Page:CTSS programmer's guide.djvu/44

 computer time required to position the appropriate history tape; if the user wishes to continue, the command will carry out the retrieval using the  command; otherwise the user is free to create a 1401 load-edit control card such that the desired file will be entered into the disk at the next disk-edit period.

System Service Changes and Supervisor Messages
Normally the CTSS system will operate for scheduled periods in blocks of time during each day. System operation will commence with messages typed on all consoles that are on (or can be remotely turned on and off) stating that the system is on and at what time the system will cease to operate again.

When the CTSS system is shut down, either on a scheduled basis or an emergency basis, the following procedure occurs. Each active user ceases to run, a cutoff message is sent to his console, all files are closed, all attached input-output units are disconnected, the user's program is created as a file of name  with class , the user is given a   command, and a message is placed in the user's console message file on the disk giving a summary of the cutoff process including the unread characters in the console input buffer.

Whenever there are individual or general disk failures as well as system changes of great importance, appropriate messages will be placed by the supervisor in each of the user console message files. A console message file (named "MESSAGE.FILE") is created for the user when necessary and is normally typed out by the  command, at which time it is deleted. During operation of his console, additional messages nay be generated; a user can read his console message file at any time with the  command, which will not delete the file. The console message files, which are created by the supervisor, use disk tracks of the user's quota.

For writing message files and for saving the user's program at the time of a system shut-down, the supervisor may have to take advantage of the feature which allows an extension of the user's track quota (see "Organization of the Disk Memory"). In this event the user should, the next time he is at a console, relieve this condition by dumping some of 30