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Intel
®
IXP45X and Intel
®
IXP46X Product Line of Network Processors—Intel XScale
®
Processor
Intel
®
IXP45X and Intel
®
IXP46X Product Line of Network Processors
Developer’s Manual
August 2006
164
Order Number: 306262-004US
Note that STM and LDM will each count as several accesses to the data cache
depending on the number of registers specified in the register list. LDRD will register
two accesses.
PMN1 counts the number of data cache and mini-data cache misses. Cache operations
do not contribute to this count. See
“Register 7: Cache Functions” on page 103
for a
description of these operations.
The statistic derived from these two events is:
Data cache miss-rate. This is derived by dividing PMN1 by PMN0.
3.7.4.3
Instruction Fetch Latency Mode
PMN0 accumulates the number of cycles when the instruction-cache is not able to
deliver an instruction to the IXP45X/IXP46X network processors due to an instruction-
cache miss or instruction-TLB miss. This event means that the processor core is stalled.
PMN1 counts the number of instruction fetch requests to external memory. Each of
these requests loads 32 bytes at a time. This is the same event as measured in
instruction cache efficiency mode.
Statistics derived from these two events:
• The average number of cycles the processor stalled waiting for an instruction fetch
from external memory to return. This is calculated by dividing PMN0 by PMN1. If
the average is high, then the IXP45X/IXP46X network processors may be starved of
the bus external to the IXP45X/IXP46X network processors.
• The percentage of total execution cycles the processor stalled waiting on an
instruction fetch from external memory to return. This is calculated by dividing
PMN0 by CCNT, which was used to measure total execution time.
3.7.4.4
Data/Bus Request Buffer Full Mode
The Data Cache has buffers available to service cache misses or uncacheable accesses.
For every memory request that the Data Cache receives from the processor core a
buffer is speculatively allocated in case an external memory request is required or
temporary storage is needed for an unaligned access. If no buffers are available, the
Data Cache will stall the processor core. How often the Data Cache stalls depends on
the performance of the bus external to the IXP45X/IXP46X network processors and
what the memory access latency is for Data Cache miss requests to external memory.
If the memory access latency for the IXP45X/IXP46X network processors is high,
possibly due to starvation, these Data Cache buffers will become full. This performance
monitoring mode is provided to see if the IXP45X/IXP46X network processors are being
starved of the bus external to the IXP45X/IXP46X network processors, which will affect
the performance of the application running on the IXP45X/IXP46X network processors.
PMN0 accumulates the number of clock cycles the processor is being stalled due to this
condition and PMN1 monitors the number of times this condition occurs.
Statistics derived from these two events:
• The average number of cycles the processor stalled on a data-cache access that
may overflow the data-cache buffers. This is calculated by dividing PMN0 by PMN1.
This statistic lets you know if the duration event cycles are due to many requests or
are attributed to just a few requests. If the average is high, the IXP45X/IXP46X
network processors may be starved of the bus external to the IXP45X/IXP46X
network processors.
• The percentage of total execution cycles the processor stalled because a Data
Cache request buffer was not available. This is calculated by dividing PMN0 by
CCNT, which was used to measure total execution time.