We do need a facility for this sort of thing, something that can act as the Blue Screen of Death or Guru Meditation for the C65GS.
However, like those, we can make it a little more distinctive.
The current model I am looking at is to have a 64x64 256-colour image in the top left of the display. To the right of this will be the message, either a version indicator on boot, or an indication of what has gone wrong, e.g., Page Fault, Illegal Action of Some Sort etc. Below this will be the lines of output.
To do this, I need to enable 16-bit text mode, so that I can have some full-colour characters on the screen. These characters use one byte per pixel instead of one bit per pixel, with the colour looked up from the 256-colour VIC-III palette. So I need a colour cube in the palette.
I also need a way to load the 4K pixels into RAM. Fortunately, recent refactoring of the ROM loading code to make it easier to load a CHARROM means I have a handy fs_readfileintomemory function in the hypervisor already that I can re-use to load the pixels.
Okay, so I can load pixels. But I need pixels to load. So I wrote a little program, pngprepare that can convert a 64x64 PNG file to the right format. Incidentally, it can also be used to create the default character ROM for the C65GS.
Along the way I also discovered a nasty little bug in the VIC-IV full-colour text mode, where the character generator accidentally skips one extra character when switching from full-colour back to normal VIC-II/III character mode on a raster line. This is pretty easy to work around, but it is rather annoying. I'll have to work my way through the character generator state machine and work out where the problem lies and fix it at some point though.
So with that done, I could create a BOOTLOGO.G65 file and put it on the SD card.
Then, with a bit of fiddling around, I had managed to massage the kickstart ROM into displaying the logo at boot time, along with the relocated version message, with the regular boot messages below, like this:
Thanks go to Adam on IRC for providing me with the initial image. The colour distortion artifacts are my fault, not his. I'll have to look into that at some point.
For those who have been reading lately, you will notice that the C64 charset has been loaded in the process, and thus is being used by kickstart at this point. If the screenshot were taken earlier in the boot process, or if CHARROM.G65 was missing from the SD card, then it would be showing our new default font, the modified VGA 8x8 one. That font is going to be replaced by an improved default font fairly soon.