Commodore 128

The Commodore 128 (also known as the C128, CBM 128 or the C=128) was the last commercially released 8-bit home computer released by Commodore Business Machines (CBM). Appearing in 1985, three years after its predecessor, the bestselling C64, the C128 was significantly expanded, featuring a redesigned case, a keyboard with a numeric keypad, 128 kB of RAM (externally expandable to 640 kB) and an 80-column RGB monitor output (driven by the 8563 VDC chip with 16 kB dedicated video RAM).

The C128 also featured two CPUs: the primary CPU (the 8502) could run at a maximum 2 MHz clock rate, whilst the secondary processor consisted of a Zilog Z80 that was used for for mode-selection of the computer upon boot-up and ensuring CP/M compatibility. However, the machine was not a multiprocessing system as the two processors could not run concurrently.

The Commodore 128 had three modes of operation: C128 Mode (native mode), CP/M Mode and C64 Mode. The native C128 mode ran at 1 or 2 MHz with the primary CPU and had both 40- and 80-column text modes available, whilst the CP/M mode used the secondary CPU in either 40- or 80-column text mode. The C64 mode was almost 100% compatible with the earlier Commodore 64 computer and allowed users to play C64 cartridges and programs. The Z80 chip was essential for all these modes as it controlled the bus on initial boot-up and checked to see if there was a CP/M boot disk, if there were any C64 or C128 cartridges present, and if the Commodore key (C64-mode selector) was active on boot-up. Based on what it found, the chip then switched to the appropriate mode of operation.

The machine had three disk drives (the 1570, 1571 and 3.5 inch 1581), which gave faster transfer speeds and allowed for more complex drive data arrangements; a feature not available to PC users. The disk drives also had more on-board RAM than the C64, and a far higher proportion was available for BASIC programming, which made it possible for BASIC program code to be stored separately from variables. This greatly enhanced the C128's ability to handle complex programs.

In 1986, an updated version of the system was released: the Commodore 128D, which featured a detached keyboard and a 1571 disk drive in the same box as the main system unit. This provided a more stylish, professional looking unit, much like that of a desktop PC.

Although the C128 sold a total number of 4 million units between 1985 and 1989, it had limited popularity compared to that of its predecessor, the C64. The unit was discontinued in 1989.

Specifications

CPUs

• MOS Technology 8502 at 1 or 2 MHz
• Zilog Z80 at 4 MHz (effectively running at 2 MHz due to stopping half the time to allow VIC-II video chip access to system bus)

RAM

• 128 kiB system RAM
• 2 kiB 4-bit dedicated colour RAM (for the VIC-II E)
• 16 kiB or 64 kiB dedicated video RAM (for the VDC)
• up to 512 kiB REU expansion RAM

ROM

• 72 kiB (28 kiB BASIC 7.0, 4 kiB MLM, 8 kiB C128 KERNAL, 4 kiB ...