What is the relationship between an MVS device, the Hercules configuration, and the host OS file (or client)?
The MVS operating system expects specific hardware devices to be attached and accessible at a specific hardware addresses. The address of a particular device is sometimes referred to as the channel and unit address, which is often abbreviated as CUU (or cuu). [The MVS versions we are legally using here date from a time when the addresses were three digits, one for the channel and two for the unit. Today’s MVS, excuse me OS/390 or zOS, utilize addresses of four digits, two for the channel and two for the unit – CCUU. The Hercules’ configuration file (and console commands) allow four digits, but you may omit the leading zero to effectively have a three digit address. MVS 3.7 and 3.8 (not to mention OS/360 MVT) will only understand 3 digit addresses.] This device type/address relationship is established during the System Generation process, when the MVS nucleus is built. The expected devices can be modified somewhat by a subsequent (mini-) System Generation process referred to as an IOgen
