Page:CTSS programmer's guide.djvu/24

 is received by the supervisor, "WAIT" is typed back. When the command is completed, "READY" is typed back. Where possible, the computer responses are in the opposite color from the user's typing. A command may be abandoned at any stage, including during the typing of the command line or during command output, by giving the "quit signal" peculiar to the console.

Command Initiation
A user starts a command when he completes a command line at his console and is automatically placed in a waiting command status. The time-sharing supervisor uses the interrupt clock feature every quantum of time to interrupt the user program being run and assign the users in waiting command status to a working status with an initial priority based on the size of the requested command program.

Program Termination
 A foreground program leaves the working status by two means. It can re-enter the supervisor in a way which eliminates itself and places the user in a dead status; alternatively, by a different entry the program can be placed in a dormant status (or be manually placed there by the user giving a quit signal). The dormant status differs from the dead status in that the user may still restart or examine his program.

Input and Output Wait States
 User input-output is through each typewriter via the supervisor, and even though the supervisor has a few lines of buffer space available, it is possible for a program to become input-output limited. Consequently, there is an input-wait status and an output-wait status, both similar to dormant, into which the user program is automatically placed by the supervisor whenever input-output delays develop. When buffers become nearly empty on output or nearly full on input, the user program is automatically returned to working status; thus waste of computer time is avoided.

Scheduling
In order to optimize the response time to a user's command or 10