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 location where his files of information are stored in the disk. Rather, the user will refer to his files only by symbolic names and logical module number. Furthermore, if the user does not specify the logical module number, it will be assigned for him by the subroutine. Each of the user's files has two names; commonly the second name is descriptive of the type of file, as "FAP," "DATA," "BSS," etc. Each name is at most six BCD characters long; the characters may be any alphanumeric characters, but special characters should not be used; the special characters are used to distinguish the names of certain files created for the user by the supervisor.

Normally different programmers on the same problem are treated as different users. To allow convenient cooperation between programmers, such as students in classes or group projects, there is a feature which makes it possible to have files common to several different programmers with the same problem number. To gain access to these common files (referred to by programmer number zero) a special supervisor call must be given which places the user in a "common file mode;" another supervisor entry allows the user to return to the normal "personal file mode." More elaborate cross-referencing of files can be accomplished by the  or   disk editor control cards described in Chapter 5.

Organization of the Disk Memory
When the user initially requests use of CTSS in his Computation Center Problem Application Form he may also request the allocation of a number of tracks on the disk. If none are requested, an initial allotment will be made automatically which may later be extended by request. The track allocation information is kept on the disk in the master file directory of all users, which in addition contains the location of each user file directory. A track usage table is also maintained on the disk.

Each user has on the disk a user file directory (UFD) which contains a 4-word entry for each of his files. The first two words contain the file name and class name of the file. The third word contains the mode of the file in the prefix, a document number (a number assigned internally to each file, for tracing and retrieval purposes) in the 24