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At both 266MHz and 300MHz clockspeeds, the Crucial PC2100 DDR
SDRAM DIMM showed only very minimal performance boost at CAS 2
over CAS 2.5. With standard SDRAM, the performance difference
between CAS 2 and CAS 3 is quite substantial, especially during
reads. One would have expected the same thing with DDR SDRAM.
However, this apparently isn't true.
CAS latency now appears to have very minimal effect on
performance. There could be several reasons for this. First, the
difference between the faster and the slower CAS latencies have
now narrowed from 1 clock cycle to only half a clock cycle. That
will account for the smaller gap in performance difference.
The 2n-bit prefetch mechanism of DDR SDRAM probably also
helped to reduce the gap by minimizing the effects of the latency.
It does this by delivering twice as much data for each data
transaction. Thus, the effect of the extra half-cycle latency
would have been halved again. This is further accentuated if burst
transactions are performed. Then, the effect of the extra
half-cycle latency would become really negligible.
In addition, while you can reduce the CAS latency from 2.5 to
2, the two other important latencies of tRCD (minimum RAS to
CAS delay) and tRP (minimum row precharge time) have
increased to 3, which is one clock cycle higher than that of
normal SDRAM. That increases the importance of the tRCD and tRP
latencies and minimizes the effect of lowering the CAS latency
from 2.5 to 2.
As such, you are unlikely to notice any performance difference
between CAS 2 and CAS 2.5 in DDR SDRAM DIMMs. For those of you who
are interested in the numbers, the greatest boost that running at
CAS 2 could give this Crucial DDR SDRAM DIMM was only a negligible
1.1%.
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