 Installing a timer interrupt handler allows a TSR to run periodically (see ISA and programmable interval timer, especially the section "IBM PC compatible").The typical method of using an interrupt vector involves reading its present value (the address), storing it within the memory space of the TSR, and installing a pointer to its own code.The stored address is called before or after the TSR has received the interrupt and has finished its processing, in effect forming a singly linked list of interrupt handlers, also called "interrupt service routines", or ISRs.