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     V9t9:  TI Emulator! v6.0 Documentation      (c) 1995 Edward Swartz
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  ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÍÍÍÍÍÍÍÍÍÍÍ VIDEO.TXT ÍÍÍÍÍÍÍÍÍÍÍÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ
  ÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ

       This file explains the mechanisms used to emulate the 99/4A's video
  chip, and how to get a nice video update speed for the emulator.


  ÍÍÍÍÍÍÍÍÍÍÍÍÚÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ¿ÍÍÍÍÍÍÍÍÍÍÍÍ
              ³ WHAT'S THE DEAL WITH "VIDEO UPDATING SPEED"? ³
  ÍÍÍÍÍÍÍÍÍÍÍÍÀÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÙÍÍÍÍÍÍÍÍÍÍÍÍ

       The simple fact is, a software program goes slower than a video
  chip.  (Except for those of you with really fast computers...  growl.)
  The 9918A chip updates the screen 60 times a second.  It takes advantage
  of the raster scan technology of computer monitor, which allows it to
  effectively redraw the screen each time.  Software, however, cannot
  possibly redraw the entire screen 60 times a second; it's inefficient as
  well as idiotic.

       Even without redrawing the entire screen, the existence of sprites
  makes it difficult to get maximum efficiency.  Sprites are pretty slow
  compared to the rest of the graphics.  The reason for this is that
  besides sprites, the 8-pixel-aligned graphics of the 9918A chip is very
  well suited to the EGA graphics mode.  Sprites, however, can go anywhere
  on the screen, making them a big problem.

       Anyway, the whole point is, video updates slow performance.  So
  there's an elegant way to handle the timing of video updates.  An
  algorithm keeps track of how much activity is going on with the video
  "chip".  If a certain minimum amount of time passes before any video
  data changes, the screen is updated.  If video data is being written at
  full speed, then after a maximum amount of time elapses, a video update
  is forced.

       In the V9t9 configuration file the variables "VideoUpdateSpeed" and
  "VideoUpdateMaxDelay" allow you to set what these minimum and maximum
  times are, in a resolution of 1/60 second.  For absolutely stinking fast
  computers, set them both to 1.  There, real 9918A updating speed.  For
  the rest of us, like those who use 386SX's, values of, say, 4 and 16 are
  nice.  This means 1/15 second is the delay required after an absence of
  video activity before the screen is updated, and 4/15 second is the
  maximum delay before doing so.

       Please note that on slower computers, the actual screen updating
  process will most likely take longer than 1/60 second in any case.
  (I.E., any case meaning any sprites.)


  ÍÍÍÍÍÍÚÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ¿ÍÍÍÍÍÍ
        ³ HOW DO YOU UPDATE THE SCREEN THAT MAKES SPRITES SO SLOW? ³
  ÍÍÍÍÍÍÀÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÙÍÍÍÍÍÍ

       Well, come on now.  It's actually pretty fast.  (Mental note --
  don't reply to the headers!)  As mentioned before, the only reason it
  takes any time at all is because it's software emulation of a hardware
  chip.

       When sprites change in any way, the characters they covered in
  their old position, and the ones they will cover in their new position,
  are drawn onto a scratch video page.  Then the sprites are drawn on the
  scratch page over those characters.  Then all the affected characters
  are copied directly onto the visible screen.

       Besides sprites and text mode, the EGA video hardware is very well-
  suited to emulating the 9918A chip.  This is because the 9918A breaks up
  graphics into 8-pixel-wide units, which corresponds nicely to the byte-
  oriented planar structure of 16-color EGA modes.  The EGA hardware can
  be set up so that when two bytes are written to video memory and two
  hardware instructions are executed, a pattern in two colors appears in 8
  adjacent pixels on the screen.  Just like the 99/4A, even in bitmapped
  mode!

       Text mode is a problem.  It has 6-pixel-wide units, which obviously
  collides with nice 8-pixel-wide structure of EGA memory.  Luckily, I
  devised a method on a 386 to speed this up, so text mode is now actually
  faster than graphics mode on those systems.  :)

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