For instance most operating systems provide files, which are pseudo devices built on top of a disk. When pseudo devices are provided, device descriptors refer to both actual and pseudo devices. Independent Another issue is whether a general set of primitives be provided that may be used for all devices or if each device should be associated with its own set of primitives. Its disadvantage is that a general set of primitives may not fit the needs of a current or future device. For instance the operation seek is applicable to disks but not to terminals.
Some devices work with single bytes while others work with blocks.
A general purpose OS will need to provide both kinds of sizes.