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For example, always-on spinning disks are online, while spinning disks that spin-down, such as massive array of idle disk ([[Non-RAID drive architectures#MAID|MAID]]), are nearline. Removable media such as tape cartridges that can be automatically loaded, as in a [[tape library]], are nearline, while cartridges that must be manually loaded are offline.
Most modern [[Central processing unit|CPUs]] are so fast that, for most program workloads, the [[wikt:bottleneck|bottleneck]] is the [[locality of reference]] of memory accesses and the efficiency of the [[CPU cache|caching]] and memory transfer between different levels of the hierarchy{{Citation needed|date=September 2009}}. As a result, the CPU spends much of its time idling, waiting for memory I/O to complete. This is sometimes called the ''space cost'', as a larger memory object is more likely to overflow a small/fast level and require use of a larger/slower level. The resulting load on memory use is known as ''pressure'' (respectively ''register pressure'', ''cache pressure'', and (main) ''memory pressure''). Terms for data being missing from a higher level and needing to be fetched from a lower level are, respectively: [[register spilling]] (due to [[register pressure]]: register to cache), [[cache miss]] (cache to main memory), and (hard) [[page fault]] (main memory to disk).
Modern [[programming language]]s mainly assume two levels of memory, main memory and disk storage, though in [[assembly language]] and [[inline assembler]]s in languages such as [[C (programming language)|C]], registers can be directly accessed. Taking optimal advantage of the memory hierarchy requires the cooperation of programmers, hardware, and compilers (as well as underlying support from the operating system):
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