VMS - Getting somewhere

Have thought a lot about the Video board since the last post.  Building a switch for this kind of thing out of discrete logic chips not practical.  I'm limited in my board size in Eagle.  I'm limited in my ability to route the thing anyway.  I have an alternative design in mind.  Instead of having a separate GPU and RAMDAC, I'll just have two GPUs which can also act as RAMDACs:

 

This removes the switching logic and now each GPU has a dedicated SRAM bank. Each GPU will be either in Render Mode or Scan-out Mode.  They will never be in the same mode.  A third AVR controls the two GPUs and tell them what mode to be in.  This coordinator chip also controls which GPU is "writing" to the R2R DAC.

I have also included a clock multiplier and 24-bit counter which will be able to quickly clear the framebuffer.  This will free up the GPU for useful drawing instead of spending 80%-90% of the time clearing the screen.  A clever GPU firmware might be able to sync on this and use the counter for drawing pixel spans of solid colours - yum.

Here is the routing so far...