Some oscilloscope tests…
I was just testing some stuff. But thanks to charles to requesting a special trigger setup, to look at the non displayed scanlines. Using Ccovell’s screen test app (it was on the flash card already, and I have no parallel port to reflash the card with anymore), I set the scope trigger to A20 address line. Chris reads the upper address range (for the VDC status reg, part of VIRQ handling code). So the scope happens to trigger on VIRQ (there’s no HIRQ happening, so it’s nice and clean).
I was able to cleanly look at the blanking scanlines in vblank. Using Ccovell’s test app, I moved the screen around (vertically). Sure enough, you can put the display to show pixels in the blanking lines. Now, this won’t hurt anything. The three vsync (inverted blank scanline) scanlines are never outputted with pixel data – the VCE cuts that off. But before and after are.
This means that there’s a possibility of manually embedding stuff in the blanking. You know, stuff like Close Captioning… ah-hahahahahah. That’s hilarious. Imagine subtitling a Japanese game or just.. anything. That really makes me laugh 😀