Cray CRAY-1 - Page 196

Ii~TERRUPT COUj~TOOl~N COUNTER
The interrupt countdown (ICO) counter is a 32-bit counter that is preset
to the contents of the interrupt interval register when the 0014j4 instruc-
tion is executed. This counter runs continuously but counts down, decrement-
ing
by one each clock period until the contents of the counter are zero. At
this time, it sets the programmable clock interrupt request. The counter
then samples the
interval value held in the interrupt interval register and
repeats the countdown to zero cycle, setting the programmable clock
interrupt request at regular intervals determined by the interval value.
When the programmable clock interrupt request is set, it remains set
until a 0014j5 instruction, clear programmable clock interrupt request,
is executed. A programmable clock interrupt request can be set only after
I the 0014j6 instruction has been executed to enable the interrupt. A pro-
grammable clock interrupt request only causes an interrupt when not in
monitor
mode; a request set in monitor mode is held until the system
switches
to user mode.
CLEAR PROGRAM~ABLE CLOCK INTERRUPT REQUEST
Following a program interrupt interval, an active programmable clock
interrupt request may be cleared by executing the 0014j5 clear program-
mable clock interrupt instruction.
Following any deadstart, the monitor program should insure the state of
the
programmable clock interrupt by clearing programmable clock interrupt
requests (0014j5) and disabling programmable clock interrupt requests (0014j7).
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