VileR wrote on 2023-03-30, 18:41:
Yeah, noticed that split banks thing as well. Interestingly the few strings of text in the BIOS don't seem to mention Taxan at all, only Paradise:
Yes, this is obviously a Paradise BIOS, but likely adapted by Taxan.
VileR wrote on 2023-03-30, 18:41:
Would be funny if it turned out to be compatible with "AT&T mode" monochrome software as well.
It is! The BIOS supports the following non-standard modes:
- 40 - 640x400 AT&T mode, 8x16 font
- 48 - 640x400 AT&T-like mode, 8x8 font
- 50 - 640x480 16-color graphics mode (but the sequencer starts with write access to the blue plane blocked. Typo?), 8x14 font
- 51 - 80x30 text mode, 8x16 font
- 52 - 640x400 16-color graphics mode, 8x14 font
- 53 - 80x25 text mode, 8x16 font
Some setting (probably the DIP switch labelled "8-9 DOT") can convert all text 80x25 text modes to 720x400 modes with a 9x16 font (like VGA), but likely slower refresh.
The Taxan EGA card does not support EGA mono operation. It only uses a single DIP switch (probably the DIP switch labelled "EGA-SDY") for monitor/card configuration, which map to the EGA configurations "EGA with EGA monitor and 350-line text mode as primary card, maybe a MDA/Hercules as secondary" and "MDA/Hercules as primare card, EGA with EGA monitor and 350-line text mode as secondary card". Interestingly, the card does not have the 16.257 MHz oscillator required not only for MDA timings (irrelevant, as this card is not designed to drive MDA monitors), but also for standard 21kHZ EGA modes. You can find a lot of pieces in the BIOS that do handle the EGAmono configuration, which is likely generic Paradise OEM code, but I can confidently say that the Taxan variation of this BIOS breaks this feature.
The PEGA2 chip supports software-assisted (but it seems a lot is handled in hardware) legacy emulation using NMI trapping, which is used by the BIOS. If I guess it correctly, it should automatically switch (hence the name autoswitch EGA) between EGA, AT&T-like CGA and Plantronics CGA. The autoswitch functionality seems can likely be disabled using the AUTO SW DIP switch.
- The CGA modes run at standard 15.6kHz/60Hz americant TV timing.
- The 640x350 color EGA modes likely operate at 22.94MHz/29.6 kHz/81Hz
- The 640x400 enhanced CGA modes possibly operate at 22.94MHz/25.15kHz/58Hz
- The 640x400 EGA graphics mode might be broken, as it uses the vertical total register from the 350-line mode
- The 640x480 color EGA modes likely operate at 25.805MHz/31.9kHz/63Hz (close enough that a VGA monitor should sync it, if converted from digital to analog)
- The 720x400 text also might be broken, again due to a bad vertical total setting.
I'm not very confident in these findings, as the 9-dot function with the DIP switch on the card seems to be a signature feature of the card - yet the BIOS parameter table for that mode contains Vtotal=374 scanlines.