Operation of the Memory Hierarchy
     A memory hierarchy typically contains the local registers of the CPU at the lowest level and may contain at succeeding levels a small, very fast, local random-access memory called a cache, a slower but still fast random-access memory, and a large but slow disk. The time to move data between levels in a memory hierarchy is typically a few CPU cycles at the cache level, tens of cycles at the level of a random-access memory, and hundreds of thousands of cycles at the disk level! A CPU that accesses a random-access memory on every CPU cycle may run at about a tenth of its maximum speed, and the situation can be dramatically worse if the CPU must access the disk frequently. Thus it is highly desirable to understand for a given problem how the number of data movements between levels in a hierarchy depends on the storage capacity of each memory unit in that hierarchy.
     This flowchart shows how a high performance CPU processes a memory request issued by the central processing unit.
Memory Hierarchy Flowchart
TLB = Translation Lookaside Buffers, PA = Physical Address
Flowchart © John Morris, 1998

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