Slight issue with the example is the size of the dual ported memory
The lcd driver is fishing the video in a round robin on 15 address lines which is 32k locations
hence substituting
MSM6255M5M5256D with CY7C007
>The ISA bus example is limited to 10 which is only a 1K or 1024 size.
its PCB is limited to dip48 ram chip, but not the overall design concept. rpi has 26 gpio, enough for 32KB sram
>If i rip out the video ram and opt for a dual port i would need an equivalent size dual port of 32kx 8 because of the processor mapping[6303]
CY7C007
>The lcd driver chip is capable of addressing 32k for video access and the display only requires 16k of video data so i think it maybe being told where to start and end.
do you have another logic analyzer to sniff SRAM content while this one is running?
>This complicates matters because it may use this to zoom the screen or add the little box overlays and text to the screen.
I think it works like this....suspend the driver chip and write new data to the video buffer. Address the driver chip and tell it where to start and end in a 16K window.
for sniffing MSM6255 even something as basic as $4 CY7C68013A board from ebay should be enough, you only need hmm 10 bits at 8MHz?
>My only hope would be to wait for the driver to suspend.As soon as it goes on its merry way for 16K locations grab the data off the bus in parallel with the graphics driver store it and buffer and reclock to a dac and vga one frame at a time using fast memory.
this isnt the nineties, just throw rpi at it and display over hdmi/vga at whatever resolution you want. Only drawback is waiting ~25 seconds for rpi to boot.
Even if you want to do it the hard way with fixed logic you dont need a dac, its B/W signal after all so one pin and resistor ladder with fixed color will suffice.
>Trouble is all the 640x200 9 inch screens are mega expensive and not worth it for an old logic analyser.
vintage hardware, vintage collectors pricing ;-)
EDIT: another alternative is
https://github.com/hoglet67/RGBtoHDMI , again rpi, but this time with cpld to grab digital mda/cga/ega signals
whole $20 worth of hardware + rpi zero
https://stardot.org.uk/forums/viewtopic.php?f=8&t=20404 (wonder if seller is the same IanB as on eevblog forum)
As a bonus its Bare Metal ARM programming, meaning no waiting for the thing to boot.
looking at
http://pdf.datasheetcatalog.com/datasheet/oki/MSM6255.pdf pages 21-24 and 30 MSM6255 is just scanning its output in reverse order on UD/LD pins. You will have to write your own code interpreting captured data.