CMS filesystem
From Just Solve the File Format Problem
The CMS minidisk filesystem is the original filesystem of the IBM mainframe z/VM operating system (previously known as VM/CMS).
It has no concept of subdirectories, all files being stored in the root directory. However, it was normal to give each user their own virtual disk, to keep each user’s files separate. Users could share disks with each other. Each disk was mounted using a drive letter A-Z, with drive A normally being each user’s own disk. Hexadecimal IO addresses were used to identify disks in a user-independent manner.
