[Coco] CoCo 3 FPGA?

[jdaggett at gate.net]

On 31 Jul 2007 at 12:54, Joel Ewy wrote:

> Why not a real 12-bit 4096-color mode as a less memory and (possibly)
> CPU intensive alternative to 15- or 16-bit modes? 
*****************

Memory wise 12 bit color will require as much memory as 16 bit unless you 
compress and eliminate the unused nibble. The issue really is not how much 
memory needed for a scren but how fast that memory can be written. 

IF  you have 640x480 with 16 bit color, 2 bytes per pixel. You need 614,400 
bytes of info. The 6809 takes a minimum of ten machine cycles to do a 
double byte load and store for a transfer. At 80nS machine cycle, 12.5MHz 
buss speed, to transfer a full screen will require ~24.6 milliseconds.  
Compared to the current Coco's 640x200 at 2 bit color of ~9.26 milliseconds 
for double byte load and store. Thee concern is time. With a verticle refresh 
of 59.94 Hz, that means each screen needs ~16.68 milliseconds to refresh. 

For 16 bit color the refresh time needs to be reduced or the data transfer 
into video memory needs to be speed up. To meet 640x480 16 bit color to 
write a full screen would need a buss speed of at least 20MHz. With the 
current setup of the CPU/GIME chip IDMA transfer would require 25nS 
dram max. To get that you need to either have SRAM or SDRAM. 

Finally you need to  have the software in SECB and/or Nitros9 su pport that. 
Having the hardware doing it is one thing. If there is no software to take 
advantage of it is then not worth the hardware effort. 

It is possible to get 20 MHz buss speeds with an FPGA. It is very much 
possible to expand many functions of the GIME chip. WHo is on board to 
upgrade software to take advantage of the  new features?


james