I'm trying to figure out if it would be possible to read data from and to a PAR from a PC while an SGL program was running. I know it sounds like a pretty weird question, but I was working on a backup routine and just thought I'd find out once and for all.
I don't see why not, as long as your program supports it. The PAR can't cause interrupts, so you have to poll the status flag every once in a while to see if the PC wants to talk. You could use the built-in communication routines, but you'd have to make some glue functions as they don't use the SuperH calling convention IIRC. Charles MacDonald's site should be useful, and there's a disassembly of the PAR communication routines at Y. Tanaka's site.