I've been working on fleshing out a supervisor mode with a goal towards being able to do multiprocessing in the unix way. The basic work is complete (protected opcodes, hardware and software interrupts that execute in supervisor mode, etc), but I'm working on the nuance now. In particular, I'm testing different ways to pass information from user space into kernel space. Since my current method of parameter passing is solely via the stack and the stack pointer swap out as part of the move to supervisor mode (supervisor stack pointer), this is mostly an exercise in C semantics now. My exception handler pushes the original stack pointer onto the supervisor stack before jumping to the exception handler, and so now I'm just working though the most sane way to reference that element (which isn't an argument to the interrupt handler!), and then use it as an index to pull out the other info on the user stack I care about.