IBM System/360 Model 91
| Company type | Computer system |
|---|---|
| Industry | Computing / Mainframe computers |
| Founded | 1964 |
| Headquarters | United States |
| Products | Mainframe computer |
The IBM System/360 Model 91 was introduced in 1964 to rival the CDC 6600. While sharing core functionalities with other high-end System/360 models, it boasted the most sophisticated internal design within the series and marked IBM's initial implementation of out-of-order instruction execution. Running on OS/360, this system was engineered for rapid data processing tasks in scientific disciplines such as space exploration, theoretical astronomy, subatomic physics, and global weather prediction.
The inaugural Model 91 was installed at the NASA Goddard Space Flight Center in 1968, establishing itself as the most powerful computing system of its era. It achieved a processing rate of up to 16.6 million instructions per second, a performance level comparable to that of Intel’s 80486SX-20 MHz or AMD’s 80386DX-40 MHz processors measured in MIPS.
Its central processing unit consisted of five independent components: an instruction unit, floating-point unit, fixed-point unit, and two storage controllers that managed overlapping memory and input/output channels. The floating-point unit made extensive use of instruction pipelining and was pioneering in applying Tomasulo's algorithm. Furthermore, this model was among the first to utilize a multi-channel memory architecture.
The power consumption of the Model 91 was approximately 74 kilowatts.
Models
The IBM System/360 Model 91 was available in four main variants, each differentiated by their primary memory setups, all utilizing IBM’s 2395 Processor Storage units.
The 91K included 2 MB of memory with a single 2395 Model 1 unit.
Both the 91KK and 91L featured 4 MB of memory; the former employed two 2395 Model 1 units, whereas the latter used one 2395 Model 2 unit.
The 91KL model offered a total of 6 MB by combining one Model 1 and one Model 2 unit from the 2395 series.
Production
A total of only fifteen Model 91 machines were manufactured, with four reserved for IBM’s own use. Estimates on the exact production or sales figures vary between ten and twenty units. Customers awaiting delivery frequently received a System/360 Model 85 as an interim replacement.
Distinctive Features Compared to Standard System/360
To enhance processing speed, the Model 91 exhibited several unique operational characteristics:
- The least significant bit in floating-point division results could differ.
- Certain exceptions were handled with reduced precision.
- The management of floating-point underflow and overflow conditions deviated from norms, arguably boosting performance.
- Memory store operations could execute out of order; a dedicated memory barrier instruction was used when strict ordering was necessary.
IBM and NASA
IBM maintained a close partnership with NASA, providing computing hardware for manned spaceflight missions. This included systems such as the IBM ASC-15 on Saturn 1, IBM ASC-15B on the Titan, IBM GDC on Project Gemini, IBM LVDC on Saturn rockets, along with various IBM System/4 Pi models supporting other programs.
The Model 91 arrived at NASA’s Goddard Space Flight Center nine months behind schedule in October 1967 and commenced routine operations in January 1968 after successfully completing federal certification.
IBM System/360 Model 95
A derivative of the Model 91, the Model 95 incorporated 1 megabyte of thin-film memory alongside 4 megabytes of core memory. NASA procured both units produced, placing one at the Goddard Space Flight Center in Maryland and the other at the Goddard Institute for Space Studies in New York City.
Although its console matched that of the System/360 Model 91, no Functional Characteristics manuals are known to exist for the Model 95.
First server connected to the internet
In 1971, UCLA employed an IBM 360/91 to deliver "production computing services" for ARPANET, which encompassed job submission, a "mailbox" feature, and FTP capabilities.
In popular culture
A Model 91 Panel is exhibited at the Living Computer Museum in Seattle, Washington and appeared in the movie Tomorrowland (2015).