MOS Technology 8502

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The MOS Technology 8502 was MOS's microprocessor used as one of the two¹ CPUs in the Commodore 128 home/personal computer. The 8502 was compatible with the C64's 6510, which was itself a compatible successor to the venerable 6502 (used in the VIC-20 among many other micros).

The 8502 could run at two clock rates; 1.02 and 2.04 MHz, i.e. the same and twice the speed of the C64's 6510. The double speed was used in the native C128 mode only, and then for applications where the 80-column RGBI-monitor output was needed (due to the cycle-stealing technique of the C128's 40-column VIC-II display chip, which allowed only ~1 MHz operation of the CPU, 40-column output was suspended when running the CPU at double speed). However, for some very quick computations (such as e.g., small "number crunching" tasks), the double speed could be employed with only a 40-column monitor present, since the display-turnoff period could then be made sufficiently short.

( ¹ The other CPU was the Zilog Z80, for use in the C128's CP/M mode )