In the 8086 architecture, which is the architecture that DOS targeted, memory references are composed of two parts: a 2-byte segment “identifier” and a 2-byte offset within the segment.

When the 8086 processor recovers from a hardware reset ... FFFF0 is really really really really close to the bottom of our memory map. Indeed, it is only 16 bytes away. So the first instruction ...

When [Ken] was examining an 8086 die, however ... then the interrupt context will wind up at some incorrect memory location. You could fix this in several ways. The way Intel did it was to ...