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In layered architecture, the operating system is broken into a number of layers (levels). The bottom layer (layer 0) is the hardware; the highest (layer N) is the user interface. Each layer is implemented only with operations provided by lower-level layers. The layers are selected so that each uses functions (operations) and services of only lower-level layer. The lowest set layer is always the hardware and the highest layer is the user interface. My question is does always a request should reach the lowest layer ( the hardware layer) so that it can be serviced? If no, then can you please give an example?

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There are many cases in which a higher-level request does not reach the lowest level. A simple example would be reading data from a file. If the file system already has the requested data cached, it will not call into the lower level disk driver to read the data.

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  • $\begingroup$ As another example, if the file system is a "RAM disk" (i.e. a file system stored entirely in core) then there is no disk hardware. It's a similar story for network devices; sometimes it's a piece of hardware (e.g. an Ethernet or WiFi device), and sometimes it's a "loopback" device which is implemented entirely in software. $\endgroup$
    – Pseudonym
    Oct 16, 2023 at 21:14

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