The 6560/6561 Video Interface Chip, usually called VIC, puts video and sound into the VIC-20. It reads a screen matrix of character numbers, finds the patterns those numbers name, and combines them with colour information. Changing a number moves a different shape into a cell; changing the pattern can alter every cell that uses it.
MOS Technology’s February 1980 data sheet describes the 6560 for NTSC timing and the 6561 for PAL. Their video timing is a real programming difference. A routine tied to raster position or a particular delay must account for the version in the machine.
Three kinds of display data
The chip’s own description separates character pointers, display characters and colour pointers:
| Data | Meaning |
|---|---|
| Screen matrix | Which character occupies each cell |
| Character patterns | The rows of dots that form a shape |
| Colour matrix | Colour information and the character’s colour mode |
The familiar 22-column, 23-row display contains 506 cells. With 8-by-8 character patterns, that is 176 by 184 dots. It is a normal arrangement, not the only layout the chip’s registers can describe. The data sheet itself uses it to explain how an index selects a pattern in RAM or ROM. (Data sheet, theory of operation; VIC-20 Programmer’s Reference Guide, VIC chip.)
That reuse is economical. A floor tile can appear dozens of times while its pattern is stored once. Giving many cells distinct patterns consumes more pattern memory, which matters on a small machine. The character graphics entry follows the same idea across other computers.
Sound shares the chip
VIC also contains three programmable tone generators and a noise generator. A common volume control sets their output level. Music software must organise notes around those available generators and keep updating the registers when the tune changes. The picture and the sound are different functions of the same chip, rather than separate graphics and audio processors.
The part also provides paddle-conversion and light-pen facilities. Their presence in the data sheet describes available hardware signals; a particular program still needs the VIC-20’s wiring and memory map to use them.
Before VIC-II
The name invites a comparison with the Commodore 64’s VIC-II. Both fetch display data, but the original VIC does not supply VIC-II’s hardware sprite system. A VIC-20 object drawn through character patterns has to fit that representation, including the work needed to erase or redraw it.
Likewise, code written around the C64’s raster-interrupt registers cannot be moved across unchanged. The shared family name is a starting point for comparison, not a register-level compatibility promise.
Sources
- MOS Technology, 6560/6561 Video Interface Chip, February 1980, especially theory of operation and register descriptions. Original scan archive.
- A. Finkel, N. Harris, P. Higginbottom and M. Tomczyk, VIC-20 Programmer’s Reference Guide, Commodore Business Machines and Howard W. Sams, first edition: chip operation in the actual computer. Manual collection.