View source for MOS Technology VIC

{{distinguish|text=its more advanced successor used in the C64 and C128, the [[MOS Technology VIC-II|VIC-II]]}}
[[Image:MOS 6561 VIC.jpg|thumb|MOS Technology VIC (PAL version 6561–101)]]
[[Image:MOS 6560 pinout.svg|thumb|right|220px|Pinout diagram of the 6560 version of the MOS VIC chip. This circuit was packaged in a standard 40-pin [[Dual in-line package|DIP]] casing.]]
<!-- Deleted image removed: [[Image:Yes VIC can.png|thumb|right|220px|MFLI [[Commodore VIC-20 demos|demo]]]] -->
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 [[VIC-20]] [[home computer]] from Commodore.  It was originally designed for applications such as low-cost [[cathode-ray tube|CRT]] [[computer terminal|terminals]], biomedical monitors, control system displays and [[arcade game|arcade]] or home [[video game console]]s.

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 Display Controller|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 [[Static random-access memory|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 [[Random-access memory|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 up to 248 × 232p/464i is possible on the [[NTSC]] machine and up to 256 × 280 is possible on the [[PAL]] machine<ref>From ftp://www.zimmers.net/pub/cbm/documents/chipdata/VIC-I.txt</ref>)
* 4 channel [[Programmable Sound Generator|sound system]] (3 square wave + "white" noise + global volume setting)
* on-chip [[Direct memory access|DMA]]
* two 8-bit [[analog-to-digital converter]]
* [[light pen]] support
Unlike many other video circuits of the era, it does not offer [[Dynamic random-access memory|dynamic RAM]] [[Memory refresh|refresh]] capabilities. Thus the VIC-20 employed the more expensive static RAM (SRAM) 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 register]]s, memory mapped to the [[memory address]] range $9000&ndash;$900F in the VIC-20 address space.  The on-chip A/D converters were used for dual [[paddle (game controller)|paddle]] position readings by the VIC-20, which also used the VIC's [[light pen]] facility.  The VIC preceded the much more advanced [[MOS Technology VIC-II|VIC-II]], used by the VIC-20's successors, the [[Commodore 64|C64]] and [[Commodore 128|C128]].

VIC-20s with expansion RAM have their video memory (550 bytes) at $1000 and when it is not present, $1E00. User-defined [[character set]]s 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 (and all subsequent Commodore machines) 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 (per the color RAM at $9400) 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 (they are turned off by writing a zero to them). The first three are square wave generators pitched half an octave from each other and the fourth is for white noise.

==Registers==

The VIC has 16 read/write registers listed below:

{| class="wikitable" style="text-align:center;"
|-
! Register
! Hexadecimal
! Bit 7
! Bit 6
! Bit 5
! Bit 4
! Bit 3
! Bit 2
! Bit 1
! Bit 0
|-
| 0
| 9000
| Interlace
|colspan="7" |Horizontal origin
|-
| 1
| 9001
|colspan="8" |Vertical origin
|-
| 2
| 9002
| VA9
|colspan="7" |Number of columns
|-
| 3
| 9003
| R0
|colspan="6" |Number of rows
| 8/16
|-
| 4
| 9004
|colspan="8" |Raster line (bits 8–1)
|-
| 5
| 9005
|colspan="4" |Video addr (bits 13–10)
|colspan="4" |Char addr (bits 13–10)
|-
| 6
| 9006
|colspan="7" |Light pen horizontal
|colspan="1" |1
|-
| 7
| 9007
|colspan="8" |Light pen vertical
|-
| 8
| 9008
|colspan="8" |Paddle X
|-
| 9
| 9009
|colspan="8" |Paddle Y
|-
| 10
| 900A
| enable
|colspan="7" |Oscillator 1 frequency
|-
| 11
| 900B
| enable
|colspan="7" |Oscillator 2 frequency
|-
| 12
| 900C
| enable
|colspan="7" |Oscillator 3 frequency
|-
| 13
| 900D
| enable
|colspan="7" |White noise frequency
|-
| 14
| 900E
|colspan="4" |Aux color
|colspan="4" |Volume
|-
| 15
| 900F
|colspan="4" |Background
|RV
|colspan="3" |Border
|}

