IBM System/360 Model 67

IBM System/360 Model 67
Company type	Mainframe computer model
Industry	Computer hardware
Predecessor	IBM System/360 Model 65M
Founded	May 1966; 60 years ago
Defunct	March 15, 1977
Headquarters	Armonk, United States
Products	Mainframe computer with virtual memory and time-sharing support

The IBM System/360 Model 67 (S/360-67) was a prominent IBM mainframe introduced in the late 1960s. Distinct from other S/360 variants, it incorporated a Dynamic Address Translation (DAT) unit—commonly referred to as the "DAT box"—which enabled virtual memory, 32-bit addressing capabilities, and support for time-sharing through the 2846 Channel Controller that facilitated channel sharing among processors. Despite these enhancements, it maintained compatibility with the broader S/360 family.

Origins

The S/360-67 was specifically designed to serve major time-sharing customers including MIT, University of Michigan, General Motors, Bell Labs, Princeton University, Carnegie Mellon University, and the Naval Postgraduate School.

In the mid-1960s, as demand for interactive computing via time-sharing grew, IBM’s initial System/360 lineup lacked effective solutions for constrained memory resources. Internal disagreements within IBM delayed the introduction of time-sharing support.

A pivotal 1966 publication by Bruce Arden, Bernard Galler, Frank Westervelt, and Tom O'Brien from the University of Michigan proposed a virtual memory framework utilizing DAT to facilitate time-sharing. Subsequently, IBM agreed to produce a customized version of the S/360-65M for Michigan incorporating DAT features that supported both virtual memory and time-sharing, although it initially lacked a dedicated operating system.

Growing interest from other institutions prompted IBM to rename this model as the S/360-67 and officially market it as a product. To meet market needs, IBM developed the IBM Time Sharing System (TSS/360) operating system to debut alongside the Model 67.

The first units of the S/360-67 were delivered in May 1966, with production ceasing on March 15, 1977.

Before launching the Model 67, IBM introduced models 64 and 66—DAT-enabled versions of models 60 and 62—but these were rapidly superseded by the Model 67 during a broader transition that replaced models 60 and 62 with model 65.

Announcement

IBM publicly unveiled the S/360-67 in its August 16, 1965 "blue letters," marking the general availability of this System/360 variant. Notable features included:

Capability for multiprocessor configurations supporting up to four processors, although only single- and dual-processor systems were manufactured
A dedicated operating system, Time Sharing System (TSS), engineered to exploit multiprocessor functionality for near-instantaneous user interactions
Dynamic program relocation through dynamic address translation (DAT), allowing swift response times for multiple concurrent users

Virtual memory

The S/360-67 pioneered virtual memory implementation via its "DAT box" (Dynamic Address Translation), inspired by the 1966 architecture proposed by Arden and colleagues. It utilized both segment and page tables, closely paralleling the virtual memory mechanisms later standardized on System/370 systems.

In contrast to other S/360 or early S/370 machines limited to 24-bit addressing, the Model 67 supported both 24- and 32-bit address spaces. Its virtual memory scheme divided addresses into 4KB pages grouped within 1MB segments, which were dynamically mapped onto physical memory. This architecture, combined with reference and change bits embedded in storage keys, facilitated demand paging whereby page faults triggered operating system interrupts for efficient memory management.

This virtual memory system addressed three critical business objectives:

Expanded address space: Extending physical memory capacity by swapping data between disk or drum storage and main memory
OS isolation: Segregating operating system memory from user applications to enhance security and reliability while maximizing available application memory
Multiple address spaces: Offering separate virtual address environments per user, laying foundational concepts for private virtual machines

These technological advancements overcame hardware memory limitations, increased system robustness, and enabled virtualization capabilities—even though full hardware virtualization was not an original design goal of the S/360-67.

Features

|} The IBM System/360 Model 67 incorporated enhancements surpassing the standard S/360 models:

It provided sixteen control registers for advanced processing control.
Dynamic Address Translation (DAT) was supported, allowing virtual addresses of either 24 or 32 bits with a hierarchical structure of segment and page tables—permitting up to sixteen segments, each containing as many as 256 pages sized at 4096 bytes.
The Extended PSW Mode introduced sophisticated interrupt masking capabilities alongside DAT functionality.
A High Resolution Interval Timer was included, offering timing precision near thirteen microseconds.
Memory protection keys were implemented with reference and change bits to safeguard storage access.
Extended Direct Control facilitated communication of external interrupts between processors in duplex configurations.
The system allowed partitioning of CPUs, memory, and input/output channels into distinct subsystems when operating in duplex mode.
Floating Addressing enabled sequential assignment of physical memory within partitioned duplex environments.
The IBM 2846 Channel Controller permitted both processors in duplex setups to access all I/O channels and manage I/O interrupts independently.
Simplex configurations could handle up to seven I/O channels, while duplex arrangements supported as many as fourteen.
New instructions were introduced for supervisor state (LMC, SMC, LRA) and problem state (BASR, BAS) operations.
Additional program interruptions were added to handle exceptions arising from segment and page translation errors.

