I found an interesting link.
I was interested in Vectrex32 and I had a talk with the author which explains to me how its card dialogs with the 6809. He uses a 2KB dual-port RAM to run 6809 code dynamically. Vectrex32 has a 32-bit CPU which can be programmed in BASIC and generates 6809 code as a coprocessor to handle video and sound stuff.
I can see several possible uses for them:
1) Full reprogrammable CGROM -> CGRAM. CPU can reprogram the full 256/512 characters through a dual-port RAM. Video generator read from it and CPU can write in it. Like PGC-1200, we could also have a two-buffer to allow a switch between video frames and have some dynamic sprites generations (CPU rewriting characters in one buffer while the video generator reading from the other, allowing smooth pixel-wise animation).
2) Making the ROMs shareable between the CPU and an external device.
3) Making all the RAMs shareable between the CPU and an external device.
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