the time from the instant that an address is presented to the memory to the instant that data have been stored or made available for use Non Random Access The time it takes to position the read– write mechanism at the desired location 13
For non-random-access memory, the following relationship holds: TN = TA + n/R TN= Average time to read or write N bits TA =Average access time n =Number of bits R =Transfer rate, in bits per second (bps) 15
MEMORY HIERARCHY Register Cache RAM Magnetic Disk Magnetic Tape 16
The Memory Hierarchy The design constraints on a computer’s memory can be summed up by three questions: How much? How fast? How expensive? 17
How much? If the capacity is there, applications will likely be developed to use it. How fast is? To achieve greatest performance, the memory must be able to keep up with the processor. That is, as the processor is executing instructions, we would not want it to have to pause waiting for instructions or operands.T How expensive? 18
A variety of technologies are used to implement memory systems, and across this spectrum of technologies, the following relationships hold: Faster access time, greater cost per bit Greater capacity, smaller cost per bit Greater capacity, slower access time 19
The way out of this dilemma is not to rely on a single memory component or technology, but to employ a memory hierarchy. As one goes down the hierarchy, the following occur: 1. Decreasing cost per bit 2. Increasing capacity 3. Increasing access time 4. Decreasing frequency of access of the memory by the processor 20
CACHE MEMORY PRINCIPLES 24
CACHE MEMORY PRINCIPLES
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