IBM System/7 Functional Characteristics - All about the IBM 1130 ...

The processor module contains two 16-bit interval timers. Each timer is a one-wordbinary counter that decrements the value of its contents once each 50-microsecond interval.Upon decrementing from 0 to —1, the timer presents an interruption request to theprocessor. The interruption request occurs when the timer has counted one more countthan the specified number. Unless the timer is stopped, it continues to count and presentsan interruption request again whenever the transition from 0 to —1 is made. Thetimer value follows the sequence —1, —2, ... , —32768, +32767, ... , +1, 0, —1, ...Though timers are automatically decremented by the machine, they are separatelycontrolled by programming. Timers can be started, stopped, read, or set to a value by I/Ocommands. A timer that is running can be read without disturbing its operation, but atimer must be stopped before it can be set to a new value.The interval timers are reset to —1 (hexadecimal FFFF) when System/7 power is turnedon. If a value is not set in a timer after system power is turned on and the timer is started,the timer will count the maximum time value (see sequence above) before presenting aninterruption request to the processor.The timers are incremented by a crystal controlled oscillator. The accuracy of thisoscillator is as follows:Nominal Accuracy10 mHz +0.0%-0.025%(Maximum loss of 21.6 seconds in 24 hours.)Calibration Accuracy9,999,055 Hz ±0.003% with a 90% confidence factorBoth interval timers are prepared simultaneously by a single prepare I/O command. Theprepare command assigns the same priority level and sublevel to both timers, with a moduleaddress of 000000 and a device address of either 000000 or 000100 for timer 0 or 1respectively. The prepare command is described further in Chapter 4 under "Input/OutputInstruction." There is no interrupt status word associated with the interval timers becausean interrupt from an interval timer has only one meaning.Set Timer0 5 8 11 16 20 26 31Op code Fun Zeros Mod DA MA0 0 0 0 1 X X X 0 0 1 0 0 0 0 0 0 0 0 0 0 0 0 X 0 0 0 0 0 0 0 01111 I I 1 1 I 1 I I I 11111 11111s•■••••■„....." •••■■••••,■■•• •••■••••■,...., •••••••■■••••••• "••■••••■■■■•,0 8-F 2 0 0 0 or 1 0 0The addressed timer (specified by the device address field) is loaded with the value residingin the index register (R), or the accumulator if R=000. This value is the number of50-microsecond intervals to be counted, after a start timer command is issued, before thetimer interrupts the system. Timers must be stopped before they are set to a new value.Condition code 2 is set if the addressed timer is already running or has an interruptionpending.+AeRnp!mpriiwn!AIMeie!,ilA pp milp gongungag