The role of device drivers

As I mentioned in Chapter 4, Configuring and Building the Kernel, one of the functions of the kernel is to encapsulate the many hardware interfaces of a computer system and present them in a consistent manner to user space programs. The kernel has frameworks designed to make it easy to write a device driver, which is the piece of code that mediates between the kernel above and the hardware below. A device driver may be written to control physical devices such as a UART or an MMC controller, or it may represent a virtual device such as the null device (/dev/null) or a ramdisk. One driver may control multiple devices of the same kind.

Kernel device driver code runs at a high privilege level, as does the rest of the kernel. It has full access to the processor address space and hardware registers. It can handle interrupts and DMA transfers. It can also make use of the sophisticated kernel infrastructure for synchronization and memory management. However, you...