MOS Technology VIC

The die

The Video Interface Chip (known as VIC or MOS Technology 6560/6561 for NTSC and PAL variants, respectively) is the video adapter IC for, and 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.

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.

Pinout

           ┌────────┐
       NC ─┤        ├─ VDD
   CHROMA ─┤        ├─ ϕ1
LUMA/SYNC ─┤        ├─ ϕ2
      R/W ─┤        ├─ OPTION
      D11 ─┤        ├─ Pϕ2
      D10 ─┤        ├─ Pϕ1
       D9 ─┤        ├─ A13
       D8 ─┤        ├─ A12
       D7 ─┤        ├─ A11
       D6 ─┤        ├─ A10
       D5 ─┤        ├─ A9
       D4 ─┤        ├─ A8
       D3 ─┤        ├─ A7
       D2 ─┤        ├─ A6
       D1 ─┤        ├─ A5
       D0 ─┤        ├─ A4
     POTX ─┤        ├─ A3
     POTY ─┤        ├─ A2
    AUDIO ─┤        ├─ A1
      VSS ─┤        ├─ A0
           └────────┘

CHROMA/LUMA-SYNC: Analog video output
R/W: Read (low) or write (high) access to registers
D0-D7: System data bus
D8-D11: Direct connection to Color RAM
POTX/POTY: ADC input from Game controller port
AUDIO: Analog audio out
ϕ1/ϕ2: Two-phase clock input
Pϕ1/Pϕ2: Two-phase clock output
A0-A13: Address bus
OPTION: Varied depending upon the model of VIC

Option pin

6560-001: ϕM — 2.0 MHz Clock for Clocked Memories
6560-101: LIGHT PEN — Negative edge triggered latch of raster position
6560-201: RESET — Reset Horizontal and Vertical counters to Vertical Sync
6560-301: BUS AVAILABLE — Pin is low when VIC is displaying data

The VIC-20 used chip model 101, with a light pen input.

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 Map

The VIC has sixteen registers, located at memory addresses $9000-$900F (36864-36879). The default register values are stored in ROM at address $EDE4 to $EDF3.

Location		Bit function	Default value		Function
hex	dec	Bit function	NTSC	PAL	Function
$9000	36864	`ABBBBBBB`	$05	$0C	Interlace mode / Screen origin (horizontal)
$9001	36865	`CCCCCCCC`	$19	$26	Screen origin (vertical)
$9002	36866	`HDDDDDDD`	$16		Screen memory offset / 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	`JJJJJJJ1`	$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 VIC 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.[1]

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-I: Screen and character memory locations

Bits 0-3: 4-bit address (see below) of character ROM. Default is 0; changes when inverse video and/or lowercase is selected
Bits 4-7: 4-bit address of video memory. Default is 15 with $9002 set to offset (the highest 512 bytes of the internal 4k) for an unexpanded or 3k expanded VIC, or 12 with $9002 set to no-offset (the bottom 512 bytes of the internal 4k) for larger expansions.
- Bits 4-7 specify a 1k window in which the video memory resides. Bit 7 of $9002 is additionally part of this address as an "offset" bit: If this bit is cleared, the video memory will start at byte 0 of the specified 1k window. If this bit is set, the video memory instead starts at byte 512 of the 1k window. This was done to increase contiguous free memory, since the VIC-20's 22x23 character grid requires just under 512 bytes. This offset bit also affects the Color RAM address: The Color RAM is a 1k RAM chip starting at address $9400. If the offset bit is cleared, the Color RAM will start at byte 0 of this block. If the offset bit is set, the Color RAM will start at byte 512 of the block.

