Ouch.

I remember I had a OS bug in my realtime class where I didn't same the ARM core's flags that get set of the compare operations.

In arm, it's two instructions to do something like `if (x > 3) { blah; }`. First do a subtract of 3 and x, and this will set the NZCV flags. If the result of `3-x` is negative, the N flag will be set.

Then do a branch if N instruction afterwards.

https://developer.arm.com/documentation/dui0801/g/Condition-...

So if there was a context switch / interrupt between the two steps, my code would take the wrong if arm. So I'd get things like

    if (x > 0) {
      y = 5 / x;
    }

throwing a divide by zero error, which was super confusing. And I could work around it by doing

    if (x > 0) {
      if (x > 0) { // For real this time
        y = 5 / x;
      }
    }

since it was very unlikely to happen twice in a row.

Eventually I fixed it, but that was fun.