About that overscan image display program. I see most of the image when it's running - just the right border seems to be wrong. I'm gonna get a bit technical:
Here is the output that is generated from my shifter model. This is a 512-pixel wide view that shows the output from the shifter at each point in time during the scanline (it ends in grey pixels).
- overscan-result.png (1.22 MiB) Viewed 4545 times
As you can see it breaks down at a point where the ST switches to high-resolution and back to low to avoid the end-of-scanline condition.
Here's the top of the same screen overlaid with shifter signals and state:
- overscan-overlaid.png (293.43 KiB) Viewed 4545 times
The light blue line is DE. The line below that is the shifter's resolution (white=hi, red=low). The line above the blue line (with the cycling intensity) is the pixel counter. The part where you can see a dark version of the image has red lines superimposed over it, which are LOADs; similar yellow lines that come after every 4th LOAD are the reload signal.
It's obvious what's happening here. While the ST is in monochrome mode for several cycles, the shifter is generating monochrome video for more than 48 mono pixels. Here's a close-up view of this section of the display:
- overscan-detail.png (350.11 KiB) Viewed 4545 times
I am expecting that the real shifter has a 2nd copy of the resolution register so that it doesn't take effect immediately.