MOS Technology VIC-II

Supratechnic, a type-in program published by COMPUTE!'s Gazette in November 1988, showcases the careful use of raster interrupts to display information outside of the standard screen borders (here: the upper and lower border).

The VIC-II is programmed by manipulating its 47 control registers (up from 16 in the VIC), memory mapped to the range $D000–$D02E in the C64 address space. Of all these registers, 34 deal exclusively with sprite control (sprites being called MOBs, from "Movable Object Blocks", in the VIC-II documentation). Like its predecessor, the VIC-II handles light pen input, and with help from the C64's standard character ROM, provided the original PETSCII character set from 1977 on a similarly dimensioned display as the 40-column PET series.

By reloading the VIC-II's control registers via machine code hooked into the raster interrupt routine (the scanline interrupt), one can program the chip to generate significantly more than 8 concurrent sprites (a process known as sprite multiplexing), and generally give every program-defined slice of the screen different scrolling, resolution and color properties. The hardware limitation of 8 sprites per scanline could be increased further by letting the sprites flicker rapidly on and off. Mastery of the raster interrupt was essential in order to unleash the VIC-II's capabilities. Many demos and some later games would establish a fixed "lock-step" between the CPU and the VIC-II so that the VIC registers could be manipulated at exactly the right moment.

The C64 shipped with the PETSCII character set in a 4k ROM, but, like the VIC-20 before it, the actual data for the characters was read from memory at a specified location. This location was one of the VIC-II registers, which allowed programmers to construct their own characters sets by placing the appropriate data in memory; each character was an 8x8 grid, a byte represented 8 bits horizontally, so 8 bytes were required for a single character and thus the complete 256-character set used a total of 2,048 bytes. Theoretically as many as eight character sets can be used if the entire 16k of video memory were filled.[8]

In addition to charsets, the VIC-II also uses 1000 bytes to store the 25 lines of 40 characters per line, one byte for each character, which in power on default configuration sits at $400-$7E8.[8] Color RAM is accessed as bits 8 to 11 of the video matrix;[9] in the 64 and 128, it is located in I/O space at $D800-$DBFF and cannot be moved from that location. It contains the values for color 1 (color 3 in multicolor mode) of each character.

The character ROM is mapped into two of the VIC-II's four "windows", at $1000-$1FFF and $9000-$9FFF, although the CPU cannot see it there (the character ROM may be switched into $D000-$DFFF where it is visible to the CPU, but not the VIC-II). Thus graphics data or video buffers cannot be placed at $1000-$1FFF or $9000-$9FFF because the VIC-II will see the character ROM there instead. Because these areas of RAM could not be used by the VIC-II graphics chip, they were frequently used for music/sound effects (the SID chip). The C64 has the ability to have RAM and ROM at the same address in memory but the CPU would "see" one and the VIC-II chip would "see" the other.

In default high-resolution character mode, the foreground of each character may be set individually in the color RAM. In multicolor character mode, color 3 is limited to the first eight possible color values; the fourth bit is then used as a flag indicating if this character is to be displayed in high-resolution or multicolor, thus making it possible to mix both types on one screen.[8] Colors 1 and 2 are set by the registers at $D022 and $D023 and are global for all characters.[8]

If Extended Background Color Mode is used, the upper two bits of the character code are used to select one of four background color registers. This allows four different background colors on the screen, but at the expense of only allowing 64 different characters instead of 256. Because this is limiting, games seldom used it.

Adding an all-points-addressable bitmap mode was one of the Commodore design team's primary goals, as the VIC-I lacked such a feature. However, in order to use as little additional circuitry as possible, they organized it in the same manner as character mode, i.e. 8x8 and 4x8 tiles. Bitmap graphics require an 8k page for the pixel data and each byte corresponds to one row of eight or four pixels. The next byte is the row underneath it and after the 8th row, returning to the top of the next tile.

In hi-res bitmaps, screen RAM is used to hold the foreground and background colors of each tile (high and low nibble of each byte). This is the only VIC-II mode that does not make any use of the color RAM at $D800 or the background color register at $D021.

Multicolor bitmap mode allows three colors per tile (the fourth is the background color as set in $D021). Colors 1 and 2 are selected by the bits in screen RAM (same as hires bitmaps) and the third is from color RAM.

Despite the high level of color detail and all-points-addressable capabilities of bitmap mode, it is generally impractical for in-game graphics due to requiring a high amount of system resources (8k for the pixel data plus considerable more CPU cycles to modify each tile) and normally cannot be scrolled. Thus, it is most commonly seen on loader and sometimes title screens.

