if a particular chip ran successfully at 2 MHz in testing it was labeled B, otherwise A. With the move to the HMOS process, most of the 8502s were capable of running at 2 MHz, the equivalent of the 6502B. Changing the running speed of a 6502-based processor is as simple as changing the input clock signal, which meant the 8502 could easily switch between 2 MHz and the 6510's 1 MHz. When the clock runs at double-speed, it faces the problem that there is not enough time for the VIC to access memory during the free half-cycles. For this reason, the 8502 could only run at double-speed full-time when being used with the 80-column VDC in the C128, which had separate memory for the display that was not being directly accessed by the CPU. When running a VIC display mode, the two chips began to share access as was the case in the C64, and this meant the CPU had to return to its normal ~1 MHz speed. Programs could disable the screen during CPU-intensive calculations to allow the CPU to run at its faster speed. A smaller speed gain, about 35%, was also possible while keeping the VIC display active by switching the CPU to 2 MHz only while the VIC is drawing the empty border along the top and bottom of the screen, since the VIC performs no RAM access during that time. The pinout is slightly different than the 6510. The 8502 has an extra I/O-pin (the built-in I/O port mapped to addresses 0 and 1 is extended from 6 to 7 bits) and lacks the ϕ2-pin that the 6510 had. The 8502 family also includes the MOS 7501, 8500 and 8501. Notes References Further reading 65xx microprocessors MOS Technology microprocessors Commodore 64 8-bit microprocessors