Beginning and End of tape

On 7-track and 9-track tape units, a reflective strip at the beginning of a tape indicates when the tape is positioned at its load point; a load point indicator bit is set in a sense byte, and is recognized by the operating system. A reflective strip also marks the logical end of the tape. If a reflective strip is encountered during a write operation, the hardware signals a unit exception condition in the interrupt to the processor.

IBM magnetic tape subsystems such as the IBM 3490, 3590, and 3592 use tape cartridges and not tape reels. Subsystems that use tape cartridges have internal mechanisms that sense the beginning and end of tape, and do not use reflective strips. The effect on software is the same as with reels.