hi,
The biggest problem is when you want to write to the screen memory. Currently i will just disable RGB output and write when needed. It is an crude solution, but it works.

Why? There are at least three solutions:
- make all write cycles to video memory at "fly back" time. Indeed, this time is not long but anyway...
- split each dot refresh cycle into two phases: a)read data from memory, b)write data in (or vice versa). Yes, at this way you need with double speed memory, however it is payment for possibility to write into video memory any time you need.
- use special designed VRAM. It is double-port memory with dual address/data/sygnal buses: one is for read, another - for write.

Regards,
Oleg