From: johnmac@fawlty.towers.oz.au (John MacLean)
Subject: Video Boards And The Toolbox
Organization: Tower Technology, Sydney
Date: Mon, 24 Aug 1992 04:35:39 GMT

These are my (hopefully practical) ideas for a new video board with new
video modes for the GS:

- Modes:
  - 640 x 200 x 16 pure colors per line
  - 640 x 400 x 16 pure colors per line
  - 320 x 200 x 256 pure colors per line
  - 320 x 400 x 256 pure colors per line
  - There would be 400 start of line registers which would be read in
    during horizontal blanking, and 400 palette registers which would
    be read during vertical blanking (both in the memory map).
    This would enable very fast scrolling and palette changes.
  - There would be 2 logical screens in 200 line mode, so that page
    flipping animation could be carried out easily.
    You could either use the first 200 lines, or the second 200 lines.

- Monitor Output:
  - The board would put out two signals, one designed for use with
    standard GS monitors (where 400 line output would be interlaced),
    and one designed for use with cheap VGA monitors.
    
- Memory:
  - Memory would be 512K of VRAM, mapped into $E2 through $E5.

- Hardware Compatibility:
  - The old video out connectors would be fed into the boards video in,
    and the new video out would be fed to your monitor.
  - The board would monitor $E0/E1/C029 and use the four unused bits to
    select either existing video out (when they are all zeros), or one
    of the new modes when they are set appropriately.
  - These unused bits would be used as:
    - Enable / Disable video board (when disabled, output goes directly
      from motherboard output).
    - 400 line / 200 line output.
    - Which 200 lines? (if in 200 line mode).

- Software Compatibility:
  - The 640 x 200 and x 400 modes could be layed out so that the memory
    was contiguous [By setting up the start of line registers appropriately].
  - In 16 color mode (640 x 200 / 400) each color would be stored in a
    nibble in the same arrangement as standard SHR 320 mode.
    [In 256 color mode each pixel would be one byte].

Best Features:
  - Hopefully can be made easily toolbox compatible (in 16 color mode)
    [Comments REALLY welcome on this one - what about interrupts?].
    You would get 16 PURE colors while in 640 x 200 mode, and could
    hopefully start using 640 x 400 mode [Maybe even with large size
    VGA monitors?]
  - 3200 mode would become obsolete - you now have 320 x 200/400 x 256 per
    scan line (easily covers the entire 4096 colors) with no processing
    overhead, and 640 x 200/400 x (16 per scanline) = 3200/4096 colors in
    640 resolution with no processing overhead.
  - With the start of scan line registers, you can easily scroll the
    screen vertically or horizontally (or some portion of it), without
    moving large chunks of data around.
    This will mean a quantum leap for what is possible with graphics in
    arcade games.
  - You can have two screens and rapidly page flip - so large shape
    animation is no longer a problem.

Questions:
  - How feasible is this from a hardware point of view?
  - How feasible is the toolbox compatibility (in 640 x 200 / 400
    with 16 pure colors)?
  - Do you think people would buy one (apart from the "I buy anything
    nifty for the GS" die hard users)?
-- 
This net: johnmac@fawlty.towers.oz.au                   Phone: +61 2 427 2999
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Snail:    Tower Technology, 1 Apollo Pl,                Home:  +61 2 449 5930
          Lane Cove, NSW 2066, Australia.
Newsgroups: comp.sys.apple2