Note on the interrupt vectors:
Since there is no memory protection, application programs may redirect interrupts to go through them, rather than to the regular handler.
This allows "hot key" programs to work - they intercept the keyboard interrupt, look for their hot key, and pass control to the regular handler if any other key is struck.
This is also the mechanism used by viruses! For example, a common virus tactic is to intercept the "run a program" DOS system call in order to infect the program before it is run.