Terminate and Stay Resident (TSR) is a computer system call in DOS  computer operating systems that returns control to the system as if the program has quit, but keeps the program in memory, to be revived later by a hardware or software interrupt. Many software vendors use the call to create the appearance of multitasking, by transferring control back to the terminated program on automatic or externally-generated events, such as pressing a certain key on the keyboard. Some TSR programs are effectively device drivers for hardware not directly supported by the operating system, while others are small utility programs offering frequently-used functionality such as scheduling and contact directories.