Mail Archives: djgpp/1996/03/19/01:51:53
On Mon, 18 Mar 1996, Christopher A Triebel wrote:
<snip>
> a specialist on DJGPP but if I understand what the non-interlaced means
> it is the way that the video card stores and retrieves information for
> the user. In non-interlaced mode all memory locations are stored
> contiguously straight through. In interlaced ever other screen line is
> contigous. In other words if you have a video buffer 64K in size the
> first 32K contains every odd numbered line, and the second 32K is every
> even numbered line. Or vice versa. I don't have a text handy, but I
> think that CGA used that approach.
>
<snip>
True, CGA *did* use an interlaced memory scheme. But DJGPP doesn't, and
never, supported CGA. The "largest interlaced mode" has to do with the
HSYNC of your monitor and your card. For example, the original IBM 8514/A
video card supported 640x480 and 1024x768 (in 1987 that was pretty good).
The catch was, most monitors didn't have enough bandwidth for 1024x768,
so IBM solved this problem by interlacing -- on pass 1, the even-numbered
rows are drawn, on pass 2, odd-numbered rows. This is on the *monitor*,
it has nothing to do with memory organization.
The NTSC (ordinary TV) specification also uses interlacing to fit all
that signal bandwidth into the 4-MHz TV channel (if you think a stock VGA
monitor with it's 18-MHz bandwidth is bad, just imagine how bad our MTV
is!!!)
The disadvantage of interlacing is that it flickers badly because the
screen refresh rate is effectively halved. Look really close at your TV
screen and you'll notice that the pixels are flashing slowly enough for
you to see. For monitors, the flashing ain't that bad, but if your
graphics screen is mostly light-colored, you'll notice it. This is why
the "largest non-interlaced mode" is important; most people wouldn't care
much for ultra-high resolution but with eye-tearing flicker.
These days though, with the VESA standard refresh rate of 72 Hz,
interlacing is getting pretty rare (the monitors are fast enough) but you
also need a fast card!!! I've only seen interlacing at 1280x1024
resolutions (on a multiscanning SVGA monitor).
Cheers,
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Orlando A. Andico
oandico AT eee DOT upd DOT edu DOT ph
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