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Chapter 44

The CRAY-1 Computer System1

Richard M. Russell

This paper describes the CRAY-1, discusses the evolution of its architecture, and gives an account of some of the problems that were overcome during its manufacture.

The CRAY-1 is the only computer to have been built to date that satisfies ERDA's Class VI requirement (a computer capable of processing from 20 to 60 million floating point operations per second) [Keller 1976].

The CRAY-1's Fortran compiler (CFT) is designed to give the scientific user immediate access to the benefits of the CRAY-1's vector processing architecture. An optimizing compiler, CET, "vectorizes" innermost DO loops. Compatible with the ANSI 1966 Fortran Standard and with many commonly supported Fortran extensions, CFT does not require any source program modifications or the use of additional nonstandard Fortran statements to achieve vectorization. Thus the user's investment of hundreds of man months of effort to develop Fortran programs for other contemporary computers is protected.


Vector processors are not yet commonplace machines in the larger-scale computer market. At the time of this writing we know of only 12 non-CRAY-1 vector processor installations worldwide. Of these 12, the most powerful processor is the ILLIAC IV (1 installation), the most populous is the Texas Instruments Advanced Scientific Computer (7 installations) and the most publicized is Control Data's STAR 100 (4 installations). In its report on the CRAY-1, Auerbach Computer Technology Reports published a comparison of the CRAY-1, the ASC, and the STAR 100 [Auerbach, n.d.]. The CRAY-1 is shown to be a more powerful computer than any of its main competitors and is estimated to be the equivalent of five IBM 370/195s.

Independent benchmark studies have shown the CRAY-1 fully capable of supporting computational rates of 138 million floating- point operations per second (MFLOPS) for sustained periods and even higher rates of 250 MFLOPS in short bursts [Calahan, Joy, and Orbits, n.d.; Reeves 1975]. Such comparatively high performance results from the CRAY-1 internal architecture, which is designed to accommodate the computational needs of carrying out many calculations in discrete steps, with each step producing interim results used in subsequent steps. Through a technique called "chaining," the CRAY-1 vector functional units, in combination with scalar and vector registers, generate interim results and use them again immediately without additional memory references, which slow down the computational process in other contemporary computer systems.

Other features enhancing the CRAY-1's computational capabilities are: its small size, which reduces distances electrical signals must travel within the computer's framework and allows a 12.5 nanosecond clock period (the CRAY-1 is the world's fastest scalar processor); a one million word semiconductor memory equipped with error detection and correction logic (SECDED); its 64-bit word size; and its optimizing Fortran compiler.


The CRAY-1 has been called "the world's most expensive love-seat" [Computer World, 1976]. Certainly, most people's first reaction to the CRAY-1 is that it is so small. But in computer design it is a truism that smaller means faster. The greater the separation of components, the longer the time taken for a signal to pass between them. A cyclindrical shape was chosen for the CRAY-1 in order to keep wiring distances small.

Figure 1 shows the physical dimensions of the machine. The mainframe is composed of 12 wedgelike columns arranged in a 270° arc. This leaves room for a reasonably trim individual to gain access to the interior of the machine. Note that the love-seat disguises the power supplies and some plumbing for the Freon cooling system. The photographs (Figs. 2 and 3) show the interior of a working CRAY-1 and an interior view of a column with one module in place. Figure 4 is a photograph of a single module.

An Analysis of the Architecture

Table 1 details important characteristics of the CRAY-1 Computer System. The CRAY-1 is equipped with 12 i/o channels, 16 memory banks, 12 functional units, and more than 4K bytes of register storage. Access to memory is shared by the i/o channels and high-speed registers. The most striking features of the CRAY-1 are: only four chip types, main memory speed, cooling system, and computation section.

Four Chip Types

Only four chip types are used to build the CRAY-1. These are 16 ´ 4 bit bipolar register chips (6 nanosecond cycle time), 1024 ´ 1 bit bipolar memory chips (50 nanosecond cycle time), and bipolar logic chips with subnanosecond propagation times. The logic chips are all simple low- or high-speed gates with both a 5 wide and a 4 wide gate (5/4 NAND). Emitter-coupled logic circuit (ECL) technology is used throughout the CRAY-1.


1Cosnm. ACM, vol. 21, no. 1, January 1978, pp. 63-72.


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