640x400x256 mode on 256K VGA
Paulo Sérgio Coelho
Hi!
Though I have tried, I have been unable to get a working 640x400
mode, at 256 colors on a plain VGA board, with the standard 256K.
I guess it will have something to do with Mode X in terms of
accessing memory, since the 64K segment won't allow more than 64K at
each time. However, I don't have any thorough discussion of mode X
here.
I would really appreciate some help in this!
Thanks!
Reunanen Markku
Paulo Sérgio Coelho (pse...@mail.telepac.pt) wrote:
: Though I have tried, I have been unable to get a working 640x400
: mode, at 256 colors on a plain VGA board, with the standard 256K.
It's not possible to get a resolution like that on a standard VGA, thanks
to IBM. Some of the registers won't allow that many bytes per scan
line.
: I guess it will have something to do with Mode X in terms of
: accessing memory, since the 64K segment won't allow more than 64K at
: each time. However, I don't have any thorough discussion of mode X
: here.
With 'Mode-X' or 'Tweaked VGA' you can get resolutions like 360x480
that work fine with a standard VGA. In some programs there have been
also horizontal resolutions up to 400 pixels, but they tend to look
terrible and won't work with all monitors.
--
# Markku Reunanen # r15...@cc.tut.fi # TTKK # Marq/Fit & L!T # Tambere #
Mark Feldman
Avery Lee wrote:
>
> pse...@mail.telepac.pt (Paulo Sérgio Coelho) wrote:
>
> >Hi!
> >
> > Though I have tried, I have been unable to get a working 640x400
> >mode, at 256 colors on a plain VGA board, with the standard 256K.
> >
> >accessing memory, since the 64K segment won't allow more than 64K at
> >each time. However, I don't have any thorough discussion of mode X
> >here.
>
> screen in 256 colors is twice as fast as the VGA can DMA from its memory.
> The VGA has two master clock divide settings and two dot clock divide
> settings based on the master clock, giving you either three or four shift
> rates (I think). The 640x480x16 and 320x200x256 modes both use the highest
> bandwidth setting. You can tweak the registers to get 640 resolution in
> 256 colors but you'll find that every other pixel is a incompletely shifted
> out copy of the first (it'll be the upper 4 bits, I think). You need an
> SVGA to have enough bandwidth.
I think he's refering to "Xtended mode", which is indeed a 640x400x256
unchained mode. Check
http://www.geocities.com/SiliconValley/2151/pcgpe.html for code and
info. It requires a card capable of "regular" 640x400x256, although some
older "extended VGA" cards supported this mode (notably Paradise, even
though it wasn't an SVGA).
----------------------------------------------------------------------
Mark Feldman mailto:pc...@geocities.com
http://www.geocities.com/SiliconValley/2151
Paulo Sérgio Coelho
r15...@cc.tut.fi (Reunanen Markku) wrote:
>Paulo S=E9rgio Coelho (pse...@mail.telepac.pt) wrote:
>: Though I have tried, I have been unable to get a working 640x400
>: mode, at 256 colors on a plain VGA board, with the standard 256K.
>
>It's not possible to get a resolution like that on a standard VGA, =
thanks
>to IBM. Some of the registers won't allow that many bytes per scan
>line.
How? I don't understand this? I'm sure this hasn't nothing to do with
horizontal timings, right?
>: I guess it will have something to do with Mode X in terms of
>: accessing memory, since the 64K segment won't allow more than 64K at
>: each time. However, I don't have any thorough discussion of mode X
>: here.
>With 'Mode-X' or 'Tweaked VGA' you can get resolutions like 360x480
>that work fine with a standard VGA. In some programs there have been
I was thinking more in terms of memory access, not just screen
resolution.
>also horizontal resolutions up to 400 pixels, but they tend to look
>terrible and won't work with all monitors.=20
The monitor is not a problem. Actually, this is not for use in a
monitor. I'm developing a driver to use a PC with a ordinary TV (PAL).
