I wrote an operating system for a number of different DSPs in 2001. The OS was C++, with different pieces written in assembly as needed. The design of the OS re

I wrote an operating system for a number of different DSPs in 2001. The OS was C++, with different pieces written in assembly as needed. The design of the OS required that we take the lowest priority interrupt to handle rescheduling as we returned from interrupts, and made sure the correct task was running, etc. At the end of that lowest priority interrupt, we wouldn't RTI if a high priority task was switching in, we'd just drop back in to user space, and handle the rest of the RTI functionality. One particular part had a fixed size hardware stack. When an interrupt occurred, it'd push stack/frame and a single register (ALU results), and begin executing the ISR. What we were seeing was occasionally we'd blow the hardware stack. Now, I've given enough data here to debug it, clearly we weren't resetting the hardware stack in the low priority interrupt, but at the time we were STUMPED. I spent a full day working and thinking about the problem, and that night, I dreamt what the answer was, came in, wrote the three lines of assembly, and it all worked perfectly. It was the first (and unfortunately not the last) time I debugged software while sleeping.