Cray CRAY-1 - Page 194

I
I/O MEMORY REQUEST CONDITIONS
The following conditions must be present for a memory request to be
processed:
1. I/O request
2.
No coincidence in rank A, B, or C
3. No scalar instruction in clock period two of a scalar sequence
4.
No fetch request
5.
No 176, 177, or 034 through 037 process
6.
No exchange sequence
7.
No 033 request
I/O MEMORY ADDRESSING
All I/O memory references are absolute. The current and limit registers
are 20 bits, allowing I/O access to all of memory. Setting of the current
and limit registers is limited to monitor mode.
REAL-TIME CLOCK
Programs can be timed precisely by using the clock period counter. This
counter
is advanced one count each clock period of 12.5 nanoseconds. Since
the clock
is advanced synchronously with program execution, it may be used
to time the program to an exact number of clock periods.
Instructions used with the real-time clock are:
0014jO
072ixx
Enter the real-time clock register with (Sj)
Transmit
(RTC) to Si
The clock period counter is a 64-bit counter that can be read by a pro9ram
through the use of the 072 instruction and can be reset only by the 0014jO
monitor instruction.
2240004
6-22
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