resend: small DOS vga improvement

[Jukka Heinonen]

Christian Costa wrote:
> Andreas Mohr wrote:
> > So until there is further information about how to do it properly,
> > I'll just remove the vga_refresh reset in the VGA_IsTimerRunning() case,
> > and of course this makes Turbo Pascal 6 usable (although screen update is
> > a bit slower than IDA, most likely due to using int 10h instead of video
> > mem and also probing I/O port 0x3da).
> 
> Sure it will help your app but others that need video sync will not work 
> properly. :-)
>
> In your case, VGA_Poll_Text is called periodicaly and should be running
> at 33 fps.
> Since 33 fps is just half the standard speed (70Hz), all should be fine.
> But is VGA_Poll_Text running at 33 fps?
> I suspect this function is too slow (maybe due to the console code).

Well, after some trivial searching, I found out that VGA register 0x3da 
reports vertical retrace, horizontal retrace and some light pen stuff:

  1xxx = Vertical retrace in progress if set
  x1xx = Light pen switched on
  xx1x = Light pen trigger set
  xxx1 = Either vertical or horizontal retrace in progress if set

The original code emulated vertical retrace handling more or less correctly.
The patch by Andreas Mohr on the other hand emulates horizontal retrace.
Unfortunately, in both cases the other retrace is emulated incorrectly.
And I have no idea what to do with light pen bits.

So, Andreas, as you now know how to do it properly, 
can you prepare a patch that:
- handles both vertical and horizontal retrace 
  (and does something with light pen)
- works even before any VGA mode has been initialized
- includes the above documentation about 0x3da bits 
  (so that the next person messing with 0x3da does have some
   starting point)
- does not use inlines unless it really makes a difference
  (getting screen to update a few milliseconds faster is not worth making
   the patch longer)

-- 
Jukka Heinonen <http://www.iki.fi/jhei/>