Interactive Application System
| Company type | Operating system |
|---|---|
| Industry | Computer software |
| Founder | Digital Equipment Corporation (DEC) |
| Fate | Discontinued |
Area served | PDP-11 users |
| Products | Operating system software |
| Owner | Digital Equipment Corporation (DEC) |
Interactive Application System (IAS) was an operating system developed by DEC for the PDP-11, originating as a derivative of RSX-11D. The system's final significant update, Version 3.0, was distributed starting in late 1979, with the ultimate release, Version 3.4, appearing in May 1990.
Overview
The RSX-11 family from DEC transitioned from early paper-tape-based editions (RSX-11A and RSX-11C) to versions supporting disk storage, where the "D" suffix denoted disk capability and "M" referred initially to "small Memory" and later to "Multi-user." IAS was engineered to accommodate a combination of simultaneous timesharing, real-time operations, and batch processing. It delivered essential system services such as interrupt management, process scheduling, and interprocess communication without striving to be a comprehensive all-in-one system. Building upon RSX-11D, IAS incorporated additional functionality specifically designed for interactive application needs.
The file versioning approach from RSX-11 was preserved in IAS (for example, MYFILE.DAT;3). Batch jobs were managed through command files that used the same syntax as interactive indirect command files, enabling multiple batch processes to execute simultaneously. The system could be set up either to utilize idle CPU time for batch tasks or to ensure a minimum CPU allocation that would not interfere with real-time activities.
IAS featured two Command Language Interpreter options: the RSX-11 MCR and the Program Development System (PDS), which represented an early form of Digital Command Language. Additionally, DEC’s Sort/Merge utility was bundled within the IAS software suite.
Performance
IAS functions across three operational modes: Real-Time, Multi-User, and Timesharing.
In Multi-User mode, resources are shared alongside Real-Time processes; Timesharing mode further incorporates simultaneous batch processing with noncritical real-time tasks and interactive user sessions. The Timesharing mode introduces Timesharing Control Primitives (TCP), facilitating communication among timesharing processes. An assessment conducted by TRW's Defense and Space Systems Group for the Tactical Operations Analysis Support Facility at Langley Air Force Base emphasized the effectiveness of the "IAS heuristic timesharing scheduler" and kernel-level subtasking support implemented through the SPAWN directive.
The heuristic timesharing scheduler optimizes task handling by analyzing past performance data and interaction patterns.
Reliability is bolstered through integrated failure recovery features embedded within both DEC hardware components and IAS software.