PID Process ID ────────────────────────────────────────────── This is a 16-bit value that uniquely identifies a running program or installed TSR. The PID or any program is the segment address of its PSP. ────────────────────────────────────────────── ■ DOS fns 52H and 50H are used by TSRs to make sure that DOS uses their PSP instead of the PSP of the program they interrupt.