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Here we see the comparative levels of memory bandwidth available to each platform. In the case of our Pentium III 800, PC133 SDRAM provides all the bandwidth the processor is capable of using. The bandwidth-hungry Pentium 4, however, is much more appropriately fed by the dual PC800 RDRAM channels of the 850 chipset. It's interesting to note as well that 845 is able to extract a significantly greater amount of write bandwidth from PC133 than 815 was.
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Cachemem also measures the latency of each memory level as observed from the processor. Here we see the Pentium 4's excellent 2-cycle L1 cache in action, as the observed L1 latency for the 2.40GHz part is a mere 0.83ns. The older Pentium III, in contrast, observes a latency of 3.75ns from its L1 cache
Despite having to wait more cycles for its L2 cache (18 vs. 13), the Pentium 4's L2 cache is still faster as well, due to the threefold increase in clock speed versus the Pentium III. The Pentium 4 can get at its L2 area in 7.5ns, versus 16.25ns for the Pentium III 800.
The main memory latency figures are somewhat more interesting. The RDRAM-equipped Pentium 4 system exhibits the lowest latency at 38.3ns. The SDRAM-powered Pentium 4 shows a much higher latency of 63.3ns. This may seem strange to some, who may have expected RDRAM to exhibit a higher latency than SDRAM. In this case, the RDRAM subsystem's latency is lower due to the fact that it is running synchronously with the Pentium 4's FSB (100/400MHz), while the PC133 SDRAM system is running asynchronously, which introduces higher latencies. PC100 SDRAM would exhibit a much lower latency (although its bandwidth would be totally inadequate). The Pentium III trails with a memory latency of 100ns.
**To calculate latencies in a time-base, first calculate the Period of the processor by dividing 1 by its clock frequency, and multiply that Period by the number of clock cycles shown on the graph. For example, the Pentium III's main memory latency is:
(1 / 800,000,000Hz) * 80 cycles = 0.0000001 secs = 100ns.
Latencies are typically expressed in Nanoseconds. One nanosecond is equivalent to 1x10^-9 seconds. Given the Pentium 4's extremely low L1 latency, though, it may soon be more practical to represent such times in picoseconds (1x10^-12).