System/360 Model 91

IBM System/360 Model 91

IBM System/360 Model 91
Announced January 18, 1966 and
withdrawn February 20, 1967.

The following is the text of an IBM Data Processing Division press backgrounder distributed on January 16, 1968.

The IBM System/360 Model 91 at NASA's Goddard Space Flight Center here, is the fastest, most powerful computer now in user operation.

It is specifically designed to handle ultra-high-speed data processing for scientific applications such as space exploration, theoretical astromony, sub-atomic physics and global weather forecasting.

The Model 91's immense computing power results from a combination of several key factors, including:

  1. Advanced circuits that switch in billionths of a second;

  2. High-density circuit packaging techniques; and

  3. A high degree of concurrency, or parallel operations.

Concurrency in the Model 91

To the user, the Model 91 is functionally the same as other large-scale System/360s. It runs under Operating System/360 - - a powerful programming package of approximately 1.5 million instructions that enable the system to operate with virtually no manual intervention.

However, the internal organization of the Model 91 is the most advanced of any System/360. Within the central processing unit (CPU) there are five highly autonomous execution units which allow the machine to overlap operations and process many instructions simultaneously.

The five units are processor storage, storage bus control, instruction processor, fixed-point processor and floating-point processor. Not only may these units operate concurrently, but each may perform several functions at the same time. In the floating-point processor, for example, as many as two additions and a multiplication can be performed simultaneously.

Because of this concurrency, the effective time required to execute instructions and process information is reduced significantly. In a conventionally organized machine, two additions and a multiplication normally would be processed serially - - one after the other - - and require a total of seven machine cycles. In the Model 91, they can be processed in only three machine cycles.

The Model 91 CPU cycle time is 60 nanoseconds. This is the time it takes to perform a basic processing operation. Its memory cycle time - - the time it takes to fetch and store eight bytes of data in parallel -- is 780 nanoseconds.

The main memory, however, is organized into 16 interleaved elements, so that the CPU may start a memory cycle with a different element every 60 nanoseconds instead of waiting the full 780 nanoseconds. This means that the CPU will rarely get a "busy signal" from the memory, and few machine cycles are wasted waiting for data.

The NASA Model 91 operates with 2,097,152 bytes of main memory interleaved 16 ways. Model 91s can accommodate up to 6,291,496 bytes of main storage.

In addition to main memory, NASA's Model 91 can store over 300 million characters in two IBM 2301 drum and IBM 2314 direct access storage units. It also has 12 IBM 2404 magnetic tape units for data analysis applications, such as the processing of meteorlogical information relayed from satellites. Three IBM 1403 printers give the system a 3,300-line a-minute printing capability. Punched card input/output is provided through an IBM 2540 card read punch.

Model 91 logic circuitry

The logic circuitry used in the Model 91 is ASLT (Advanced Solid Logic Technology). ASLT switching speeds - - 1.8 nanoseconds - - are three times faster than the fastest SLT circuits and result in the Model 91 machine cycle time of 60 nanoseconds.

Externally, an ASLT module closely resembles SLT - - the basic technology of other System/360s. But with the protective covering removed, the ASLT unit looks like a two-story SLT device - - two interconnected, half-inch square substrates with componentry on both surfaces of each substrate. The component density is greater, too.

The dense ASLT circuit packaging reduces the distance signals must travel from one circuit to another, further increasing internal operating speed of the system. The Model 91 circuits operate at speeds of four to five nanoseconds including the 1.8 nanosecond switching time.

The concurrency features of the Model 91, combined with the high speed circuits, provide a ten-fold increase in arithmetic capability over the System/360 Model 75, the fastest computer now in IBM's product line.

Model 91 technical facts

Typical arithmetic operations per second

Floating Point (1 word = 32 or 64 bits):
    Multiply (1 word x 1 word) up to 5,530,000/sec.
    Add (1 word + 1 word) up to 16,600,000
    Divide (1 word divided by 1 word) up to 1,385,000
Fixed Point (1 word = 32 bits):
    Multiply (1 word x 1 word) up to 1,569,000/sec
    Add (1 word + 1 word) up to 16,600,000
    Divide (1 word divided by 1 word) up to 450,000
Speed Data width (bits)
Machine cycle 60 nanoseconds  
Logic circuits 4-5 nanoseconds
including a switching
time of 1.8 nanoseconds
*Memory cycle 780 nanoseconds 64
    16 general purpose
    4 floating point

60 nanoseconds
60 nanoseconds


* The memory is interleaved 16 ways, effectively allowing data to enter the CPU from main storage at a rate equivalent to the basic machine cycle.