Internal I/O Port
The 6510 has an 8 bit I/O port built in. Only the lower 6 bits (P0…P5) are exposed on the 6510 (as used on the Commodore 64) whilst all 8 bits are exposed on the 6510-1 and 6510-2 processors by sacrificing the RDY & NMI signals.
The Data Direction Register is at address 0x0000 whilst the data port is at address 0x0001.