Burroughs MCP - File System

File System

The MCP provides a file system with hierarchical directory structures. In early MCP implementations, directory nodes were represented by separate files with directory entries, as other systems did. However, since about 1970, MCP internally uses a 'FLAT' directory listing all file paths on a volume. This is because opening files by visiting and opening each directory in a file path was inefficient and for a production environment it was found to be better to keep all files in a single directory, even though they retain the hierarchical naming scheme. Programmatically, this makes no difference. The only difference visible to users is that an entity file can have the same name as a directory. For example, "A/B" and "A/B/C" can both exist; "B" can be both a node in a file and a directory.

Files are stored on named volumes, for example 'this/is/a/filename on myvol', 'myvol' being the volume name. This is device independent, since the disk containing 'myvol' can be moved or copied to different physical disk drives. Disks can also be concatenated so that a single volume can be installed across several drives, as well as mirrored for recoverability of sensitive data. For added flexibility, each program can make volume substitutions, a volume name may be substituted with a primary and secondary alternate name. This is referred to as the process’ FAMILY. For instance, the assignment “FAMILY DISK = USERPACK OTHERWISE SYSPACK” stores files logically designated on volume DISK onto the volume USERPACK and will seek files first on volume USERPACK. If that search has no success, another search for the file is done on volume SYSPACK. DISK is the default volume name if none is specified.

Each file in the system has a set of file attributes. These record all sorts of meta data about a file, most importantly its name and its type (which tells the system how to handle a file, like the more limited four-character file type code on the Macintosh). Other attributes have the file's record size (if fixed for commercial applications), the block size (in multiples of records that tells the MCP how many records to read and write in a single physical IO) and an area size in multiples of blocks, which gives the size of disk areas to be allocated as the file expands.

The file type indicates if the file is character data, or source code written in particular languages, binary data, or code files.

Files are protected by the usual security access mechanisms such as public or private, or a file may have a guard file where the owner can specify complex security rules.

Another security mechanism is that code files can only be created by trusted compilers. Malicious programmers cannot create a program and call it a compiler – a program could only be converted to be a compiler by an operator with sufficient privileges with the 'mc' make compiler operator command.

The MCP implements a Journaling file system, providing fault tolerance in case of disk failure, loss of power, etc. It is not possible to corrupt the file system (except by the operating system or other trusted system software with direct access to its lower layers).

The file system is case-insensitive and not case-preserving unless quotes are added around the name in which case it is case-sensitive and case-preserving.