VIC-II sprites are either 24x21 monochrome or 12x21 multicolor. Similar to character graphics, the latter have one individual color for each sprite and two global ones. VIC-II has eight sprites, each of which uses 64 bytes of memory to store but, with certain limitations, it can display many more. Sprite multiplexing is a common method of getting more than eight on screen (although there still is a maximum of eight per scan line). The VIC-II scanline counters are polled until the desired point is reached on screen, after which the program quickly changes the sprite coordinates. This programming trick and other workarounds could result to over twenty sprites onscreen once. For a demo, though, the limit is considerably more flexible.

In theory the maximum number of different sprites visible at the same time is 256 (assuming the VIC-II's entire 16k page was filled). They are addressed by using a block number to refer to each sprite pattern in memory beginning with 0 and going to 255 ($FF) depending on their position in the video page. (if the second video bank (numbered as 0 1 2 and 3) is used, Block 0 would refer to the sprite stored at $4000 and Block 255 would be at $7FC0).

Each sprite may be double-sized vertically, horizontally or both. This does not make the sprite bigger (except visually) or add more pixels to the sprite, but merely upscales the existing pixels.

Because the horizontal position register for sprites is one byte and limited to a maximum value of 255, it could not cover the entire 320 pixels of the VIC-II's screen area, so an additional register called the Most Significant Byte Flag is provided for this.

$D01E and $D01F contain the Background and Sprite-to-Sprite Collision registers. The former is rarely used because it cannot provide information on the specific background object the sprite is touching.

$D01B contains the Sprite To Background priority register, which is used to govern whether a sprite moves behind or in front of background objects. When a sprite enters the same space as another sprite, the lower-numbered ones will always pass over the higher numbered ones.

In order to scroll a character screen, the VIC-II is set to 38-column and/or 24-line mode via the registers at $D011 and $D016. This creates an off-screen buffer where the row of characters to be scrolled is placed. By adjusting the scroll bits in the above-mentioned registers, one row may be moved on-screen after which it repeats unless a new row is put in the buffer. Color RAM is scrolled simultaneous with screen RAM and works the same way.

VIC-II scrolling is a relatively complicated, CPU intensive task, although it was not uncommon for C64 game programmers to cheat by designing graphics so that the color RAM can remain static. Another standard trick is to cover the bottom or top 25% of the screen with a score counter to reduce the amount of scrolling that has to be performed. Finally, it is usually necessary to use an extra 1k piece of RAM to write character data to and then "blit" it into the screen RAM to prevent screen tearing, although this cannot be done with color RAM.

Late in the C64's commercial lifespan, an exploit known as VSP (Virtual Screen Positioning) was discovered that involved manipulation of the control bits in $D011 to produce fast scrolling at a much lower CPU cycle cost than the standard scroll registers, however it required careful, cycle-exact coding and did not work reliably on some VIC-II revisions, also it can only be used for horizontal scrolling. It is notably used in Mayhem in Monsterland.

Utilization of raster interrupts is an essential part of C64 game programming. In the computer's power-on default state, the first MOS Technology CIA chip generates a maskable interrupt (IRQ) 60 times per second (NTSC) or 50 (PAL) which sends the CPU to the kernel IRQ handler at $EA31. The handler acknowledges the CIA's IRQ, updates the clock, scans the keyboard, and blinks the cursor in BASIC.

Games normally disable the CIA's interrupt and instead set up the VIC-II to generate interrupts when a specific scanline is reached, which is necessary for split-screen scrolling and playing music. The game remaps the IRQ vector at $0314/$0315 to its raster handler which performs these functions and then optionally executes a JMP $EA31 instruction to return control to the kernel.

Some games use only one IRQ; however, nested ones are more common and improve program stability. In this setup, the IRQ is remapped to the second routine and so forth for each one until the last one restores it to the address of the first IRQ. When nested IRQs are used, only one JMP $EA31 instruction is needed in the chain and the others can be ended with JMP $EA81, which simply goes to the end of the kernel handler. Also it is not uncommon for games to switch out the kernal and use their own IRQ handler instead. The NMI can be used for an additional interrupt thread, although undesirable side effects can result from accidentally pressing the Restore key as it triggers an NMI if pressed.

The VIC-II may also generate a raster interrupt from the collision registers, but this feature is rarely used.