So far I was able to get working, mode 12h (640x480), and also
800x600(!), among 2 other resolutions: 720x540 (my TV's full screen,
I'll probably replace this by the 800x600 mode), and also 768x432, a
16:9 mode, with the same number of active lines as PalPlus. Note that
my TV (37cm/14") only shows about 680 pixels horizontally.
Now, timings here aren't that important, since we must always have
the same timing, which is quite slow in comparison to what monitors
use.
It would be nice, then, to have this resolution on a standard VGA.
The only feature I'll use, non-standard, is interlacing, which I use
on my OAK087 and my sister's S3 Virge.
Therefore, the real resolution I'm after isn't 640x400 but
640x200x256 colors. But this probably doesn't make a difference, or
does it?
Bye! And thanks for the reply!
Arve Holmboe
>mode, at 256 colors on a plain VGA board, with the standard 256K.
>
>accessing memory, since the 64K segment won't allow more than 64K at
>each time. However, I don't have any thorough discussion of mode X
>here.
>
>
>Thanks!
The video mode you refer to can be set via VESA. Get hold of
Scitech Display doctor and the VESA specs.
As far as I know this VESA mode is not a planar mode. Pixels are
manipulated in the same way as in VGA mode 13 hex with the addition
of doing bankswitching between pages in video mem.
Cheers,
--
_______________________________________________________________
Arve Holmb›, Tel: +47 66 79 45 00
Information & Graphics Systems AS Fax: +47 66 79 45 90
N-1371, Asker, Norway email: ar...@igs.no
o o o o o o
_____ o
____==== ]OO|_n_n__][.
[________]_|__|________)<
oo oo 'oo OOOO-| oo\_
+--+--+--+--+--+--+--+--+--+--+--+
Herman Dullink
> The video mode you refer to can be set via VESA. Get hold of
> Scitech Display doctor and the VESA specs.
> As far as I know this VESA mode is not a planar mode. Pixels are
> manipulated in the same way as in VGA mode 13 hex with the addition
> of doing bankswitching between pages in video mem.
VESA mode 0100h (640x400, 8-bit) is indeed not a planar mode. But the
VESA interface supports Mode-X, so you are able (and allowed) to create
your own 640x400, 8-bit Mode-X VESA mode. This is what SDD has done, and
why their driver is 8K (and mine only a few hundred bytes :-).
Herman
Paulo Sérgio Coelho
Mark Feldman <pc...@geocities.com> wrote:
>> pse...@mail.telepac.pt (Paulo S=E9rgio Coelho) wrote:
>>=20
>> > Though I have tried, I have been unable to get a working 640x400
>> >mode, at 256 colors on a plain VGA board, with the standard 256K.
>> >
>> > I guess it will have something to do with Mode X in terms of
>> >accessing memory, since the 64K segment won't allow more than 64K at
>> >each time. However, I don't have any thorough discussion of mode X
>> >here.
>I think he's refering to "Xtended mode", which is indeed a 640x400x256
I was? :-) Oh, well, I guess I was then... ;-)
>unchained mode. Check
>http://www.geocities.com/SiliconValley/2151/pcgpe.html for code and
>info. It requires a card capable of "regular" 640x400x256, although some
>older "extended VGA" cards supported this mode (notably Paradise, even
>though it wasn't an SVGA).
Great! I'll try it tonight! Thanks!
Bye!
Herman Dullink
>It's not possible to get a resolution like that on a standard VGA, thanks
>to IBM. Some of the registers won't allow that many bytes per scan
>line.
> How? I don't understand this? I'm sure this hasn't nothing to do with
>horizontal timings, right?
needs two clocks per 8-bit pixel. This halves the horizontal resolution
from 640 to 320. With some tweaking with the registers, you can get
360x480, 8-bit without crossing standard VGA timings.
>With 'Mode-X' or 'Tweaked VGA' you can get resolutions like 360x480
>that work fine with a standard VGA. In some programs there have been
> I was thinking more in terms of memory access, not just screen
>resolution.
Once you have 640x400 initialised (SVGA, VESA), you can use Mode-X
methods to access the 640x400 = 256KB.
> my TV (37cm/14") only shows about 680 pixels horizontally.
