MOS Technology VIC

MOS Technology VIC

The VIC (Video Interface Chip), specifically known as the MOS Technology 6560 (NTSC version) / 6561 (PAL version), is the integrated circuit chip responsible for generating video graphics and sound in the Commodore VIC-20 home computer. It was originally designed for applications such as low cost CRT terminals, biomedical monitors, control system displays and arcade or home video game consoles.

The chip was designed by Al Charpentier in 1977 but Commodore could not find a market for the chip. In 1979 MOS Technology began work on a video chip named MOS Technology 6564 intended for the TOI computer and had also made some work on another chip, MOS 6562 intended for a color version of the Commodore PET. Both of these chips failed due to memory timing constraints (both required very fast and thus expensive SRAM, making them unsuitable for mass production). Before finally starting to use the VIC in the VIC-20, chip designer Robert Yannes fed features from the 6562 (a better sound generator) and 6564 (more colors) back to the 6560, so before beginning mass production for the VIC-20 it had been thoroughly revised.

Its features include:

• 16 kB address space for screen, character and color memory (only 5 kB points to RAM on the VIC-20 without a hardware modification)
• 16 colors (the upper 8 can only be used in the global background and auxiliary colors)
• two selectable character sizes (8×8 or 8×16 bits; the pixel width is 1 bit for "hires" characters and 2 bits for "multicolor" characters)
• maximum video resolution depends on the television system (176 × 184 is the standard for the VIC-20 firmware, although at least 224 × 256 is possible on the PAL machine)
• 4 channel sound system (3 square wave + "white" noise + global volume setting)
• on-chip DMA
• two 8-bit A/D converters
• light pen support

Unlike many other video circuits of the age, it does not offer dynamic RAM refresh capabilities. Thus the VIC-20 employed the more expensive static RAM chips; but with the low RAM capacity of that machine, just 5 KB in the main box, that didn't matter too much. Memory expansions for the VIC-20 either used SRAM as well or implemented their own refresh circuit.

The VIC was programmed by manipulating its 16 control registers, memory mapped to the range $9000–$900F in the VIC-20 address space. The on-chip A/D converters were used for dual paddle position readings by the VIC-20, which also used the VIC's lightpen facility. The VIC preceded the much more advanced VIC-II, used by the VIC-20's successors, the C64 and C128.