emulation of a cbm8032 screen on a serial terminal (need some advices)

Hi

I'm thinking to the emulation of the cbm8032 screen on a pocketerm modified

I'thinking to modify the pocketerm as follow:
- spi bus to communicate with the avr 128A1 xplained.
  (apparently with the propeller it is possible to reach about 1 Mhz but not
beyond)
- font modified in the propeller firmware to support commodore graphic
characters.
- font switching via a command send to the terminal.  
- keyboard code modified to create commodore codes 
  (not really worked on this par but I have a few idea coming from my old
emulator)

The fastest writing loop on the cbm screen is probably this sequence:

Loop   sta $(xx),y    6  cycles
	 iny            2  cycles
       bne            3  cycles

so a t 1 Mhz 83333 char /sec
(1000000/12 * 10) = 833330 bits /sec   (8 bits + start + stop)

if the addess change:  from the previous write 
  jocker + line + col

but I guess that the writing in the screen is rarely random...
  
 while we are on the same line, we send the byte one after the others...
   char + char + char
   
but if the char is the same the char + a jocker + a rep count is sent.
   char + escape code + rep count

all write a not immediate but after a short time so there is a chance
to create a string with a repeat count.

My problem is the scroll, the scroll is really expensive, I'm thinking to
Way to detect a scroll with a code not too expensive but without any
Modification in commodore kernel.

If I could find a way to detect it, I could send a scroll up/down command
To the propeller

Any idea for a scroll detection ?

Thanks

--
Didier










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