What about 768x576? Oh, and what kind of VGA->TV converter do you use,
VGA->SCART or VGA->composite? If SCART, how's the picture quality?
> It would be nice, then, to have this resolution on a standard VGA.
>The only feature I'll use, non-standard, is interlacing, which I use
>on my OAK087 and my sister's S3 Virge.
Oh, if you already didn't know it, many S3 card don't have 640x400...
> Therefore, the real resolution I'm after isn't 640x400 but
>640x200x256 colors. But this probably doesn't make a difference, or
>does it?
No, just some minor vertical timing/settings.
Herman
Paulo Sérgio Coelho
Psi...@concentric.net (Avery Lee) wrote:
>pse...@mail.telepac.pt (Paulo S=E9rgio Coelho) wrote:
>
>>Hi!
>>
>> Though I have tried, I have been unable to get a working 640x400
>>mode, at 256 colors on a plain VGA board, with the standard 256K.
>>
>> I guess it will have something to do with Mode X in terms of
>>accessing memory, since the 64K segment won't allow more than 64K at
>>each time. However, I don't have any thorough discussion of mode X
>>here.
>
>No, that's not the problem. The problem is that 640 pixels across the
>screen in 256 colors is twice as fast as the VGA can DMA from its =
memory.
>The VGA has two master clock divide settings and two dot clock divide
>settings based on the master clock, giving you either three or four =
shift
4 settings: 25.175Mhz, 28.322Mhz, 12.5875Mhz and 14.1610Mhz.
However, in terms of clocking, screen sync, I don't think that's the
problem. At least my attempts gave a steady picture, only the picture
was interleaved with black areas, about 1 character-wide picture for 3
or 4 black character-squares.
But I don't know about bandwidth. Does VGA really use DMA?
>rates (I think). The 640x480x16 and 320x200x256 modes both use the =
highest
>bandwidth setting. You can tweak the registers to get 640 resolution in
>256 colors but you'll find that every other pixel is a incompletely =
shifted
>out copy of the first (it'll be the upper 4 bits, I think). You need an
>SVGA to have enough bandwidth.
Nope, I didn't got that behaviour. A lot different, actually.
Bye!
Paulo Sérgio Coelho
"Herman Dullink" <dul...@castel.nl> wrote:
> I have no idea what he meant, but I know that the standard VGA card
> needs two clocks per 8-bit pixel. This halves the horizontal resolution
> from 640 to 320. With some tweaking with the registers, you can get
> 360x480, 8-bit without crossing standard VGA timings.
Well, what I'm trying to do isn't according to VGA standard timings,
it's more to do with PAL standard timings, which means 15625Hz
horizontal frequency, at 50Hz vertical frequency.
That is less than half that of standard VGA, which is just NTSC
non-interlaced, if you ask me...
> >With 'Mode-X' or 'Tweaked VGA' you can get resolutions like 360x480
> >that work fine with a standard VGA. In some programs there have been
> > I was thinking more in terms of memory access, not just screen
> >resolution.
> Once you have 640x400 initialised (SVGA, VESA), you can use Mode-X
> methods to access the 640x400 =3D 256KB.
That's the catch. I don't, I can't relie on VESA. Just imagine I have
a PS/2 with it's VGA card. This is what I mean by standard VGA. And
because I can only have a 64K "window", if I wanted to access memory,
I couldn't do it linearly, so I must do it through planes, `a la
16-color modes, which is, from what I know, the way Mode X does it.
> > my TV (37cm/14") only shows about 680 pixels horizontally.
> What about 768x576? Oh, and what kind of VGA->TV converter do you use,
> VGA->SCART or VGA->composite? If SCART, how's the picture quality?
What about 768x576?
Supposedly, TVs can show 576 lines, but mine only shows 524 lines.
I've drawn a line on the top of the screen (one more up and it would
disappear), and another one at the bottom. One less the other gave 524
(lines). I did the same thing horizontally, hence the 680 figure.
Other TVs can (and do, I have 2 more here at home that show a bit more
picture than mine; but they are bigger too) show more lines and more
columns.
