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The execution time is the time for a cache access, and the memory stall cycles include the time to service a cache miss and access lower levels of memory. If the access latency, miss rate and miss penalty are known, the average memory access time can be calculated with: = +
A cache miss is a failed attempt to read or write a piece of data in the cache, which results in a main memory access with much longer latency. There are three kinds of cache misses: instruction read miss, data read miss, and data write miss.
AMAT's three parameters hit time (or hit latency), miss rate, and miss penalty provide a quick analysis of memory systems. Hit latency (H) is the time to hit in the cache. Miss rate (MR) is the frequency of cache misses, while average miss penalty (AMP) is the cost of a cache miss in terms of time. Concretely it can be defined as follows.
= miss ratio = 1 - (hit ratio) = time to make main-memory access when there is a miss (or, with a multi-level cache, average memory reference time for the next-lower cache) = latency: time to reference the cache (should be the same for hits and misses)
A cache hit occurs when the requested data can be found in a cache, while a cache miss occurs when it cannot. Cache hits are served by reading data from the cache, which is faster than recomputing a result or reading from a slower data store; thus, the more requests that can be served from the cache, the faster the system performs.
A hash-rehash cache and a column-associative cache are examples of a pseudo-associative cache. In the common case of finding a hit in the first way tested, a pseudo-associative cache is as fast as a direct-mapped cache, but it has a much lower conflict miss rate than a direct-mapped cache, closer to the miss rate of a fully associative cache. [9]
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 ...
The trace cache continues to feed the fetch unit until the trace line ends or until there is a misprediction in the pipeline. If there is a miss, a new trace starts to be built. The Pentium 4's execution trace cache stores micro-operations resulting from decoding x86 instructions, providing also the functionality of a micro-operation cache ...