Willard The C-64 used different key combinations on the keyboard to print the graphic characters in strings in the C-64 code. Is there something like ALT code to print these Unicode characters in BASIC engine code?
Not really. I hacked something into the original ESP8266 firmware so you can use the I-don't-even-remember-which modifier key to enter characters above 127, but I don't think that still works in the SDL version.
What would be very easy is to add a keyboard layout for entering graphics characters to /sys/kbd. The question is what that should look like, though. Unlike the Commodore devices contemporary PC keyboards don't have graphics characters printed on the key caps, and I guess it would be very hard to memorize the layout unless you're a PETSCII artist. (OTOH, there is the keyboard layout popup which might help with that.)
I was also thinking about something like the emoji menu that mobile device input methods have, but that would be a bigger effort to implement...
Willard I noticed there are graphic characters in Print strings in the Tetris game code to print the playing screen in the program. How did these graphic characters get inserted in the Print strings?
I don't remember anymore how they originally got in there, but I'm sure I used a script to convert them from the legacy encoding to UTF-8.
