Comment by hermitdev

> because the simulator had a bug

I had something similar happen when I was taking microcomputers (a HW/SW codesign class at my school). We had hand-built (as in everything was wire wrapped) 68k computers we were using and could only download our code over a 1200-baud serial line. Needless to say, it was slow as hell, even for the day (early 2000s). So, we used a 68k emulator to do most of our development work and testing.

Late one night (it was seriously like 1 or 2 am), our prof happened by the lab as we were working and asked to see how it was going. I was project lead and had been keeping him apprised and was confident we were almost complete. After waiting the 20 minutes to download our code (it was seriously only a couple dozen kb of code), it immediately failed, yet we could show it worked on the simulator. We single-stepped through the code (the only "debugger" we had available was a toggle switch for the clock and an LED hex readout of the 16-bit data bus). I had spent enough time staring at the bus over the course of the semester that I'd gotten quite good at decoding the instructions in my head. I immediately saw that we were doing a word-compare (16-bit) instead of a long-compare (32-bit) on an address. The simulator treated all address compares are 32-bit, regardless of the actual instruction. The real hardware, of course, did not. It was a simple fix. Literally one-bit. Did it in-memory on the computer instead of going through the 20-minute download again. Everything magically worked. Professor was impressed, too.