How to simulate a hardware interrupt?
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I'm not sure about how to simulate one, but I remember using the standard PC keyboard interrupt to trigger events in a C++ (state machine) program assignment I wrote. My lecturer just loaded it onto his PC with all his work on it to try it out and it worked and importantly then returned normal function to the keyboard interrupt I'd used - Interrupt 9. I used a book 'The undocumented PC for information which is/was very good for this sort of stuff. As you mention a USB device, try looking at devices that provide Inputs and Outputs - they often have one or more inputs that can be set to trigger an interrupt when their input state is changed - by say a switch. I've used mostly internal PC I/O cards which can trigger interrupts but also USB devices which provide the same functionality. Serial Port connected devices also have/had this functionality - even if your PC may no longer have a 9 pin serial port.