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Index > OS Construction > WAIT &H3DA, 8 | VGA issues

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DOS386



Joined: 08 Dec 2006
Posts: 1901
DOS386
RB wrote:

Code:
Bitfields for CGA status register:
Bit(s)  Description     (Table P0818)
 7-6   not used
 7  (C&T Wingine) vertical sync in progress (if enabled by XR14)
 5-4        color EGA, color ET4000, C&T: diagnose video display feedback, select
         from color plane enable
 3 in vertical retrace
 (C&T Wingine) video active (retrace/video selected by XR14)
 2   (CGA,color EGA) light pen switch is off
     (MCGA,color ET4000) reserved (0)
    (VGA) reserved (1)
 1        (CGA,color EGA) positive edge from light pen has set trigger
        (VGA,MCGA,color ET4000) reserved (0)
 0      horizontal retrace in progress
      =0  do not use memory
       =1  memory access without interfering with display
      (VGA,Genoa SuperEGA) horizontal or vertical retrace
     (C&T Wingine) display enabled (retrace/DE selected by XR14)
SeeAlso: #P0817,#P0819,#P0762
    


Code (QuackBASIC) Laughing :

Code:
WAIT &H3DA, 8
    


Well, I'd like to know how this bit exactly works: does the archaic "CGA" bit work on VGA and VESA also, is it reliable at all, at what time exactly is it 1 or 0 ?

Is there a better way to do - to find out the exact Y position on the screen at given time ?

_________________
Bug Nr.: 12345

Title: Hello World program compiles to 100 KB !!!

Status: Closed: NOT a Bug
Post 29 Aug 2007, 06:15
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f0dder



Joined: 19 Feb 2004
Posts: 3170
Location: Denmark
f0dder
It works for VGA and better too, I used it for vertical retrace waiting before backbuffer->frontbuffer flip back in the DOS days - can't imagine that it wouldn't be supported even on the latest cards.

Some cards support(ed?) firing an interrupt at retrace time, but I have no idea whether this is still a supported feature or not.
Post 29 Aug 2007, 10:35
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DOS386



Joined: 08 Dec 2006
Posts: 1901
DOS386
> Some cards support(ed?) firing an interrupt at retrace time

Don't need an interrupt ... but a more precise measurement of the "position" on screen being "fired" out to the monitor.
Post 29 Aug 2007, 11:26
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f0dder



Joined: 19 Feb 2004
Posts: 3170
Location: Denmark
f0dder
I wonder if that's possible... you sync pretty good with the vertical (ie, full-screen) retrace, and it's possible to synchronize with horizontal retrace as well (yeah, copper bars are possible on x86), but you need some very accurate timing to do better synchronizing than that, I guess, and that's not very easy with the gazillion different configurations possible on PC system.
Post 29 Aug 2007, 12:26
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rugxulo



Joined: 09 Aug 2005
Posts: 2341
Location: Usono (aka, USA)
rugxulo
I've heard some newer gfx cards don't support EGA, but I dunno exactly which ones.
Post 30 Aug 2007, 03:18
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DOS386



Joined: 08 Dec 2006
Posts: 1901
DOS386
> I've heard some newer gfx cards don't support EGA

Not a big problem for me ...

Nevertheless: Nobody knows how to find out the vertical position more exactly or there is none ? And how the bit 3 exactly works (when it switches from 0 to 1 and back) ?
Post 09 Sep 2007, 23:47
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