When a context switch occurs between two processes, the hardware page table in the MMU must be updated with the memory mapping of the new process. In fact, at any given moment, a particular virtual memory address means completely different things to different processes. Virtual address 1000 in my process is completely different from virtual address 1000 in your process. Of course changing the hardware page table mappings is something that can only be done in kernel mode. We will learn more about this when we cover Memory Management in a few weeks.
Signals are a primitive form of interprocess communication.
Hardly any information is sent along with the signal. All you know is that it was sent. Processes can handle or ignore signals. From the executing process, a software interrupt is entirely synchronous whereas device-generated interrupts appear asynchronous. There are a fixed number of both signals and interrupts.