I can setup a screen bigger than the showing area, of course, such as
the 800x600 mode, or the 768x576 mode you mentioned, but some of it,
on the top, bottom and sides will not be displayed. That's the well
known overscan.
I designed and built a VGA to composite video circuit, based around a
MC1377 chip. I should've gone to a better, modern, chip, but I had
already bought this one.
I built the SCART circuit (the one from Tomi Engdahl), but because I
was new to those things, I swapped the emmiter and the colector of the
2 transistors on that circuit. Thus, it never worked. I ended up
dismantling it before knowing this. Then, in November I got a much
needed delay line, so I built the circuit right away.
The reason I went composite and not SCART, is that I only have
composite video equipment on my room (the other 2 TVs I mentioned have
SCART but are too big to be carried around) and because it seems
normal (or all) VCRs won't receive in RGB.
The picture quality is very good, I was very suprised. I don't have a
SCART circuit to test this on the same TV, but I expect it [SCART] to
be better than composite of course. Still, since the circuit can be
tuned for color crossover, I can get good compromises.
Text is read easily, though I haven't tried something like 132
columns. I'll probably do that when I come to do the VESA part of my
driver. It might be possible. At 28.322Mhz, If I setup a screen width
of 1809 pixels (201 chars at 9 pixels), I'll endup with 50Hz too, and
now I have more than 132 chars available for this. With interlacing,
60 text lines are more than likely possible. 30 lines per frame, at 8
lines per char, is 240 lines.
> > It would be nice, then, to have this resolution on a standard VGA.
> >The only feature I'll use, non-standard, is interlacing, which I use
> >on my OAK087 and my sister's S3 Virge.
> Oh, if you already didn't know it, many S3 card don't have 640x400...
Once again, remember this is to be done based on a standard VGA.
Besides, 640x400 is just a matter of setting the display to 400 lines
instead of 480, leaving all other registers the same. Nothing more.
Bye!
Paulo Sérgio Coelho
Psi...@concentric.net (Avery Lee) wrote:
>pse...@mail.telepac.pt (Paulo S=E9rgio Coelho) wrote:
>
>> 4 settings: 25.175Mhz, 28.322Mhz, 12.5875Mhz and 14.1610Mhz.
>
>I think there's two more settings needed for the really low bandwidth
>modes... CGA graphics perhaps.
At least I don't have found them. Isn't 12Mhz low enough for you?
Anyway, I ran a program I did, which snoops into the VGA register
set, displays their contents, and does a sum up at the end. On
320x200x256 colors, I got this:
Miscellaneous Output (3CCh/3C2h):99 01100011b VGA
Clock:25.175Mhz
[...]
Clocking Mode (01h):1 00000001b
[...]
Horizontal Display End (01h):79(+1) 640 pixels
[...]
Vertical Display End (12h):143(+1) 400 lines
[...]
Resolution: Graphics 320x200 256 colours
Dot Clock: 25.187Mhz * Line Frequency: 31.484Khz * Frames Per Second:
70.121Hz
Video Bandwidth: 20.149Mhz .. 25.187Mhz
For 320x200 at just 4 colors, I got this:
[...]
Resolution: Graphics 320x200 4 colours
Dot Clock: 12.593Mhz * Line Frequency: 31.484Khz * Frames Per Second:
70.121Hz
Video Bandwidth: 10.074Mhz .. 12.593Mhz
From this figures, it seems possible to get the screen wider (like
1200), and still the timings would still be fine for TV use (which is
what I'm after).
>One problem you'll find is that nearly every VGA card emulates the
>registers slightly differently. I have a Western Digital 16-bit VGA =
here
Yeah, those things are really annoying...
>> But I don't know about bandwidth. Does VGA really use DMA?
>
>Yes, according to the DMA that I know. There's no external device
>transferring the data; the CRTC generates the addresses which go =
directly
>to the display memory address bus, and the data goes straight to the
>display generation hardware.
Oh, so this is DMA strictly inside the VGA card, nothing to do with
the PC motherboard's DMA.
