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''This entry is copied from english Wikipedia''

Commodore, specifically two designers; Fred Bowen and Bil Herd, were determined to avoid the problems of the Plus/4, making sure that the eventual successors to the C64—the Commodore 128 and 128D computers (1985)—were as good as, and fully compatible with the original, as well as offering a host of improvements (such as a structured BASIC with graphics and sound commands, 80-column display capability, and full CP/M compatibility). The decision to make the Commodore 128 compatible with the C64 was made quietly by Bowen and Herd , software and hardware designers respectively, without the knowledge or approval by the management in the post Jack Tramiel era. The designers were careful not to reveal their decision until the project was too far along to be challenged or changed and still make the impending Consumer Electronics Show (CES) show in Las Vegas. Upon learning that the C128 was designed to be compatible with the C64, Commodore’s marketing department independently announced that the C128 would be 100% compatible with the C64, thereby raising the bar for C64 support.

In 1986, Commodore released the Commodore 64C (C64C) computer, which was functionally identical to the original, but whose exterior design was remodeled in the sleeker style of the Commodore 128 and other contemporary design trends. The modifications to the C64 line were more than skin deep in the C64C with new versions of the SID, VIC and I/O chips being deployed—with the core voltage reduced from 12 V to 9 V. In the United States, the C64C was often bundled with the third-party GEOS GUI-based operating system. The Commodore 1541 disk drive received a matching face-lift resulting in the 1541c. Later a smaller, sleeker 1541-II model was introduced along with the 800KB 3.5-inch capable 1581.