Editing Incompatible Timesharing System (section)

==Significant technical features==
ITS introduced many then-new features:
* The first [[device independence|device-independent]] [[graphics terminal]] output; programs generated generic commands to control screen content, which the system automatically translated into the appropriate character sequences for the particular type of terminal operated by the user.
* A general mechanism for implementing [[virtual device]]s in software running in user processes (which were called "jobs" in ITS).
* Using the virtual-device mechanism, ITS provided transparent [[Distributed file system|inter-machine filesystem access]]. The ITS machines were all connected to the [[ARPANET|ARPAnet]], and a user on one machine could perform the same operations with files on other ITS machines as if they were local files.
* Sophisticated [[Process management (computing)|process management]]; user processes were organized in a [[tree (data structure)|tree]], and a superior process could control a large number of inferior processes. Any inferior process could be frozen at any point in its operation, and its state (including contents of the registers) examined; the process could then be resumed transparently.
* An advanced [[software interrupt]] facility that allowed user processes to operate asynchronously, using complex [[interrupt handling]] mechanisms.
* [[PCLSRing]], a mechanism providing what appeared (to user processes) to be quasi-[[atomic operation|atomic]], safely-interruptible [[system call]]s. No process could ever observe any process (including itself) in the middle of executing any system call.
* In support of the AI Lab's robotics work, ITS also supported simultaneous [[real-time computing|real-time]] and time-sharing operation.