new GFX card

I prefer a 4 bit/pixel build up in sprites and tiles, with prefix bits in the SPAT / Name Table.
This way I could reuse sprite patterns with different pallette and use more colors on one single layer.

Having checked a quick reference of V9990's pattern mode screens, it's safe to say that my proposed specs by far exceed that of V9990, provided I increase the pattern buffer.

Gotta keep in mind though that on V9990 both pattern and sprite patterns are shared. In my design these would be separate memories.

                  V9990(P1)     VDPX
Res:	         256x212       256x212
Map Patterns:     8160          2048 (8x8)
Sprite Patterns:  ^shared       256 (16x16)
Sprites:	         125/16scan    128
Map Layers:       2(ScA/B)      4 (or even 8)
Map Size:         64x64         User defined
Pattern Colors:   16/32         256
Sprite Colors:    16/48         256
Palette:          4x32K         16.7M

That's as far as pattern mode goes though. VRAM commands and additional modes are to be defined. My first goal would be to implement this screenmode, using a total of 256KB video memory. If we were to double VRAM to 512KB, we'd end up with 4096 256-color patterns and 512 256-color sprites.
No excotic trickery with palette bits in registers to somewhat create more than 16 colors on-screen, but straight forward 256-color patterns/sprites and one master palette.

I think this could have merit

Excellent !!

256 colors is more than enough ...

How is the vram access done ? through IO ports or memory mapped access? the latter is my prefered method...IO is usually pretty slow...

That would be 16KB block (1 page) memory mapped in the slot the thing is plugged into. Can even select the slot page (0 ~ 3 although 3 would rarely if ever be used).

That VSU thing I've looked at seems sort of strange, so many separate VDP chips on it, primarily for backward compatibility I suppose.

My idea was to leave the MSX computer unharmed by any hardware mods. Just route its video out into the video-in for the VDPX cart, and its video-out to a monitor (RGB Scart -> DIN type of cable to be included in the package). Output DIN -> RGB as well as normal RF jack on it. This kills the need to support backward compatibility completely as the MSX' own VDP isn't really replaced.

Simply by activating a VDPX screenmode would switch the video signal from the video-in to the VDPX output.

i'm already in contact with a japanese guy, who did some graphics in a msx game recently. let's hope if this thing run some custom made pictures to produce some kind of a game using it. please contact me later when something is ready to run in the real machine.

Anyways, I'll fire up ISE some time, design the necessary internals and see how FPGA resources are used. If the project appears to be viable, I'll probably make a prototype cart, allowing me to better estimate cost. If the FPGA to be used has plenty resource blocks left, I can perhaps include the 4bpp sprite/pattern style with multiple palettes as well.

I have to get my MSX2 back from the tons of boxes in my storage room again

looking forward to hear about your progress...it sounds promising !!