The STIC (General Instruments AY-3-8900) provides the video display for the Intellivision Master Component. It is capable of displaying a background resolution of 160x96 pixels and up to 8 hardware sprites, referred to as Moveable OBjects or MOBs.
The STIC has a fair number of registers visible in the CPU's memory map. The System RAM appears to decode addresses $00 - $7F to the STIC, although the STIC itself only responds to the range $00 - $3F. Furthermore, that address space is not entirely populated. The following table lists the known STIC registers:
|$00 - $07||MOB X Coordinate Registers|
|$08 - $0F||MOB Y Coordinate Registers|
|$10 - $17||MOB Attribute Registers|
|$18 - $1F||MOB Interaction (collision) Registers|
|$20||Active Display Strobe|
|$21||Mode Select Strobe|
|$22 - $27||Unused/Unknown|
|$28 - $2B||Color Stack|
|$2D - $2F||Unused/Unknown|
|$30||Horizontal Delay (Scrolling)|
|$31||Vertical Delay (Scrolling)|
|$32||Border Extension Control|
|$33 - $3F||Unused/Unknown|
The background area is rendered by the STIC as a series of "cards", which are blocks of 8x8 pixels. The background is composed of 20 cards horizontally and 12 cards vertically for a total of 240 cards. The overall resolution of the background is 159x96. (The rightmost column does not get displayed.)
The cards to be displayed by the STIC are loaded by the program into an area of memory referred to as the BACKground TABle or BACKTAB. The BACKTAB is located within a portion of the 16-bit RAM memory space from $0200-$02EF. While drawing the display, the STIC reads each card from the BACKTAB and draws it onscreen. How the STIC interprets each card in the BACKTAB depends on whether the STIC is in Foreground/Background mode or Color Stack mode.
The program selects the STIC's Foreground/Background (FGBG) mode by writing to the STIC register at location $21 during VBlank Period 1. This sets the STIC in FGBG mode. The STIC stays in this mode until the program accesses location $21 again. While in this mode, the STIC interprets each value in the BACKTAB as follows:
Unlike Color Stack mode below, the Foreground/Background mode places no restrictions on where the background color changes. With this flexibility comes a couple disadvantages:
- The pastel colors (8 - 15) cannot be used as foreground colors for cards in the BACKTAB.
- For images sourced from GROM, both the BACKTAB and the MOBs are limited to GROM cards 0 through 63. (GRAM has only 64 images, and so this restriction has no effect on images sourced from GRAM.)
- Colored Squares mode cannot be invoked from FGBG mode.
Color Stack Mode
The program selects the STIC's Color Stack mode by reading from the STIC register at location $21 during VBlank Period 1. This sets the STIC in Color Stack mode. The STIC stays in this mode until the program accesses location $21 again. The following diagram shows the way the STIC interprets each word of BACKTAB in general while in this mode:
Note that the behavior of the STIC is slightly different when cards come from GROM versus GRAM. Notably, foreground colors are restricted to the primary colors (0-7) for cards coming from GROM. Also, two additional bits become available as flags for cards coming from GRAM.
When rendering the background in color stack mode, the STIC selects the foreground color from the value stored in BACKTAB, and the background color from the current value on the top of the Color Stack. The Color Stack registers, at locations $28 - $2B define this stack of colors. The Color Stack is really more like a circular queue: Initially, the STIC picks the background color from the first entry on the stack. Whenever the STIC sees the 'Advance' bit set, it moves to the next entry on the stack prior to rendering the card. If the STIC was already on the last entry (at location $2B), it wraps back to the first entry.
In this mode, both foreground and background colors can come from the entire 16-color palette, at least for images from GRAM. This allows producing very colorful screens, but with some restriction on where the background color changes.
Color Stack mode also supports a special display mode for individual cards, called Colored Squares mode. Programs indicate which cards display as Colored Squares cards by setting bit 12 to '1' and bit 11 to '0'. This also means that programs cannot display cards from GROM with a pastel foreground color, since that is what this encoding would otherwise specify. The STIC interprets Colored Squares cards as follows:
Each quadrant of a Colored Square card can be one of 8 colors. The squares have these properties:
- Colors 0 through 6 display directly from the primary color set.
- Color 7 displays the current color on the top of the color stack.
- Colors 0 through 6 behave like "on" pixels and interact with MOBs.
- Color 7 does not interact with MOBs.
- The color stack never advances on a Colored Square card.
The STIC is capable of rendering up to 8 moveable objects, or MOBs, which may be either 8x8 or 8x16 in size. The STIC can stretch each MOB horizontally by 2x, and can stretch each MOB vertically by 2x, 4x, or 8x. Note that each MOB pixel is displayed by default (if no stretching is enabled) as the same width of each background pixel, but only half the height of the background pixels. The STIC can also mirror each MOB in the horizontal direction, the vertical direction, or both.