The third episode takes us into the 1960s — a time of major breakthroughs in operating systems.
For the first time, hardware and software were unified, and concepts such as multitasking and multi-user access emerged.
The key systems of this era were IBM’s OS/360 and the experimental MULTICS project, co-developed by MIT, Bell Labs, and General Electric — both of which defined the standards for memory management, security, and user interaction.
Context – The Need for Unification and Multitasking
Before the 1960s, the mainframe market was fragmented — every model had its own operating system.
This led to high maintenance costs and limited capabilities in single-user, batch-only environments.
In 1964, IBM launched the System/360 — a family of compatible machines, from small to large, all using a unified architecture and instruction set.
This created a need for one operating system to support the entire family: OS/360.
OS/360 – One OS for the Whole Hardware Range
OS/360 was not a single system, but a family of operating systems tailored to different hardware tiers and user groups:
| Version | Description | Notes |
|---|---|---|
| PCP | Primary Control Program | Simple single-task system for early installs |
| MFT | Multiprogramming with Fixed number of Tasks | Fixed partitions, supported multitasking |
| MVT | Multiprogramming with Variable number of Tasks | Dynamic memory allocation, advanced management |
There was also DOS/360 for smaller System/360 models — a simplified version of the OS.
IBM System/360 Model 40 by U.S. Department of Agriculture, Public domain
Key innovations in OS/360:
- Multitasking – multiple programs could run simultaneously by dividing memory.
- Advanced I/O handling – broad support for peripherals and spooling.
- JCL (Job Control Language) – a scripting language to define job parameters and queues.
- Memory protection and dynamic allocation – especially in the MVT variant.
OS/360 left a deep mark on the development of mainframe OSes and directly led to modern IBM systems like z/OS.
MULTICS – A Vision of Secure, Shared-Time Systems
At the same time, MIT, Bell Labs, and General Electric were co-developing MULTICS (Multiplexed Information and Computing Service),
a pioneering OS built for multi-user, time-sharing environments.
Key features of MULTICS:
- Time-sharing – many users could be logged in simultaneously, sharing computing resources.
- Hierarchical file system – advanced access controls and layered security.
- Virtual memory – allowing programs to use large address spaces.
- Modular architecture – separation between kernel and user-space.
- Security rings – defined user privilege levels and access to system resources.
Though MULTICS never gained commercial success, it had a massive influence on the development of OS architecture and security models.
Competition and Legacy of Multi-User Systems
Despite their differences in commercial adoption, OS/360 and MULTICS were highly influential:
| Feature | OS/360 | MULTICS |
|---|---|---|
| Launch Year | 1964 | 1969 |
| Processing Model | Batch + multitasking | Multi-user, time-sharing |
| Memory Architecture | Partitioned + dynamic allocation | Virtual memory |
| Security | Basic protection (MVT) | Advanced ring-based security |
| Adoption | Widely used, IBM standard | Limited installations (experimental) |
Ideas from OS/360 evolved into MVS and OS/390; MULTICS inspired the design of UNIX.
Other Notable Systems of the Era – Advancing Multitasking and Time-Sharing
CTSS (Compatible Time-Sharing System) (1961–1966)
Developed at MIT, it introduced interactive multi-user sessions on the IBM 7094.
GE COS / GCOS
General Electric’s OS that competed with IBM, enabling simultaneous task execution.
Burroughs MCP (Master Control Program) (1961)
Possibly the first OS with a high-level language kernel (ALGOL), featuring memory protection and resource control.
Conclusion
The years 1964–1969 laid the groundwork for modern operating systems.
IBM’s OS/360 unified its hardware ecosystem and pioneered batch multitasking with memory and device control.
Meanwhile, MULTICS set new standards in time-sharing, security, and modularity — concepts that later shaped UNIX and its descendants.
In the next episode, we’ll explore UNIX and BSD, tracing their roots back to the collaborative, multi-user systems of the late 1960s.
