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Unformatted text preview: operate at around 3 MHz for random accesses and at 6 MHz for sequential (page mode) accesses. The CISC microprocessors of the time could access memory at most at 2 MHz, so memory bandwidth was not being exploited to the full. RISC processors, being rather simpler, could be designed to operate at clock rates that would use all the available memory bandwidth. Neither of these properties is a feature of the architecture, but both depend on the architecture being simple enough to allow the implementation to incorporate it. RISC architectures succeeded because they were simple enough to enable the designers to exploit these organizational techniques. It was entirely feasible to implement a fixed-length instruction load-store architecture using microcode, multi-cycle execution and no pipeline, but such an implementation would exhibit no advantage over an off-the-shelf CISC. It was not possible, at that time, to implement a hard-wired, single-cycle execution pipelined CISC. But it is now! Clock rates As footnotes to the above analysis, there are two aspects of the clock rate discussion that require further explanation: The Reduced Instruction Set Computer 27 1980s CISC processors often had higher clock rates than the early RISCs, but they took several clock cycles to perform a single memory access, so they had a lower memory access rate. Beware of evaluating processors on their clock rate alone! The mismatch between the CISC memory access rate and the available bandwidth appears to conflict with the comments in 'Complex Instruction Set Computers' on page 20 where microcode is justified in an early 1970s minicomputer on the grounds of the slow main memory speed relative to the processor speed. The resolution of the conflict lies in observing that in the intervening decade memory technology had become significantly faster while early CISC microprocessors were slower than typical minicomputer processors. This loss of processor speed was due to the necessity to switch from fast bipolar technologies to much slower NMOS technolog...
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- Spring '09