The operating system kernel needs to provide a complete I/O control process and prepare a large number of various data structures to manage the flow of I/O between various modules.

These modules include: VFS directory layer, Page Cache layer, file system layer, network processing layer, common block layer, I/O scheduler layer, block device driver layer, external I/O channel controller driver layer. These layers together form a large and complex I/O protocol stack.

I'm sure you already have an idea of how programs use I/O devices. The most common I/O devices fall into three categories: network card devices, storage devices（st2400mm0129）， and interactive devices such as keyboards/mice. These devices can be connected to the system through PCIE or USB ports. The question is, how is the data sent to them generated and sent all the way to the device driver and to the device?

If a user mode program wants to read the contents of sector 0 of a hard disk fan, it cannot directly manipulate the hard disk, because it cannot call the functions provided by the hard disk device driver at all, it can only delegate the kernel code to do this through the read system call.

In the parameters of the read system call, the user side program must at least tell the kernel: which device to read, which bytes of the device to read from, where to put the data read back into memory, and other parameters (such as what way to read when reading, etc.). The same goes for reading files.