MobyGamer wrote on 2021-12-03, 20:19:

Grzyb wrote on 2019-06-29, 18:54:

Jo22 wrote:

I've also got an "Olivetti" labled PVGA1 card (related to WD90C00) with Olivetti BIOS..
Maybe this will be useful later on ?

Can you try setting mode 40h via int 10h on that card?

I have a PVGA1A card in my 286-based AT&T (Olivetti) system and it works with 640x400x2 mode inits. I disassembled the Paradise VGA BIOS on that card to learn why, and found that the BIOS will redirect any int 10h calls setting the mode to 40h (640x400x2) to the host BIOS. So, to answer the mystery, these cards don't actually fully support 640x400x2 on their own because they rely on a host AT&T computer to set the mode properly.

Ah, that explains a lot! 😃 Thank you very much for your help and writing back! 🙂👍

So if the Olivetti 40h BIOS calls are reverse engineered should be possible to enable the AT&T mode on the PVGA cards?

I've found something very interesting for this topic. The ROM listing for the AT&T 6300 Plus is available online (http://bitsavers.org/pdf/att/6300/ATT_Persona … ue_1_-_1986.pdf), so it's possible to understand how the mode 40h and 48h are set.

Just go to the page 202.

I'll try to create a test utility and see if this really works on more video cards.

Seems to me that referring to this Windows 2.x/3.x/GEM driver for a never-has-been 640x400 CGA mode thread would be a good idea.

MobyGamer wrote on 2021-12-03, 20:19:

I have a PVGA1A card in my 286-based AT&T (Olivetti) system and it works with 640x400x2 mode inits. I disassembled the Paradise VGA BIOS on that card to learn why, and found that the BIOS will redirect any int 10h calls setting the mode to 40h (640x400x2) to the host BIOS. So, to answer the mystery, these cards don't actually fully support 640x400x2 on their own because they rely on a host AT&T computer to set the mode properly.

Old Toshiba laptops like T3100/T3200/T1200XE were equipped with 640x400 LCD/plasma screens and were compatible with this Olivetti/AT&T "double-CGA" mode. With 32bit 386(SX) CPUs, Toshiba switched to VGA chips based on PVGA1A architecture (like WD90C22). I was curious if Toshiba allowed to run the same AT&T-mode software without changes on its VGA laptops.

The answer is: Yes, at least the first generation of VGA portables from Toshiba do support the AT&T 640x400 video mode by default. It is not necessary to set the VGA chip into a specific mode first. I used my HPGL viewer for DOS written in Quick Basic 4.5* to test it. It runs just like on the pre-VGA Toshiba laptops. I will later try also Windows 3.1 in the AT&T mode.

AT&T 640x400 mode (notice the thin font and unused fist 40 lines of pixels indicating a 400-line mode):

VGA 640x480 mode (now the whole screen is used):

Program setup:

*) modded to not check for Olivetti string in BIOS when deciding if the 640x400 should be allowed - necessary on any compatible Compaq/Toshiba machine

I've tried Word 5.5 for DOS on Toshiba T2200SX (WD90C22) because it has integrated support for many video cards including the AT&T 6300. It works with the AT&T video driver and allows switching from the text mode into graphics mode with two custom fonts - 80x25 characters and 80x50 characters (it looks like a text-mode but it is not):

I also tried Quick Basic 4.5 on a Toshiba T3200SX (1989) with a VGA plasma screen. It behaves the same way as T2200SX:

Even the built-in driver in Windows 3.1 works somehow. There is only an issue that uses two very similar shades instead of black and "white". This is the only program where I found this behavior: