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In computing, memory latency is the time (the latency) between initiating a request for a byte or word in memory until it is retrieved by a processor. If the data are not in the processor's cache, it takes longer to obtain them, as the processor will have to communicate with the external memory cells. Latency is therefore a fundamental measure of the speed of memory: the less the latency, the faster the reading operation.
Latency should not be confused with memory bandwidth, which measures the throughput of memory. Latency can be expressed in clock cycles or in time measured in nanoseconds. Over time, memory latencies expressed in clock cycles have been fairly stable, but they have improved in time.
Memory latency is also the time between initiating a request for data and the beginning of the actual data transfer. On a hard disk drive, latency is the time it takes for the selected sector to come around and be positioned under the read/write head.
- Crucial Technology, "Speed vs. Latency: Why CAS latency isn't an accurate measure of memory performance" 
- Overview of the different kinds of Memory Latency
- Article and Analogy of the Effects of Memory Latency
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