V9978... what could it have been?

I have wondered why there is no high color and or high resolution pattern modes in the 9938, would this be useful?

Umh, this retro-compatibility is "a too big effort", manuel. If you think a bit it was already happened.
look at v9938. the requirement of compatibility forced designers to make non optimal solutions. with the huge bandwidth available on v9938 the performances could have been a lot better.
Look at the choices done in the sprite system. The main reason of having a SCT was the compatibility with TMS that have monochromatic sprites.
The SMS changed a lot in the new sprite system and this resulted in a better sprite system. But of course a lot of things changed, for example the way the SMS VDP fetch and interprets sprite data (look at the sprite layout in SMS SPT)

And take into consideration that in the case of v9978 this could be worst. you have a new chip like V9990 that is required to maintain compatibility not only with TMS but with two chips, the TMS and the V9958/38. A lot of garbage means a lot of compromises.
IMHO this chip never existed because of the difficulties of reconcile a V99x8 chip already full of quirks and compromises (due to heritage of TMS) with a new clear architecture like the V9990.

for example look at the protocol needed to set the VRAM PTR on V99x8 and on V9990.
Not only the V9990 one is better engineered, but is more clear, logic and powerful. the v99x8 is a crappy one that exists as is because of the TMS compatiility. the choice to encode in the second byte you send to VDP two bits that semantically are not addresses but more a selection of mode of work of the VDP was a real disaster and precluded a more intelligent use of the byte only for address setup purpose.
They maintained this crappy thing because of compatibility and this result a big overhead for the CPU needing to split a register in bits, sending some to one register and other to another. (My GOD)

The V9938 sprite system introduced the OR color trick because of inability to have a proper colored sprite, but the V9938 had a potential of doing a good sprite system like SMS. It happened in this way because of compatibility

I suspect that even if yamaha succeeded in V9978, the tricks they would have forced to use also could have some negative impact on the V9990 parts in terms of limitations.

To be is more logical to think the V9958 as a two system on a chip, maybe internally superimposed or switchable when required. Essentially two cores:
Do you need compatibility? switch on the v9958 core by setting a register. The V9990 is disabled and the output is for v9958 core in the V9978 chip.
Do you need a proper VDP ? switch on the V9990 and shutdown the V99x8 in the V9978....

I do not see a better solution. full compatibility in a single chip is a goal to much ambitious given the existing situation.

what is/was the limitation?? - with a 21Mhz clock it doesn't seem impossible...

Screen 4 ?

I would like to have the table names as in screen2/4 for screen 5 !!!
Sometimes I don't understand with screen4 because it was similar to screen 2 but with palette + multicolor sprites ?

Bye bye
DanySoft

Interesting, what do you expect will happen if one try to use V9990 with panasonic MSX2+ ?
Aren't those ports the so-called swithed ports ( which are dependant on the manufacturer) so no conflict?