The Model 67 featured an internal cycle time of approximately two hundred nanoseconds and a magnetic core memory cycle time of seven hundred fifty nanoseconds, aligning with the performance of the S/360-65. Positioned within the System/360 range, it fell between the Model 30 and Model 195. Processor storage operations could read or write between one and eight bytes per cycle. For floating-point calculations, the system incorporated a sixty-bit parallel adder along with a serial adder dedicated to floating-point exponent arithmetic, decimal arithmetic, and instructions involving variable field lengths.

New components

The S/360-67 introduced four distinct new elements:

2067 Processing Unit Models 1 and 2,
2365 Processor Storage Model 12,
2846 Channel Controller, and
2167 Configuration Unit.

These components, along with the 2365 Processor Storage Model 2, 2860 Selector Channel, 2870 Multiplexer Channel, and various other System/360 control units, maintained compatibility with the S/360-67. Although Carnegie Tech utilized a 360/67 paired with an IBM 2361 LCS, this configuration was not included in IBM’s price list and likely did not support duplex operation.

Basic configurations

IBM provided three fundamental configurations for the System/360 model 67:

Simplex—comprising a single IBM 2067-1 processor, two to four IBM 2365-2 Processor Storage units (ranging from 512K to 1M bytes), up to seven data channels, and associated peripherals (marketed as IBM System/360 model 67–1).
Half-duplex—featuring one IBM 2067-2 processor, two to four IBM 2365-12 Processor Storage units (512K to 1M bytes), one IBM 2167 Configuration Unit, one or two IBM 2846 Channel Controllers, up to fourteen data channels, plus peripherals.
Duplex—consisting of two IBM 2067-2 processors, three to eight IBM 2365-12 Processor Storage units (768K to 2M bytes), one IBM 2167 Configuration Unit, one or two IBM 2846 Channel Controllers, a maximum of fourteen data channels, and peripherals.

Systems configured as half-duplex could be upgraded to duplex by adding a second IBM 2067-2 processor and increasing storage capacity if required. Both half-duplex and duplex models were classified under the IBM System/360 model 67–2 designation.

Operating systems

When the S/360-67 was launched in August 1965, IBM introduced TSS/360, a time-sharing operating system project that ultimately was canceled in 1971 after numerous setbacks. Subsequently, IBM adapted TSS/360 into TSS/370, which was offered as a Program Request for Price Quotation (PRPQ) through three releases before being discontinued. The discontinuation of TSS/360 paved the way for alternative operating systems that exploited the unique capabilities of the S/360-67.

The Michigan Terminal System (MTS), developed at the University of Michigan, debuted on the Model 67 in January 1967 and incorporated virtual memory by October of that year. Support for multi-processor configurations on duplex S/360-67 systems was added in October 1968.

CP/CMS, created at IBM's Cambridge Scientific Center, was the pioneering virtual machine operating system. Initially an unsupported research initiative involving external collaborators, it eventually became a fully supported product line (VM/370 and today’s z/VM). Based on CP/CMS, VP/CSS was developed by National CSS to provide commercial time-sharing services.

Legacy

The S/360-67 left a profound impact. Despite the failure of TSS/360, IBM was taken aback by the expansion of a time-sharing user community around the S/360-67 platform. Systems such as CP/CMS, MTS, and MUSIC gained traction. Numerous commercial enterprises, academic institutions, and service bureaus adopted the system, taking advantage of IBM’s limited time-sharing support while collaborating on resource sharing and source code enhancements to establish a generation of time-sharing centers.

The distinctive features of the S/360-67 were initially missing from IBM’s subsequent System/370 series; however, the 370/145 incorporated an associative memory primarily used for paging purposes. This omission reflected internal divisions within IBM regarding the prioritization of time-sharing versus batch processing—with batch processing initially prevailing.

Nevertheless, increasing customer demand alongside the success of multiple S/360-67 time-sharing deployments—including emerging vendors like National CSS and Interactive Data Corporation (IDC)—motivated IBM to integrate virtual memory capabilities into the S/370 lineup by 1972. The microcode for the 370/145 was enhanced to utilize associative memory for virtual address translation, validating advancements achieved during the S/360-67 project. The lasting success of IBM’s VM family and virtualization technologies owes much to this foundational platform.

In its technical overview of the z196 mainframe released in 2010, IBM acknowledged that its software virtualization roots trace back to the System/360 model 67.

External links

A. Padegs, "System/360 and Beyond", IBM Journal of Research & Development, vol. 25 no. 5, pp. 377–390, September 1981
IBM System/360 System Summary, thirteenth edition, January 1974; pages 6–13 to 6-15 focus on the Model 67
IBM System/360 Model 67 Reference Data
Photographs of a dual-processor IBM 360/67 at the University of Michigan Computing Center from the late 1960s or early 1970s featured in Dave Mills' article on the Michigan Terminal System (MTS)
Visuals of an IBM S/360-67 at Newcastle University (UK)
TSS/360 Concepts and Facilities
Time-sharing in the IBM System/360 Model 67