4-bit addresses

Byte $9005 (36869) is comprised of two 4-bit addresses for the character memory and video memory. These addresses refer to sixteen 1k blocks. Values in the range of 0-7 are addresses in the $8000-$9FFF range, and values in the range of 8-15 are in the $0000-$1FFF range:

0: $8000, 1k character ROM, capitals + glyphs
1: $8400, 1k character ROM, capitals + lowercase
2: $8800, 1k character ROM, inverse capitals + glyphs
3: $8C00, 1k character ROM, inverse capitals + lowercase
4: $9000, 1k VIC/VIA registers
5: $9400, 1k RAM used as Color RAM
6: $9800, 1k Expansion port
7: $9C00, 1k Expansion port
8: $0000, 1k RAM, zeropage, datasette buffer, and other various uses
9: $0400, 1k Expansion port
10: $0800, 1k Expansion port
11: $0C00, 1k Expansion port
12: $1000, 1k RAM
13: $1400, 1k RAM
14: $1800, 1k RAM
15: $1C00, 1k RAM

The character memory always starts at byte 0 of the selected block, while the video memory can be offset by 512 bytes by setting bit 7 of $9002. While these are the values that can be chosen via $9005, in practice, much isn't usable for a variety of reasons:

Attempting to set the character and video locations to the same addresses, without offsetting the video memory, will result in the VIC reading the same memory to both determine which character is drawn, and what that character looks like. While the VIC succeeds in doing this, it would likely be impossible to create a readable display this way.
Setting the video address to 0-3 will cause the VIC to read the character ROM as video memory, resulting in a random array of characters.
Setting either value to 4 will cause the VIC to read its own registers, as well as the registers of both VIAs. Since these values don't in any way correspond to text characters, this will garble the display.
Setting either value to 5 will cause the VIC to read the same bytes as Color RAM and whatever has been set to this address. If the character ROM is placed here, whichever half is being used for Color RAM will be read as character data, resulting in some garbled characters. However, since the characters are either 8 or 16 bytes apiece, this is still usable, it merely limits the character table to 128 or 64 usable characters. However, setting the video RAM to this location will never work, as the offset bit of $9002 always applies to both. If the character memory is at the default address, this will create the humorous effect of characters appearing as @ or the letters A-F depending upon the foreground color of that character, since @ followed by A-F are the first seven letters in the character ROM.
Values of 6, 7, 9, 10, or 11 correspond to the expansion port. Having the VIC read from the expansion port could be a very valuable tool; aside from extra RAM, ROM chips holding character ROM could be placed on cartridges and read directly by the VIC, much like the video system of Nintendo video game consoles. Unfortunately, due to a design flaw, this isn't possible.[2] The VIC and CPU share the address and data buses, and access these on alternating clock cycles. Conflicts are avoided by using 74245 bus transceivers, which alternately disconnect the VIC or CPU from the buses. However, the physical topology of the motherboard is as follows:

6502 CPU <--> Expansion port <--> Transceiver <--> Onboard RAM/ROM <--> Transceiver <--> 6560/6561 VIC

As a result, whenever the VIC has control of the buses, the expansion port remains connected to the CPU and isolated from the VIC. This means the data lines "float" unconnected for these address ranges whenever the VIC is active, resulting in random read values, resulting in garbled text.

A value of 8 selects the lowest 1k of system memory, containing the zeropage and other items. It's not completely unusable– the highest 191 bytes of this range are the datasette port buffer, and seven bytes below that are unused. As a result, a very small screen or character set can be placed here. Only the highest 12 or 24 characters (depending upon regular or double-height) are thus usable if the character memory is placed here, and a screen would be limited to about 198 overall characters, a screen of 18x11 or similar. Also, since the screen memory must start no higher than byte 512, there will always be garbled text above the usable screen space, though this can be hidden by raising the top of the screen or setting the appropriate bytes in Color RAM to match the background or border colors.

Using values of 5 (color RAM) for character memory, and 8 (zeropage) for video memory, are a simple way to free up the entire system memory as a contiguous block while using modified characters. However, the limits this setup imposes requires tricky programming to overcome while writing quality software.

N-U: 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: Master 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-Z: Color/inverse

WWWW: Auxiliary color for Multicolor character mode
XXXX: Background color
Y: Inverse; swaps the specified background and individual color RAM (except for multicolor characters)
ZZZ: Border color

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

These values may be used in the appropriate registers and also stored in color RAM. Colors 8-15 may only be used in the Auxiliary and Background registers, as foreground colors in the Color RAM and the background color use three-bit values.

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]

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.

External links

archive . 6502 . org / datasheets/mos_6560_6561_vic.pdf
www . atarimagazines . com / compute/issue38/105_1_PART_III_VISITING_THE_VIC-20_VIDEO.php