Page 54 - UNI 101 Computer Science Handout.

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3.2 The Hierarchy of Storage

Computer storage is often conceptualized hierarchically, based upon the speed with which data can be

accessed. The table in Figure below shows this hierarchy, together with some typical access times. At
the top of the hierarchy are the CPU registers used to hold data for the short term while processing is

taking place. Access to registers is essentially instantaneous, since the registers are actually a part of the

CPU. Cache memory, if present, is the fastest memory outside the CPU. You recall from Chapter 8 that

cache memory is a small fast memory that is used to hold current data and instructions. The CPU will

always attempt to access current instructions and data in cache memory before it looks at conventional
memory.

There may be as many as three different levels of cache. The CPU accesses the data or instruction in

conventional memory if cache memory is not present. Next in the hierarchy is conventional memory.

Both conventional and cache memory are referred to as primary memory. Both provide immediate
access to program instructions and data by the CPU and can be used for the execution of programs. The

data throughput rate of memory is determined primarily by the capability of the bus and interfaces that

connect memory to the CPU. Rates well in excess of 1 GB/sec are common in modern computers. Below
the level of conventional memory, storage in the hierarchy is not immediately available to the CPU, is

referred to as secondary storage, and is treated as I/O. Data and programs in secondary storage must

be copied to primary memory for CPU access.1 Except for flash memory, access to secondary storage is

significantly slower than primary storage. Disks and other secondary storage devices are mechanical in

nature, and mechanical devices are of necessity slower than devices that are purely electronic. The
location of the desired data is usually not immediately accessible, and the medium must be physically

moved to provide access to the correct location. This requires a seek time, the time needed to find the

desired location. Once the correct data is located, it must be moved into primary memory for use. The

54 Academic Year 2025/2026