IBM System/360 Model 85
| Company type | Computer model |
|---|---|
| Industry | Computer hardware |
| Founded | January 1968 |
| Headquarters | Armonk , United States |
| Products | Mainframe computer |
The IBM System/360 Model 85 represents a premium-tier computer within the System/360 series, equipped with sophisticated features. It was introduced in January 1968 and began shipping by December 1969. Due to an economic downturn that impacted governmental budgets, IBM manufactured approximately only 30 units. Despite its limited quantity, the Model 85 played a crucial role in closing performance gaps until later models such as the 360/91 were released.
Models
The 360/85 was available in four distinct versions: I85 (512K), J85 (1M), K85 (2M), and L85 (4M), primarily distinguished by their memory sizes. Memory options included configurations of either two or four IBM 2365 Processor Storage modules or one of two IBM 2385 Processor Storage variants, providing a range from 512K to 4M bytes of core memory. The I85 employed two-way interleaved memory, whereas the higher-end models supported four-way interleaving, which improved memory access efficiency and overall system throughput.
Advanced/special features
- The console design is L-shaped: one section houses the Main Control Panel featuring a CRT display, while the other contains two screens labeled "Microfiche Document Viewer" and "Indicator Viewer."
- IBM referred to its CPU cache as "high-speed buffer storage," which effectively reduced system storage cycle times to between one-third and one-fourth of those for main memory, depending on the variant. This cache, positioned between the CPU and main storage as a "Level 1" cache, was implemented using high-speed static memory and came in capacities of 16 KB or 32 KB. Notably, the System/360 Model 85 was the first commercial system to incorporate cache memory.
- The machine utilized monolithic integrated circuit technology, featured wide data paths, an internal buffer with an access time of approximately 80 nanoseconds and a minimum size of 16K bytes, along with a high-speed multiply function.
- Floating-point operations were enhanced by support for extended-precision 128-bit quadruple-precision floating-point arithmetic.
- Included both Read-only and Writable Control Storage; it was the second model in the System/360 lineup to offer writable control storage, introduced on January 30, 1968.
Emulation
With the inclusion of the 709/7090/7094 Compatibility Feature and corresponding emulator software, the 360/85 could execute programs originally developed for IBM 709, 7040, 7044, and 7094 systems, thereby broadening its applicability within enterprise computing environments.
Gateway to the future
The System/360 Model 85 anticipated many elements of the System/370 series, particularly influencing the IBM System/370 Model 165 unveiled in 1970. It introduced MST circuitry that was subsequently incorporated into early System/370 models and brought forward advanced capabilities such as 128-bit floating-point arithmetic and block multiplexor channels that became foundational in System/370 architecture.
Its instruction fetch and decode functions were handled by a hardwired I-unit, while instruction execution was managed through microcode in the E-unit. This combination contrasted with fully hardwired designs like those found in the System/360 Model 75 and Model 91. Additionally, most high-end System/370 systems employed horizontal microcode techniques, with the IBM System/370 Model 195 as an exception.