MOS Technology VIC

The die

The 6560 Video Interface Chip (VIC) is the namesake of the VIC-20. This chip handles the graphics and sound, interacts with light gun/pen, and converts analog paddle signals to a digital value.

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.

History of development

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.

Features

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)
• 4 channel sound system (3 square wave + "white" noise + global volume setting)
• two 8-bit A/D converters
• light pen support
• on-chip DMA and address generation

The 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)

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.

Versions

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

Comparison of PAL and NTSC versions

Chip	6560-101	6561-101
System	NTSC-M	PAL-B
Cycles per line	65	71
Lines per frame	261	312
Lines per frame interlaced	525	not available
Crystal	14318181 Hz	4433618 Hz
Bus clock	crystal/14	crystal/4
Screen width	210	233
Screen height	233	284

Memory access limits

The VIC cannot access memory connected using the Cartridge port, only RAM in the ranges $0000-03FF and $1000-$1FFF, as well as the character ROM at $8000-8FFF, can be read.[1]

Memory Map

The default register values are stored in ROM at address $EDE4 to $EDF3.

Location		Bit function	Default value		Function
hex	dec		NTSC	PAL
$9000	36864	ABBBBBBB	$05	$0C	Interlace mode / Screen origin (horizontal)
$9001	36865	CCCCCCCC	$19	$26	Screen origin (vertical)
$9002	36866	HDDDDDDD	$16		Screen memory location / Number of columns
$9003	36867	GEEEEEEF	$2E		Raster value (lowest bit)/ Number of rows / Double character size
$9004	36868	GGGGGGGG	variable		Raster value
$9005	36869	HHHHIIII	$F0		Screen memory location / Character memory location
$9006	36870	JJJJJJJJ	$00		Light pen position (horizontal)
$9007	36871	KKKKKKKK	$00		Light pen position (vertical)
$9008	36872	LLLLLLLL	$FF		Paddle 1
$9009	36873	MMMMMMMM	$FF		Paddle 2
$900A	36874	NRRRRRRR	$00		Bass sound switch & frequency
$900B	36875	OSSSSSSS	$00		Alto sound switch & frequency
$900C	36876	PTTTTTTT	$00		Soprano sound switch & frequency
$900D	36877	QUUUUUUU	$00		Noise switch & frequency
$900E	36878	WWWWVVVV	$00		Auxiliary color / Composite sound volume
$900F	36879	XXXXYZZZ	$1B		Screen color / Reverse mode / Border color

A : Interlace mode

In this mode, the videochip will draw 525 interlaced lines instead of the 261 non-interlaced lines in the normal mode. This bit has no effect on the PAL version.[2]

To turn off in BASIC:

   POKE 36864, PEEK(36864) AND 127

To turn off in ML:

   lda $9000
   and #%01111111
   sta $9000

To turn on in BASIC:

   POKE 36864, PEEK(36864) OR 128

To turn on in ML:

   lda $9000
   ora #%10000000
   sta $9000

B : Horizontal position

Moves the screen along the X axis half characters (4 hires pixels).

C : Vertical position

Moves the screen along the Y axis two dots per increment. The VIC can be tricked into single-line offsets by setting the horizontal position to 0 in the first raster line.

D: Number of columns

The number of character columns, normally set to 22. Values greater than 31 will be trimmed. 27 columns fill a NTSC screen, 30 columns fill a PAL screen. However, to keep all columns visible on any CRT you shouldn't use more than 24 columns on NTSC systems and no more than 26 columns on PAL systems. The total number of characters displayed may exceed 512 if enough internal RAM is available.

E: Number of rows

On NTSC systems the maximum number of rows is 28, on PAL it is 32. Greater values probably won't work on every CRT. The total number of characters displayed may exceed 512 if enough internal RAM is available.

F: Double character size

When set this will double the character size to 16 lines. This can be used to display bitmapped screens where every pixel can be modified.

G: Raster value

This holds the number of the line that is currently drawn to the screen. It's also used to synchronize the light pen. The single bit in register $9003/36867 is the lowest bit of the raster value.

H: Screen memory location

Determines where the character index values are stored in memory. The single bit in $9002/36869 is bit 9 of the screen address. The other bits are bits 10-13 of the screen address. Keep in mind that bit 9 is also bit 9 of the color memory location. If it's set to 0 color memory starts at $9400/37888, otherwise it starts at $9600/38400.

To the CPU, bit 13 is bit 15 but inverted.

Values for bit 9 set to 1

These are the possible screen locations for unexpanded VICs or VICs with a 3K memory expansion (and color memory at $9600/38400), default in bold:

Value (bits 10-13)	Location		Contents
	hex	dec
0	$8200	33280	character ROM
1	$8600	34304	character ROM
2	$8A00	35328	character ROM
3	$8E00	36352	character ROM
4	$9200	37376	I/O block 1 (VIC and VIAs) / alternative color RAM location
5	$9600	38400	normal color RAM
6	$9A00	39424	I/O block 2
7	$9E00	40448	I/O block 3
8	$0200	512	RAM
9	$0600	1536	RAM (3K memory expansion, not accessible)
10	$0A00	2560	RAM (3K memory expansion, not accessible)
11	$0E00	3584	RAM (3K memory expansion, not accessible)
12	$1200	4608	RAM
13	$1600	5632	RAM
14	$1A00	6656	RAM
15	$1E00	7680	RAM

