Operations randomly fail under DCPU Toolchain's dtemu

[unbibium]

If you run DCPU-Basic under the emulator supplied with the DCPU Toolchain, then it will come close to working, but fail quickly. The startup screen will probably display slightly wrong, and if you load the directory and list it, one of the lines might display the line number with extra decimal places or something. If you try to load and run a program, it will crash eventually.

As near as I can tell, this must have something to do with interrupt handling. I was able to reduce the symptoms a bit by pushing EX to the stack during interrupts, but that didn't fix everything.

I may end up rewriting the whole kernel, or making it swappable, so that I can isolate this problem.