Com 226 comp trouble shooting II theory book
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- Memory Speed and Wait States
| Memory Considerations Memory has become far more important than just a place to store bits for the microprocessor. It has proliferated and specialized to the point where it is difficult to keep track of all the memory options and architectures that are available. This part of the chapter reviews established memory types, and explains some of the current memory architectures. Memory Speed and Wait States The PC industry is constantly struggling with the balance between price and performance. Higher prices usually bring higher performance, but low cost makes the PC appealing to more people. In terms of memory, cost-cutting typically involves using cheaper (slower) memory devices. Unfortunately, slow memory cannot deliver data to the CPU quickly enough, so the CPU must be made to wait until memory can catch up. All memory is rated in terms of speed—specifically, access time. Access time is the delay between the time data in memory is successfully addressed, to the point at which the data has been successfully delivered to the data bus. For PC memory, access time is measured in nanoseconds (ns), and current memory offers access times of 50 tons. 70-ns memory is extremely common. The question often arises Can I use faster memory than the manufacturer recommends The answer to this question is almost always Yes but rarely does performance benefit. As you will see in the following sections, memory and architectures are typically tailored for specific performance. Using memory that is faster should not hurt the memory or impair system performance, but it costs more and will not produce a noticeable performance improvement. The only time such a tactic would be advised is when your current system is almost obsolete, and you would want the new memory to be useable on anew, faster motherboard if you choose to upgrade the motherboard later on. Await state orders the CPU to pause for one clock cycle to give memory additional time to operate. Typical PCs use one wait state, although very old systems might require two or three. The latest PC designs with high-end memory or aggressive caching might be able to operate with no (zero) wait states. As you might imagine, await state is basically a waste of time, so more wait states result in lower system performance. Zero wait states allow optimum system performance. Table 23-9 illustrates the general relationship between CPUs, wait states, and memory speed. It is interesting to note that some of the fastest systems allow the most wait states. This flexibility lets the system support old, slow memory, Download 1.89 Mb. Share with your friends:
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