Values for bit 9 set to 0

This are the possible screen locations for more 8K+ expanded VICs (with color RAM at $9400/37888), default in bold:

Value (bits 10-13)	Location		Contents
	hex	dec
0	$8000	32768	character ROM
1	$8400	33792	character ROM
2	$8800	34816	character ROM
3	$8C00	35840	character ROM
4	$9000	36864	I/O block 1 (VIC and VIAs) / alternative color RAM location
5	$9400	37888	normal color RAM
6	$9800	38912	I/O block 2
7	$9C00	39936	I/O block 3
8	$0000	0	RAM
9	$0400	1024	RAM (3K memory expansion, not accessible)
10	$0800	2048	RAM (3K memory expansion, not accessible)
11	$0C00	3072	RAM (3K memory expansion, not accessible)
12	$1000	4096	RAM
13	$1400	5120	RAM
14	$1800	6144	RAM
15	$1C00	7168	RAM

I: Character memory location

Value	Location		Contents
	hex	dec
0	$8000	32768	character ROM upper case normal characters
1	$8400	33792	character ROM upper case reversed characters
2	$8800	34816	character ROM lower case normal characters
3	$8C00	35840	character ROM lower case reversed characters
4	$9000	36864	I/O block 1 - VIC and VIAs
5	$9400	37888	color RAM with memory expansion in block 1 / normal color RAM
6	$9800	38912	I/O block 2
7	$9C00	39936	I/O block 3
8	$0000	0	RAM
9	$0400	1024	RAM (+3K memory expansion, not accessible)
10	$0800	2048	RAM (+3K memory expansion, not accessible)
11	$0C00	3072	RAM (+3K memory expansion, not accessible)
12	$1000	4096	RAM
13	$1400	5120	RAM
14	$1800	6144	RAM
15	$1C00	7168	half RAM, half ROM of upper case normal characters since the 14-bit address space of the VIC wraps around

NOPQRSTU: Sound switches and frequencies

When the sound switches are set to 1 the sound channel associated with it is turned on. The frequencies for the channels are calculated like this:

frequency = clock / (127 - value)

To get the register value for a particular frequency do:

value = 127 - (clock / frequency)

The 'clock' varies depending on the channel and on the version of the VIC you are using:

Channel	NTSC clock	PAL clock
bass	3995	4329
alto	7990	8659
soprano	15980	17320
noise	31960	34640

Internally, the square wave sound generators are realized as 8-bit linear feedback shift registers (LSFR). When they're active, one bit is shifted out, inverted, output and inserted at the other end, at the rate programmed with the low 7 bits of the frequency register. The noise generator seems to be a 13-bit LSFR.[3]

V: Composite sound volume

This determines the loudness of all channels combined, ranging from 0 (off) to 15 (full volume). With high volume settings and more than one channel running clipping might occur. See DC offsets and clipping. For the smallest amount of clipping the volume should be set to a value of 8.[4]

W: Auxiliary color

This color is used with multicolor characters only, but all 16 possible colors can be used.

X: Screen/background color

Here as well, all 16 colors can be chosen from.

Y: Reverse mode

When set to 0 character and screen color are swapped.

Z: Border color

Only colors 0-7 can be used here.

Colors

Overview

The VIC produces a 16-color YPbPr composite video palette. The palette lacks any intermediate shade of gray, and it has only four levels of luminance above the zero (black).[5]

Color codes

Value	Color	Y	Pb (rel.)	Pr (rel.)
0	black	0	0	0
1	white	1	0	0
2	red	0.25	-0.383	0.924
3	cyan	0.75	0.383	-0.924
4	purple	0.5	0.707	0.707
5	green	0.5	-0.707	-0.707
6	blue	0.25	1	0
7	yellow	0.75	-1	0
8	orange	0.5	-0.707	0.707
9	light orange	0.75	-0.707	0.707
10	pink	0.5	-0.383	0.924
11	light cyan*	1	0.383	-0.924
12	light purple	0.75	0.707	0.707
13	light green	0.75	-0.707	-0.707
14	light blue	0.5	1	0
15	light yellow*	1	-1	0

The colors marked with an asterisk (*) are out of the RGB gamut.[6]

Colors 8-15 may only be used in the Auxiliary and Background registers.

Multicolor codes

These are the bit pairs used in multicolor characters (color RAM values with bit 3 set to 1):

Value	Color
00	screen/background color
01	border color
10	character color from color RAM
11	auxiliary color

Advanced sound programming techniques

Digitized sound output

4-bit resolution digitized sounds can be played by writing samples to the volume register. See One hour of digitized music on the VIC.

The rather quiet output can be amplified with a HF carrier.

Manipulating waveforms

The waveforms of the three square wave oscillators can be modified by rapidly turning an oscillator on and off after letting it rest for a while. See 15 new waveforms for the VIC-20.

Advanced graphics mode programming techniques

With help of just-in-time graphics data manipulation (and great loss of CPU performance), resolutions far beyond the original VIC design can be established. See the New Frontiers in VIC-Hires-Graphics Series.

Die shot

The Visual 6502 project offers a 6561 die shot for reverse engineering purposes.