The VIC-II has a 14-bit address bus and can use any of the four 16k segments of the C64's memory space for video data. To manage this, two additional address bits are contributed by port bits of CIA. $0000-$3FFF is the power-on default. The second segment ($4000–$7FFF) is typically the best choice for programming from BASIC as it is the only segment that is completely free RAM with no ROMs or I/O registers mapped into it. The fourth segment ($C000–$FFFF) is also a good choice provided that machine language is used, as the kernel ROMs must be disabled to gain read access by the CPU, and it avoids having discontiguous program code and data that would result from using $4000-$7FFF. Note that graphics data may be freely stored underneath the BASIC ROM at $A000-$BFFF, the kernel ROM at $E000-$FFFF or I/O registers and color RAM at $D000–$DFFF, since the VIC-II only sees RAM, regardless of how the CPU memory mapping is adjusted; character ROM is visible only in the first and third segment, thus if segment two or four is used, the programmer must supply his own character data. The screen RAM, bitmap page, sprites, and character sets must all occupy the same segment window (provided the CIA bits aren't changed via scanline interrupt). The last six bytes of system memory ($FFFA-$FFFF) contain the IRQ, NMI, and reset vectors so if the top of memory is used to store a character set or sprite data, it will be necessary to sacrifice one character or sprite to avoid overwriting the vectors.

The VIC-II has 47 read/write registers listed below:

The VIC-II chip has a fixed 16-color palette, shown above.

In multicolor character mode (160×200 pixels, which most games used) characters had 4×8 pixels (the characters were still approximately square since the pixels were double width) and 4 colors out of 16 colors. The 4th color was the same for the entire screen (the background color), while the other 3 could be set individually for every such 4×8 pixel area. Two colors were loaded from the active text screen, and the third was loaded from color RAM. Sprites in multicolor mode (12×21 pixels) had three colors: two shared among all sprites and one individual. The artist had to pick shared colors such that the combination with individual colors led to a colorful impression. Some games reloaded shared colors during the raster interrupt; for example, the game Turrican II's underwater area (which was vertically distinct) had different colors. Others, such as Epyx's Summer Games and COMPUTE!'s Gazette's Basketball Sam & Ed, overlaid two high-resolution sprites to allow two foreground colors to be used without sacrificing horizontal resolution . Of course, this technique reduced the number of available sprites by half.

On PAL C64s, the PAL delay line in the monitor or TV which averages the color hue, but not the brightness, of consecutive screen lines can be used to create seven nonstandard colors by alternating screen lines showing two colors of identical brightness. There are seven such pairs of colors in the VIC chip.

The C64's team did not spend much time on mathematically computing the 16 color palette. Robert Yannes, who was involved with the development of the VIC-II, said:

I'm afraid that not nearly as much effort went into the color selection as you think. Since we had total control over hue, saturation and luminance, we picked colors that we liked. In order to save space on the chip, though, many of the colors were simply the opposite side of the color wheel from ones that we picked. This allowed us to reuse the existing resistor values, rather than having a completely unique set for each color.[10]

Note that early versions of the VIC-II used in PAL C64s has a different color palette than later revisions.[11]

The full palette of sixteen colors is shown below:

MOS 8566 VIC-IIe pinout

The 8564/8566 VIC-IIe in the Commodore 128 used 48 pins rather than 40, as it produced more signals, among them the clock for the additional Zilog Z80 CPU of that computer. It also had two extra registers. One of the additional registers was for accessing the added numerical keypad and other extra keys of that computer; this function was added to the VIC merely because that proved to be the easiest place in the computer to add the necessary three extra output pins. The other extra register was for toggling between a 1 MHz and a 2 MHz system clock; at the higher speed the VIC-II's video output is merely displaying every second byte in the code as black hires bit-pattern on the screen, suggesting use of the C128's 80-column mode at that speed (via the 8563 VDC RGB chip). Rather unofficially, the two extra registers were also available in the C128's C64 mode, permitting some use of the extra keys, as well as double-speed-no-video execution of CPU-bound code (such as intensive numerical calculations) in self-made C64 programs.[12] The extra registers were also one source of minor incompatibility between the C128's C64 mode and a real C64 - a few older C64 programs inadvertently wrote into the 2 MHz toggle bit, which would do nothing at all on a real C64, but would result in a messed-up display on a C128 in C64 mode.

The VIC-IIe has the little-known ability to create an additional set of colors by manipulating the registers in a specific way that puts the color signal out of phase with what other parts of the chip consider it to be in.

Using the specific behavior of the VIC-IIe's test bit, it is furthermore capable of producing a real interlace picture with a resolution of 320×400 (hires mode) and 160×400 (multicolor mode).