Raven. A homemade Atari-like computer

[luciodra]

There's no hope for my 5420, is there? :cry:

[Mikerochip]

There's no hope for my 5420, is there? :cry:

Is it not already working?

[agranlund]

There's no hope for my 5420, is there? :cry:

(I don't have one but there's a chance it is already working? though i suspect really quite slowly as you go up in resolutions)

I've made the driver activate Cirrus linear framebuffer from GD5422 and up, but unless I'm mistaken it looks like maybe even the GD5420 had support for that (?) I will change the driver this evening to assume GD5420 has support for that and you could try with your card. (*)

Keep in mind that blitter and other hardware 2D acceleration features didn't appear until GD5426.
But the older GD542x cards were still good MS-DOS performers so in the context of Atari they're probably as good a pick as an unaccelerated ET4000.


(*)
Linear framebuffer is required for VDI to work, it does not know of bank-switching concepts from early PC days.
So.. without hardware linear framebuffer the driver makes a "virtual" linear framebuffer that performs old-school bank switching automatically and behind the scenes. This is slow and I wouldn't want to use it for anything larger than maybe 640x480x8pp.
Or maybe even, I wouldn't want to use it at all, but it was very fun and rewarding to implement and get that system working.
Cards that are old enough to not have linear framebuffer support are also usually quite slow in themselves so you are getting double punished right there :)

[luciodra]

The card works in black and white, and responds quickly. It seems much more responsive than the ET4000. Also the video output has the best signal. But in color it either doesn't start or the screen isn't usable.

[Atarian Computing]

It's almost time we had a chart!!

You've added so many different cards now :O
(Impressive!!)

I'm actually working in this and other stuff. Currently am swamped with other stuff though.

Cirrus and WDC cards can as of now do 1280x720x8bpp :)
Tested with GD5426, GD5434, WD90C31, WD90C33. Your mileage may vary.

This is great progress! Thanks!

[agranlund]

@Atarian Computing thank you!
I feel missing out on that 720p resolution was quite a big drawback of the homemade driver so I’m really happy to have had that resolved now :)

[agranlund]

The card works in black and white, and responds quickly. It seems much more responsive than the ET4000. Also the video output has the best signal. But in color it either doesn't start or the screen isn't usable.

I rechecked the GD542X databook and the GD5420 does not have LFB support, they introduced that feature with the GD5422.
So I think the answer is no love for the GD5420.

Screenshot 2026-02-10 at 19.28.33.png

So you'd be getting slow auto-bankswitching at best, which is next to pointless except for it being pretty cool that it works.
Alternativly sticking to standard lowres VGA modes which completely fit inside the 64Kb VGA window.
But since you mention the driver isn't working with your card, chances are the bank switch code or possibly something else is not quite correctly done for that model.

Out of curiosity, what does "screen isn't usable" mean?
Since you mentioned that as a separate thing from doesn't start I'm assuming you have something on the screen but it's garbage somehow?
And out of even more curiosity, does standard VGA modes work? (640x480 in 16 color and/or 320x200 in 256 color?)
(And I feel silly for asking this, but it's a GD5420 and not the GD5402 correct?)

[luciodra]

Out of curiosity, what does "screen isn't usable" mean?
Since you mentioned that as a separate thing from doesn't start I'm assuming you have something on the screen but it's garbage somehow?
And out of even more curiosity, does standard VGA modes work? (640x480 in 16 color and/or 320x200 in 256 color?)
(And I feel silly for asking this, but it's a GD5420 and not the GD5402 correct?)

Actually at 640x480x4 and 800x600x4 the video card outputs color. I tried to use some games but no video output.
At 320x200x16 the screen remains black, but it could depend on some monitor parameters.
At 256 colors there are a lot of artifacts:

IMG_2883.jpg

IMG_2884.jpg

[agranlund]

I wonder, is there a possibility of back porting this to work with a standard Mega STE/TT enabled Nova machine?
I feel like a lot of ST users might benefit from, even the old crappy ISA cards.

Backporting is probably not advisable since it relies on running the x86 vgabios and that's a bit of a dealbreaker on a stock 68000.
But maybe snippets of information can be usueful as reference.
One would have to make card setup and mode change code from scratch and in "bare metal" though which is far from a weekend thing.

Perhaps though, by targetting a more modern chip like the Cirrus you'll make your life easier. These are all-in-one chips with minimal external components -- much less vendor variations to support compared to something like the ET4000 that relied on external clock generator and ramdac.

Either get an ET4000, and 24bit/16/15/8/4 bit DAC, or, give a kidney for a Mach32. Or two kidneys, for a Mach64 :P
if I could get a GD-5420 for a few quid, and it'd give me 256 colours at 800x600, I'd get one for my mATX ST :O

Yeah the asking price for some of these cards is nuts.
Even the ET4000 feels overpriced due to popularity. They are no doubt great MS-DOS cards since they saturate the bus, but so does a bunch of others too. And all the ones in the top pack end up at near enough same framerate being bus-limited. Perhaps it just became the most well known one, or was one of the first ones to do so?


For the price of GD5420 I would probably aim for something slightly higher would I be in the realm of making some kind of driver aimed at reasonable priced cards for stock machines I think (I am not, so it's pure speculation :) )
It feels like there is quite a wide span of different cards ending up at roughly the same price point. Everything from really slow ones to sometimes even fast accelerated cards.
I think I ended up paying ~30 eur for a notoriously bad Trident card (which I wanted out of nostaliga and for testing). And that's not too far off from what I bought the WD90C33 for..


The accelerated WDC cards can sometimes be had for cheap since they're not as recognised as Cirrus.
But still they are in the same ballpark as GD5426 in terms of performance.
I grabbed my WD90C33 for ~50 eur which is cheaper than a lot of unaccelerated cards. Same MS-DOS performance as ET4000 but with hardware acceleration to boot.

One can sometimes find a good price for Cirrus cards too. The GD5428 isn't as well known as the more common 26 and 29 models so can sometimes pop up for cheap. There's one on ebay right now for 65 eur or best offer which is quite a good price I think.

There's a GD5426 for 55 eur as well, but listed as a 512Kb model. It looks like it has 1MB of chips on it so perhaps some of it is broken.
Or, with luck, it could also be that the MS-DOS utility shown saying 512Kb is misstaken or that it's only true in his very specific context; running MS-DOS on his particular PC with his particular PC Bios settings etc..

(I bought an ET4000/W32i for less than usual where the seller was showing that exact same MS-DOS utility reporting it as a 512Kb card. That card had 1MB of ram chips on it and all of it was perfectly usable for me)

That GD5426 looks a bit dodgy with the writing of OK on one of the ram chips and what looks like an X drawn on the cirrus itself though.. so maybe it is just a broken 1MB card working as 512Kb.

[agranlund]

At 256 colors there are a lot of artifacts:

Hmmn, that looks like it could be something caused by the bank-switch code not working as intended.
I tested that stuff on my GD5426 and perhaps there are differences between the models that I did not know about and take into account.