Page:CTSS programmer's guide.djvu/29

 region and reached via dummy subprograms which issue the appropriate supervisor call

Time-Used Messages
Since usage time and computation time are different from real time, the user may periodically want to know how much computer time he has used. He may request from the supervisor a message of the following form: MIN. USED = 12.070 + 2.005 IN 24.011 + 5.725 In this example, the user has logged 2.005 minutes of computer time since he last requested a time message, which was 5.725 minutes (real time) ago; at that time he had used 12.070 in 24.011 minutes; he has now used a total of 14.075 minutes since he logged in 29.736 minutes ago. Comparison of these sets of figures affords an estimate of his rate of service.

The time-request is made by typing in a special message at any level of console input; the message convention is initially set to "TIME" followed by carriage return. If the user wishes he may by use of the  command either change the time-request message to another set of characters or discontinue the facility altogether.

Interval Timer Clock
To facilitate running programs for a limited amount of time, the CTSS IBM 7090/94 has an interval timer clock available. This clock is completely under control of the supervisor; its action is as follows: location 5, memory A, is incremented in the units portion every 1/60 sec; whenever it overflows on a count of 2$35$, an interrupt occurs which, if the clock is enabled, causes a trap to location 7 and the trap location to be stored in location 6.

The supervisor uses this clock both for interrupting programs and for time accounting. Base-time and day-of-the-month information are obtained from the on-line printer clock. The supervisor can, however, simulate the interrupt clock behavior for each user. By supervisor calls similar to those of MITMR (memo CC-193), the user can program for nested interrupts and computation time readings. 15