<p>What interrupt would you hook from DOS to get the real-time clock?</p>

Read up on the
Intel 8259
family of Programmable Interrupt Controllers.  According to this, it's interrupts 8 (master) and 112 (slave).  Here's a very technical document on the 8259A:
http://pdos.csail.mit.edu/6.828/2008/readings/hardware/8259A.pdf