I now have implemented a stripped down version of the VT240 display code (no DMA scroll mode, no line erase mode...).
The rest turns out to be a lot harder than i thought.
Main problem: macros for VSYNC and PALETTE still appear to be wrong.
. So VSYNC IRQs do not fire and i can't get hold of the second palette. Could an expert have a look?
i added a bootable image
, as well as the Pacman freeware for testing purposes. Pacman runs in medium resolution, on color ("pacman") and monochrome displays ("pacmono"). It was chosen because it needs no IRQs and follows well known patterns.
mess64 -window -debug rainbow -frameskip 10 -flop1 c:\msys64\_DISKS\bootbar.img -flop4 c:\msys64\_DISKS\FDSTD_COLOR.img
A newly invented DIP should, in theory, switch between grey shades on monochrome and R G B ("color mode"; the green gun is actually wired to MONO out from the _graphics option_). Dual monitor support (R G B plus Mono) was added late in the design. Few commercial programs (AutoCad 2.6) support it. A homebrew cable and a patch to avoid the internal switch ('diagnostic_w', m_ONBOARD_GRAPHICS_SELECTED) usually make the difference.
Although not entirely correct, my proposition is to restrict NEC GDC7220 output to the right hand side.