* add a Signal struct which records its val and prev val, and can be used for state change things like interrupts
  the int controller coild have multiple for different prioritis, and could maybe be activated by multiple sources
* the remaining issue with the shell is that putchar_buffered requires the TX interrupt and TX enable/disable commands to work in order to trigger a write


* how will you add the cpu, which is not addressable, but is steppable, and also interruptable... what else?
* should you simulate bus arbitration?
* if the bus had an Rc<RefCell<Box<dyn AddressableDevice>>>, then it could be like AddressSpace used to be, with it's own Addressable, and then you could
  access one of multiple busses to do an op on it, rather than a single bus per machine
* can you get rid of the duplicate read/write functions in System?

* how do you pass in a mutable system to a mutable device?  At the very least, I need the AddressSpace, but that needs access to the devices/memory segments.
  I could make System contain refcells for submutability



* check all instructions in the docs

* make tests for each instruction