STIC

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Revision as of 05:07, 11 January 2005 by Mr z (talk | contribs) (Foreground/Background Mode)
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The STIC 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.

Background

The background area is rendered by the STIC as a series of "cards", which are blocks of 8x8 pixels. Thus the background is composed of 20 cards horizontally and 12 cards vertically for a total of 240 cards.

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. During each screen refresh, 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.

Foreground/Background Mode

The program selects to have the STIC display the BACKTAB in Foreground/Background (FGBG) mode by writing to register $21 during VBlank Period 1. This sets the STIC in FGBG mode until the program reads from $21 during VBlank Period 1. While in this mode, the STIC will then interpret each value in the BACKTAB as follows:

     15 14 13 12 11 10 09 08 07 06 05 04 03 02 01 00
      ?  ?  B  b  t  b  b  n  n  n  n  n  n  f  f  f
where: bBbb The background color (off bits), either pastel or primary colors (0 - 15). Note that bit 13 of the BACKTAB word becomes bit 2 of the color number. t Indicates where the STIC will source the image from: 0 = GROM, 1 = GRAM NN Ignored by the STIC in FGBG mode. Programs can use these as flag bits. nnnnnn Actual # of the card corresponding to it's position in GRAM or GROM, (0 - 63). fff is the foreground color, primaries only (0 - 7)

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

Moveable Objects

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.

Registers