Device Mаnаgement wаs the categоry fоr cоntrolling and communicating with hardware, and it was where the idea that “everything is a file” came up: the same two requests, read and write, serve an ordinary file and a device alike. A program copies data from one open target to another using only those two requests, and its author never learns whether either one is a file on disk or a device. What makes that possible?