Editing Commodore 128 (section)

== Architecture and Processor ==

The Commodore 128 features a revolutionary dual-processor architecture that was unprecedented in the home computer market. Instead of the single 6510 microprocessor of the C64, the C128 incorporates a two-CPU design<ref>{{cite web |url=https://en.wikipedia.org/wiki/Commodore_128 |title=Commodore 128 - Wikipedia |publisher=Wikipedia |date=2025-06-06 |access-date=2025-06-09}}</ref>.

The primary processor is the then-new 8502 microprocessor, which is completely backward-compatible with the C64's 6510, but can run at double the speed if desired<ref>{{cite web |url=https://en.wikipedia.org/wiki/Commodore_128 |title=Commodore 128 - Wikipedia |publisher=Wikipedia |date=2025-06-06 |access-date=2025-06-09}}</ref>. The 8502 operates at about 1.02 or 2.04 MHz memory clock<ref>{{cite web |url=https://www.c64-wiki.com/wiki/Commodore_128 |title=Commodore 128 - C64-Wiki |publisher=C64-Wiki |date=2019-10-27 |access-date=2025-06-09}}</ref>, providing significantly improved performance over the C64's 1 MHz operation.

The secondary processor is a Zilog Z80 CPU which allows the C128 to run CP/M, as an alternative to the usual Commodore BASIC environment<ref>{{cite web |url=https://en.wikipedia.org/wiki/Commodore_128 |title=Commodore 128 - Wikipedia |publisher=Wikipedia |date=2025-06-06 |access-date=2025-06-09}}</ref>. The Z80 runs at 4 MHz (running at an effective 2 MHz because of wait states to allow the VIC-II video chip access to the system bus)<ref>{{cite web |url=https://en.wikipedia.org/wiki/Commodore_128 |title=Commodore 128 - Wikipedia |publisher=Wikipedia |date=2025-06-06 |access-date=2025-06-09}}</ref>.

Herd gave the reason for the 128's inclusion of a Z80 processor as ensuring this "100% compatibility" claim, since supporting the C64's Z80 cartridge would have meant the C128 supplying additional power to the cartridge port<ref>{{cite web |url=https://en.wikipedia.org/wiki/Commodore_128 |title=Commodore 128 - Wikipedia |publisher=Wikipedia |date=2025-06-06 |access-date=2025-06-09}}</ref>.

The system includes a MOS Technology 8722 Memory Management Unit that controls 8502/Z80 processor selection and ROM/RAM banking<ref>{{cite web |url=https://en.wikipedia.org/wiki/Commodore_128 |title=Commodore 128 - Wikipedia |publisher=Wikipedia |date=2025-06-06 |access-date=2025-06-09}}</ref>.