The simplest way to execute a TSR is to transfer control to it explicitly from another program. Because the TSR in this case does not solicit processor control, it is said to be passive. If the calling program can determine the TSR's memory address, it can grant control via a far jump or call. More commonly, a program activates a passive TSR through a software interrupt. The installation section of the TSR writes the address of its resident code to the proper position in the interrupt vector table (see Section 7.4, “DOS Interrupts”). Any subsequent program can then execute the TSR by calling the interrupt.
Passive TSRs often replace existing software interrupts. For example, a passive TSR might replace Interrupt 10h, the BIOS video service. By intercepting calls that read or write to the screen, the TSR can access the video buffer directly, increasing display speed.
Passive TSRs allow limited access since they can be invoked only from another program. They have the advantage of executing within the context of the calling program, and thus run no risk of interfering with another process, which could happen with active TSRs.