There will be two buffers: one that gets filled as characters are received via the RS232, and one that contains the characters that have to be shown. The receiving end will work like the circular RS232 buffer I made earlier -- except that the receiving buffer will not be circular, and will discard any characters that exceed the length of the buffer. With a special starting and opening character (00 and FF?), the computer can signify the start and end of a single string that is to be displayed. I'm leaning towards a buffer length of 64 characters -- long enough to show "Ghost in the Shell - Tachikoma Special" and an episode number. ;)
The display buffer will be as large as the reception buffer. With indirect adressing I will create a 'window' of 6 characters to display. A timer (or delay loop) will shift the window onwards, causing the scrolling action. When the last character is reached, the buffer will be padded with spaces.
If, after scrolling through the display buffer, there is another text waiting in the reception buffer, the reception buffer is copied into the display buffer before the display routine is called again. This also means that there is no way for the machine to interrupt the display routine -- maybe I would need that?
I'm also thinking of building the 7-segment display as well, just because I can. It would be cool if the computer could query the display module as to its capabilities (numbers-only or text), but that's for the (far?) future.