The IBM 2310 disk drive stored sectors of 320 words (640 bytes) plus a one-word sector address. A cylinder consisted of two tracks on the top and bottom surfaces of the 2315, or of one platter on the 1316 disk pack used in the 2311. Each disk cylinder contained eight sectors. A sector was logically divided by the monitor into sixteen "disk blocks" of 20 words each (40 B); the disk block was the unit of allocation for files. The system distinguished between "system cartridges", which contained the monitor and utilities along with user data, and "nonsystem cartridges", which contained user data only. A system cartridge contained the cartridge id and the "cold-start program" (bootstrap code) in sector 0 followed by a "communications area" and the resident monitor in sectors one and two. Sectors three through five contained the "System Location Equivalence Table" (SLET)—a directory of all phases of all monitor programs. Other control information filled out the first track.