That's also why Q-Link had the signature "squiggly menus" that acted as the AJAX spinner of the day... it drew the menus by changing the background color at a c

That's also why Q-Link had the signature "squiggly menus" that acted as the AJAX spinner of the day... it drew the menus by changing the background color at a certain raster line, and the Commodore's serial routines (completely implemented in software) had to disable interrupts while they sent or received data. I seem to recall Ray Kelm worked around this for the CNet/Image BBS sysop console in his custom high-speed serial routines, but I may be conflating stories. (Commodore's serial routines couldn't go far above 2400bps; Ray managed to get 9600 working reliably.)