How do I implement the text mode.
The usual method is to send the video data Byte to the 8 higher address inputs of the character ROM to select the character pattern, and the low bits of the video line counter to the low ROM address inputs (eg. A0-3 for an 8x16 font) to select the pattern row. The character ROM then outputs 8 bits which are latched and ...
(Preface, this is RC.SE, not EE.SE, so less appropriate for such questions - so I wont go into schematic details)
a) Video RAM Access
As usual there are many ways. The most common are synchronizing CPU speed and video to
use 'the' other half. Or separate both RAMs and access only during pause. Or use a dedicated video chip already offering separation.