The Commodore 128 (C128, CBM 128, C=128) home/personal computer was Commodore Business Machines's (CBM) last commercially released 8-bit machine. Introduced in January of 1985, it appeared three years after its predecessor, the bestselling C64. The C128 was a significantly expanded successor to the C64.
The C128 had three modes of operation: C128 Mode (native mode), which ran at 1 or 2 MHz with the 8502 CPU and had both 40- and 80-column text modes available; CP/M Mode, which used the Z80 second CPU in either 40- or 80-column text mode; and C64 Mode, which was very nearly 100% compatible with the earlier computer. None of these modes would have been possible as implemented on the C128 without the Z80 chip. The Z80 controls the bus on initial boot-up and checks to see if there are any C64/C128 cartridges present, and if the Commodore key (C64-mode selector) is active on boot-up. Based on what it finds, it will switch to the appropriate mode of operation.
C128 had the MOS 6581 SID (or, in the C128DCR, the MOS 8580 SID synthesizer chip which contained 3 voices (ADSR-controllable), Standard SID waveforms (triangle, sawtooth, variable pulse, noise, and certain combined modes), multi-mode filter and 3 ring modulators.
The C64's graphics and sound capabilities were generally considered excellent. The C128 also had twice the RAM of the C64, and a far higher proportion was available for BASIC programming, due to the new MMU bankswitching chip. This feature made it possible for BASIC program code to be stored separately from variables, greatly enhancing the machine's ability to handle complex programs. The C128's greater hardware capabilities, especially the increased RAM, screen display resolution, and serial bus speed, made it the preferred platform for running the GEOS graphical operating system.