Single IRQ line
Today the Mock Turtle uses two IRQ lines: one for the host message queue and one for the serial interface.
I suggest to use a single IRQ line for the Mock Turtle and then in software handle the distinction by using the status registers.
Why I suggest this?
Because the schema works well when there is an IRQ controller in the middle that collects those IRQ from multiple sources (e.g. the VIC from general cores). But this is not always the case for simple designs.
Not doing this may introduce a useless dependency with an IRQ controller.