tri-state and the CPU can be halted cleanly.
Description
The 6510 and variants were based on the same core as the 6502, and are opcode compatible, including undocumented opcodes.The parallel port was provided by using several formerly unused pins, eliminating some, and re-arranging others. In the original 6502, pins 5, 35 and 36 were not connected. Pin 3, formerly the phase-1 clock out, was eliminated, as most roles did not require it. This left only CLKIN, moved to pin 1 from 37, and CLKOUT on its original pin 39. The SO pin, which was connected to the overflow flag in the processor status register, was eliminated as few applications made use of it and the new parallel port could provide similar functionality. The final pin to be removed was the VSS on pin 1, the original 6502 had it on both pin 1 and pin 21, on the opposite side of the chip, but only one was needed.
The pins were also re-arranged. The VSS on pin 1 became clock in, while the other pins on the right side all moved up to fill the space from the removed clock out on pin 3 and unused pin 5. This put address bus pins A0 to A13 on one side, instead of 0 through 11 on the 6502, removing two from the left side. On the left side, the SO and two unconnected pins were removed, while clock in moved to pin 1 and the two address pins to 19 and 20, leaving pins 29 through 24 to be available for the parallel port pins, P0 through P5.