Non-Preemptable IO Resources Non-preemptable IO resources are resources that cannot betaken away from a process that is currently using them without having some negative effect. An example of a non-preemptable resource would be a CD/DVD drive. If a process is reading from or writing to an optical storage device, it is difficult to take that resource away from the process without corrupting whatever is being read or written to the drive. Deadlocks Deadlocks occur when multiple processes are holding IO resources, and they each require resources that currently in use by another process that is unwilling to release the resource. When this occurs, neither process is able to make any progress. For example, one process may currently hold control of the optical storage (CD/DVD) drive, and may need to send something to the printer. A second process may hold control of the printer, and may need to access data from the CD/DVD drive. If neither process is willing to give up control of the resources it currently holds, then neither process will be able to proceed. Block IO Devices Some IO devices send and receive data from the computer system in blocks of characters. Such devices are referred to as block IO devices. The operating system manages reading and writing data to block devices by using a data buffer system. Data buffers are allocated to hold a single block of characters. When the buffer is full, the data in the buffer is then sent to or from the IO device in one chunk. Common examples of block IO devices are hard disks, optical storage drives, and memory regions. Character IO Devices Some IO devices send and receive data from the computer one character at a time. Such devices are called character IO devices. Common examples include virtual terminals and serial modems. Character transmission to character devices is unbuffered. Optical storage devices are non-preemtable
