Since the number of interrupts available is
chip specific and the interrupt vector table always ends at the last
byte of memory, the interrupt numbers corresponds to the interrupt
vectors in reverse order of address. For example, interrupt 1 will
use the interrupt vector at 0xfffc, interrupt 2 will use the interrupt
vector at 0xfffa, and so on. However, interrupt 0 (the reset vector
at 0xfffe) is not redefinable in this way; instead see section
for details on customizing startup.