When an interrupt is triggered in real mode, the CPU looks in the IVT to find the address of the interrupt procedure in DOS or the BIOS, and then calls it in a way that's much much slower and slightly different than a normal call. (interrupt procs must be terminated with an iret) Apps can revector interrupts to whatever they please, allowing realtime keyboard lookups and such. Much more complicated in protected mode. (IDTs and LDTs and virtual86s], argh !)