Bye!
Paulo Sérgio Coelho
Psi...@concentric.net (Avery Lee) wrote:
>>Video Bandwidth: 20.149Mhz .. 25.187Mhz
>>
>> For 320x200 at just 4 colors, I got this:
>>
>>Video Bandwidth: 10.074Mhz .. 12.593Mhz
>>
>> From this figures, it seems possible to get the screen wider (like
>>1200), and still the timings would still be fine for TV use (which is
>>what I'm after).
BTW, the formula to calculate the above bandwidths wasn't devised by
me. I got it from the VGADOC 4b graphics programming package. I don't
entirely know how it works...
>Wait... if you're trying to set up the VGA to output video at NTSC/PAL
>rates, you should have no problem getting 640 pixel wide resolution at
>8-bit depth. You might have to set some timing register extension bits
No problem. On my test program, which I'm using to make new weird
modes (like 16:9 ones), I have a procedure which takes 5 parameters
for horizontal sync and another 5 for vertical sync.
Like: h_sync(1280,1440,1568,1776,1808) and
v_sync(200,249,259,310,313), which will make the board 640x400 (fake,
but with interlace it's real) at 256 colors.
The parameters are to be read : Display End/Start horizontal
blanking, Start Horizontal Retrace, End Vertical Retrace, End Blaking,
Horizontal Total. Same thing with V_SYNC, except horizontal is now
vertical. With this, I can easily make a new mode, and let the boring
stuff to the procedure.
Anyway, last night on a DOC which talked about Mode X, they said to
turn Chain 4 off to make the board access memory through planes. I did
that, set the dot clock for 28.322Mhz (note: 25.175Mhz also works,
though I haven't tried, but timings are well in reach), and
reprogramed the CRTC through the above procedure. Oh, first of all I
called mode 13h, from BIOS, of course :-)
Since I don't have a Mode X routine for drawing a line or point, I
had to just output a number to memory. It does use each pixel, except
it seems to write it bottom-top, and it seems to divide the screen in
4 parts. Weird.
>We're talking about two different things. You're talking about the dot
>clock, which determines how fast dots are clocked out to the display
>drivers. I'm talking about memory bandwidth, how fast the memory =
subsystem
>can physically (physically? how else would it be?) push bytes out to =
the
>display hardware. At a 31.5KHz scan rate, there is not enough bandwidth=
to
>supply the video hardware with a byte per pixel at 640 pixel resolution.
I really don't know much about this (bandwidth, and I really program
the CRTC based on the current values (if Display End makes for 640
dots, and the screen IS 640 dots wide, I know that to get 800 dots, I
need to get 99 there (99+1=3D100x8=3D800 pixels).
Now, on the 320x200 mode, I get there 640, so it seems the value is
doubled. To get 640, I need to get double the values.
>Now, NTSC/PAL at 15.7KHz is a different story... you might have to use =
the
>28MHz clock instead of the 25MHz clock for timing, though, since VGA =
video
>gets pretty close to the horizontal sync and you'd end up with massive
>overscan without switching to the 28MHz clock. But you should have no
No, at least on my crappy TV it doesn't even have overscan (at
28Mhz). Okay, I have just made a change. I've setup the 25Mhz clock
and have changed the CRTC setup for it. Indeed there is overscan. Not
massive, but some. Anyway, at 28Mhz the screen seems more 4:3 ratio,
so it's better.
BTW, there is no problem getting this into NTSC timings: just replace
the 313 on the V_sync line to 262 and the blanking for 260 and you
should be fine. I can't test it on my TV (won't sync vertically, and
the V_sync knob is inside the TV), but there is probably some work to
do on the vertical retrace to make it center on-screen.
>Okay, now you've got me interested. I've got this NEC multisync 3D here
>that can sync down to NTSC/PAL rates... time to try a little TV video =
out
>the VGA.
