Significant Technical Features of The OS Itself
ITS introduced many revolutionary features:
- It had the first 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 devices in software which ran in user processes (which were called "jobs" in ITS).
- Using this mechanism, it provided transparent inter-machine filesystem access (almost certainly the first operating system to do so). The ITS machines were all connected to the ARPAnet, and a user on one could perform the same operations on files on other ITS machines as on local files.
- Sophisticated process management; user processes were organized in a 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 restarted transparently.
- An advanced software interrupt facility that allowed user processes to operate asynchronously, using complex interrupt handling mechanisms.
- PCLSRing, a mechanism which provided what appeared (to user processes) to be quasi-atomic, safely interruptible system calls. 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 and time-sharing operation.
Many of these, and numerous other significant advances, were later picked up by other operating systems.
Read more about this topic: Incompatible Timesharing System
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