== Color palette ==
The full palette of sixteen colors was generated based on variations of [[YPbPr]] signals:
{| class="wikitable sortable" style="border-style: none" border="1" cellpadding="5"
! style="width:152px"| Number — name || [[YPbPr|Y]] || [[YPbPr|Pb]] (rel.) || [[YPbPr|Pr]] (rel.)
|- style="color:white; background:#000;"
|| 0 — black || 0 || 0 || 0
|- style="color:black; background:#fff;"
|| 1 — white || 1 || 0 || 0
|- style="color:white; background:#782922;"
|| 2 — red || 0.25 || −0.3826834 || 0.9238795
|- style="color:black; background:#87d6dd;"
|| 3 — cyan || 0.75 || 0.3826834 || −0.9238795
|- style="color:white; background:#aa5fb6;"
|| 4 — purple || 0.5 || 0.7071068 || 0.7071068
|- style="color:white; background:#55a049;"
|| 5 — green || 0.5 || −0.7071068 || −0.7071068
|- style="color:white; background:#40318d;"
|| 6 — blue || 0.25 || 1 || 0
|- style="color:black; background:#bfce72;"
|| 7 — yellow || 0.75 || −1 || 0
|- style="color:white; background:#aa7449;"
|| 8 — orange || 0.5 || −0.7071068 || 0.7071068
|- style="color:black; background:#eab489;"
|| 9 — light orange || 0.75 || −0.7071068 || 0.7071068
|- style="color:white; background:#b86962;"
|| 10 — light red || 0.5 || −0.3826834 || 0.9238795
|- style="color:black; background:#c7ffff;"
|| 11 — light cyan || 1 || 0.3826834 || −0.9238795
|- style="color:black; background:#ea9ff6;"
|| 12 — light purple || 0.75 || 0.7071068 || 0.7071068
|- style="color:black; background:#94e089;"
|| 13 — light green || 0.75 || −0.7071068 || −0.7071068
|- style="color:white; background:#8071cc;"
|| 14 — light blue || 0.5 || 1 || 0
|- style="color:black; background:#ffffb2;"
|| 15 — light yellow || 1 || −1 || 0
|}
<small>Note: Y has 5 possible values (0, 0.25, 0.5, 0.75 and 1); Pb and Pr have 7 possible values (-0.9238795, −0.7071068, −0.3826834, 0, 0.3826834, 0.7071068, 0.9238795) </small>

== VIC IC list ==

*MOS Technology 6560 [[NTSC]]
*MOS Technology 6561E [[PAL]] Ceramic version, used in early VIC-20's
*MOS Technology 6561-101 [[PAL]]

==See also==
* [[Video Display Controller]]
* [[Motorola 6847]]
* [[List of home computers by video hardware]]
* [[VIC-II|MOS VIC-II]] as used in the [[Commodore 64]]

== References ==
* Bagnall, Brian (2005). ''On The Edge: The Spectacular Rise and Fall of Commodore''. Variant Press. {{ISBN|0-9738649-0-7}}.

;Specific
<references />

== External links ==
* [https://raw.githubusercontent.com/cbmeeks/VIC-20/master/6561.txt VIC Chip info from Rick Melick's VIC-20 Tribute Page]
* [http://www.zimmers.net/anonftp/pub/cbm/documents/chipdata/6560.zip MOS VIC datasheet (GIF format, zipped)]

{{MOS Video/Sound}}

[[Category:MOS Technology integrated circuits]]
[[Category:Graphics chips]]
[[Category:Sound chips]]
[[Category:VIC-20]]