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 intentional. The user number of another console must be determined by supervisor subroutine calls which give the desired user number on the basis of the console location, problem number and/or programmer number.

Disk Memory Control
For a good understanding of the disk control subroutine, the following list of considerations concerning the use of the disk may be helpful.


 * 1) The user is able to write and maintain permanent program and data files on the disk.
 * 2) System programs (commands and standard library) are permanently recorded on the disk.
 * 3) The user has only symbolic reference to his files.
 * 4) The user is able to read and write many files simultaneously, and, for the sake of efficiency, may specify in which logical module a new file is to be written. In this way much unnecessary seek time may be avoided by using two separate modules for a simple read-write operation.
 * 5) The user is not able to reference any files not authorized to him.
 * 6) The user is able to initiate files in different modes, such as temporary files, permanent files, or permanent read-only files.
 * 7) In order to utilize the maximum storage capacity of the disk file and to be compatible with any future I.B.M. programs using the disk, Format 1 (i.e., a single record per track) is used.

A high level of user information protection can be achieved using the 1301 disk unit with the CTSS 7090. Complete protection is maintained (up to machine error and user identification errors) of all information on the disk unit. All users and systems use a single standard supervisor subroutine for reading and writing the disk. Because of the input-output trapping and memory protection features, mishaps will not occur if the user executes wild transfers, tries to store spurious information in the subroutine, or tries to execute explicit instructions for writing the disk.

It is desirable, from the point of view both of programming and of disk administration, that the user have no notion of the absolute 23