such as Solaris Containers, typically each chroot environment needs its own /dev; these mount points will be visible on the host OS at various nodes in the global file system tree. By restricting the device nodes populated into chroot instances of /dev, hardware isolation can be enforced by the chroot environment (a program can not meddle with hardware that it can neither see nor name—an even stronger form of access control than Unix file system permissions). MS-DOS managed hardware device contention (see terminate-and-stay-resident program) by making each device file exclusive open. An application attempting to access a device already in use would discover itself unable to open the device file node. A variety of device driver semantics are implemented in Unix and Linux concerning concurrent access. Unix and Unix-like systems Device nodes correspond to resources that an operating system's kernel has already allocated. Unix identifies those resources by a major number and a minor number, both stored as part of the structure of a node. The assignment of these numbers occurs uniquely in different operating systems and on different computer platforms. Generally, the major number identifies the device driver and the minor number identifies a particular device (possibly out of many) that the driver controls: in this case, the system may pass the minor number to a driver. However, in the presence of dynamic number allocation, this may not be the case (e.g. on FreeBSD 5 and up). As with other special file types, the computer system accesses device nodes using standard system calls and treats them like regular computer files. Two standard types of device files exist; unfortunately their names are rather counter-intuitive for historical reasons, and explanations of the difference between the two are often incorrect as a result. Character devices Character special files or character devices provide unbuffered, direct access to the hardware device. They do not necessarily allow programs to read or write single characters at a time; that is up to the device in question. The character device for a hard disk, for example, will normally require that all reads and writes be aligned to block boundaries and most certainly will not allow reading a single byte. Character devices are sometimes known as raw devices to avoid the confusion surrounding the fact that a character device for a piece of block-based hardware will typically require programs to read and write aligned blocks. Block devices Block