Sixteen contiguous interrupt service fetch packets (ISFPs), each corresponding to a condition in the interrupt flag register (IFR). The IST resides in memory accessible by the programme memory system. The IST must be aligned on a 256-word boundary (32 fetch packets x 8 words/fetch packet). Although only 16 interrupts are defined, space in the IST is reserved for 32 for future expansion. The IST’s location is determined by the interrupt service table pointer (ISTP) register.