The VIC , specifically known as the MOS Technology 6560 / 6561, is the integrated circuit chip responsible for generating video graphics and sound in the Commodore VIC-20home computer. It was originally designed for applications such as low costCRTterminals, 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. Before finally starting to use the VIC in the VIC-20, chip designer Robert Yannes fed features from the 6562 and 6564 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
Unlike many other video circuits of the era, it does not offer dynamic RAMrefresh capabilities. Thus the VIC-20 employed the more expensive static RAM chips. This is likely the reason why the machine was sold with just 5 KB of RAM. 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 memory address 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 light pen facility. The VIC preceded the much more advanced VIC-II, used by the VIC-20's successors, the C64 and C128. VIC-20s with expansion RAM have their video memory at $1000 and when it is not present, $1E00. User-defined character sets must be placed within the first 5k of system RAM. The default PETSCII character ROM is at $8000 and each character takes 8 bytes to store. Up to 128 characters may be used at any one time. While the PET had a backslash in its character set, this was replaced on the VIC-20 with a British pound sign. Programmable characters are the only way of creating graphics and animation on the VIC as the chip does not have sprites or an all-points-addressable bitmap mode. Of the 16 colors in the palette, eight may be used for the foreground and border while the others are limited to the background and auxiliary multicolors. The MSB of the color RAM is a flag used to indicate if that character is multicolor or high resolution. Due to the extreme blockiness of the former, most VIC-20 games use hires characters. The VIC does not support scrolling or raster interrupts like on the VIC-II, but the scanline counters could be polled for a specific point on the screen to produce raster effects. This feature was rarely used in games except for a few titles like Imagic's Demon Attack. Sound programming on the VIC is done by placing a frequency value in one of the four registers at $900A-$900D. The first three are square wave generators pitched half an octave from each other and the fourth is for white noise.
