(BOS) An early IBM operating system.

According to folklore, BOS was the predecessor to TOS on the IBM 360 and it was IPL'd from a card reader. It may have been intended for very small 360's with no disks and limited tape drives.

BOS died out really early as disks such as the 2311 and 2314 became common with the IBM 360, whereas disks had been a real luxury on the IBM 7090.