Great! If you want, I can mail you my test program. It sets interlace
for Oak and S3. I tried Matrox but the VGADOC TXT on it wasn't sure
how to turn interlace on, and last night on a Millenium I tried, it
didn't work. I've also Genoa interlace support, but I also don't know
if it works. Will test this afternoon on a friend's card. Also on a
Diamond Stealth, which uses the S3 chipset, supported and working (on
a S3 Virge anyway).
These are all PAL timings, and I have 640x480, 720x540, 800x600,
768x432, 960x540 and 640x400x256 colors.
For reasons I told in another post, I won't use this later mode on my
driver. Mode X is still not a standard... :-)
Bye!
Mark Feldman
Avery Lee wrote:
>
> Mark Feldman <pc...@geocities.com> wrote:
>
> >I think he's refering to "Xtended mode", which is indeed a 640x400x256
> >http://www.geocities.com/SiliconValley/2151/pcgpe.html for code and
> >info. It requires a card capable of "regular" 640x400x256, although some
> >older "extended VGA" cards supported this mode (notably Paradise, even
> >though it wasn't an SVGA).
>
> have the bandwidth or settings for 640 resolution in 256 colors.
Actually some plain vanilla VGA's can indeed do 640x400x256. The
Paradise "extended VGA" card I refered to did it in BIOS (which I then
unchained), but on some cards you can supposedly tweak the registers to
get it. Check out Rob Schmidts TWEAK utility for the appropriate
register settings. I seem to remember it not working on a lot of cards
though, my Imagine 128 Series 2 doesn't like it at all.
Cheers,
Wolv
Herman Dullink <dul...@castel.nl> wrote in article
<01bbfef9$91fb54e0$0100a8c0@herman>...
> >It's not possible to get a resolution like that on a standard VGA,
thanks
> >to IBM. Some of the registers won't allow that many bytes per scan
> >line.
> > How? I don't understand this? I'm sure this hasn't nothing to do with
> >horizontal timings, right?
> I have no idea what he meant, but I know that the standard VGA card
> needs two clocks per 8-bit pixel. This halves the horizontal resolution
> from 640 to 320. With some tweaking with the registers, you can get
> 360x480, 8-bit without crossing standard VGA timings.
>
>
> >With 'Mode-X' or 'Tweaked VGA' you can get resolutions like 360x480
> >that work fine with a standard VGA. In some programs there have been
> > I was thinking more in terms of memory access, not just screen
> >resolution.
> Once you have 640x400 initialised (SVGA, VESA), you can use Mode-X
> methods to access the 640x400 = 256KB.
>
> > my TV (37cm/14") only shows about 680 pixels horizontally.
> What about 768x576? Oh, and what kind of VGA->TV converter do you use,
> VGA->SCART or VGA->composite? If SCART, how's the picture quality?
>
> > It would be nice, then, to have this resolution on a standard VGA.
> >The only feature I'll use, non-standard, is interlacing, which I use
> >on my OAK087 and my sister's S3 Virge.
> Oh, if you already didn't know it, many S3 card don't have 640x400...
>
> > Therefore, the real resolution I'm after isn't 640x400 but
> >640x200x256 colors. But this probably doesn't make a difference, or
> >does it?
> No, just some minor vertical timing/settings.
>
>
> Herman
>
>
scuse' me but isn't 256k on a video card just a little outdated?
Paulo Sérgio Coelho
"Wolv" <Raymond....@thezone.net> wrote:
>>=20
>scuse' me but isn't 256k on a video card just a little outdated?
Not a little. A lot. But that isn't an excuse for forgetting about
them. Being outdated doesn't mean they aren't around. I have here 2
PCs, and one of them has a 256K VGA. I know other people who also have
256K VGAs, and there's loads more throughout this world.=20
So, if you want to do a program to work on everyone's board, you'll
work assuming they have the least common denominator: a 256K VGA
board. You can assume other things, of course, more memory, VESA, etc,
but then people would leave your software and use the other guy's
which does support their old boards.
Besides, VGA boards are all compatible at the standard VGA registers,
which means if your program wishes to work on all (or almost all)
boards in existence, you can't work with chipset specific registers,
or assume more than 256K or even other clocks than the standard 25 and
28Mhz ones.
Bye!