View Full Version : Backwards compability of VGA cards

January 9th, 2011, 11:31 AM
Standard VGA cards have almost perfect compatibility with EGA, but their compatibility with 6845-based cards is severely limited: MDA and CGA modes are supported only at BIOS level, and there's no compatibility with Hercules Graphics Card (a.k.a. MGA) at all.
Some older VGA cards, however, also have special modes for register-level compatibility with MDA/HGC and CGA.
Cards with compability modes
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! Manufacter !! Chipset !! Setup !! Compability modes !! CGACOMP tests failed !! Other issues
| ATI || ATI18800-1 || VInstall, VSetup or VConfig || Hercules, non-standard hercules, MDA, CGA, Tweaked CGA text-mode and EGA || Brown is dark yellow in med-res graphics mode, no snow, no cursor in the cursor-shape test, some characters differ from the CGA font, tweaked text-mode don't work, vertical refresh is detected as ~70Hz, row-column reprogramming don't work, display positioning don't work and interlaced mode don't work || The dedicated tweaked-mode patch fixes tweaked text-mode and cursor shapes, but it uses a different font and compability for low-intensity palettes of med-res graphics mode breaks.
| Chips and Technologies || 82C451 || Unknown || data sheet (http://www.bitsavers.org/pdf/chipsAndTech/82C451_VGA_Controller_Jul90.pdf) claims there are EGA, CGA, MDA, HERCULES emulation modes || Not tested || Unknown
| Cirrus Logic || CL-GD610 || Unknown || data sheet (http://www.bitsavers.org/pdf/cirrusLogic/CL-GD_610_620_Flat_Panel_CRT_Enhanced_VGA_Controller_ Jul89.pdf) claims there are CGA and HGC || Not tested || Unknown
| Cirrus Logic || CL-GD620 || Unknown || data sheet (http://www.bitsavers.org/pdf/cirrusLogic/CL-GD_610_620_Flat_Panel_CRT_Enhanced_VGA_Controller_ Jul89.pdf) claims there are CGA and HGC || Not tested || Unknown
| Oak || OTI077 || vgamode.exe || CGA, MGA, EGA - all with LOCK option || medium-res graphics background select/foreground select/palettes, top-and-bottom dual line cursor, strikethrough cursor, snow, vertical refresh (70 Hz instead of 60), horizontal refresh, interlacing, display positioning || issues with non-standard HGC modes
| Trident || TVGA8800CS || Onboard switches 1 and 2 defines the startup mode || MDA (maybe Hercules), CGA and EGA || Not tested || Any mode will only provide it's native sync-rates.
| Trident || TVGA8900D || svm89 (http://ftp.serdica.org/public/drivers/Video/Trident/8900C/svm89.exe), svm.exe || EGA, CGA (+warm boot option), HERC || border/overscan color select, top-and-bottom dual line cursor, strikethrough cursor, snow, interlacing || random issues with HGC emulation - eg. Checkit doesn't work, but some games using non-standard modes do work
| Western Digital || PVGA1A || http://bbs.actapricot.org/files/area34/vgaplus.exe || CGA, MDA, Hercules Half, Hercules Full, also allows to lock the mode so it survives warm-boot || Snow, Vertical refresh (70 Hz instead of 60), Interlacing, Display positioning || Hercules non-standard modes (used in many games) exhibits problems.

Chipsets without compability modes
Acumos AVGA2/Cirrus Logic CL-GD5402
Cirrus Logic CL-GD5422
Tseng ET4000

Category:ISA cards