Page:CTSS programmer's guide.djvu/50

 However, in order that the operator may effectively perform these functions, the address of the console keys is continually checked by the Time-Sharing Supervisor. (The background system cannot use the address section of the keys.) When the supervisor finds certain codes, it simulates the following functions: Depressing the "Load Cards" button. Depressing the "Load Tape" button. Depressing the "Start" button. Initiating the "standard error procedure." The "standard error procedure" consists of storing the instruction counter in a specified location, and transferring control to some address which is the start of a post-mortem routine or a return to the background system monitor. The background system should therefore specify two locations for use by the Time-Sharing Supervisor. This is done by the following call to a supervisor subroutine: TSX      DEFERR,4 PZE      ERRILC,,ERRTRA where  contains:. is the name of a supervisor subroutine which defines the error procedure. is the address where the instruction counter will be stored, and control will be transferred to location.

is the subroutine used to read the date and time from the special Computation Center on-line printer clock. The original version of the program uses the "load channel" instruction (which is illegal), and is not interruptable. A new, interruptable version will be available.

The interval timer clock will be simulated for use by the background system. This clock will only operate when the background system is in control and should therefore be used to determine the amount of time run. MITMR, the Center's timer program (CC-193), may be used in the normal manner. In general on the 7090, when an interrupt occurs, the interrupted program is resumed at the location specified in the appropriate lower core register. When the 7090 is stopped by an  instruction, 36