Bus-based systems add another layer of complication to performance analysis. Platform-level performance involves much more than the CPU. We often focus on the CPU because it processes instructions, but any part of the system can affect total system performance. More precisely, the CPU provides an upper bound on performance, but any other part of the system can slow down the CPU. Merely counting instruction execution times is not enough.
Consider the simple system of Figure 4.28. We want to move data from memory to the CPU to process it. To get the data from memory to the CPU we must:
• read from the memory;
• transfer over the bus to the cache;
• transfer from the cache to the CPU.
Figure 4.28 Platform-level ...