Editing Critical section (section)

=== Critical sections in relation to peripherals ===

Critical sections also occur in code which manipulates external peripherals, such as I/O devices. The registers of a peripheral must be programmed with certain values in a certain sequence. If two or more processes control a device simultaneously, neither process will have the device in the state it requires and incorrect behavior will ensue.

When a complex unit of information must be produced on an output device by issuing multiple output operations, exclusive access is required so that another process does not corrupt the datum by interleaving its own bits of output.

In the input direction, exclusive access is required when reading a complex datum via multiple separate input operations. This prevents another process from consuming some of the pieces, causing corruption.

Storage devices provide a form of memory. The concept of critical sections is equally relevant to storage devices as to shared data structures in main memory. A process which performs multiple access or update operations on a file is executing a critical section that must be guarded with an appropriate file locking mechanism.