Cray Technology

The circuit technology used in CRAY machines has always been relatively conservative. The technology used in their construction, on the other hand, has always been spectacular. The CRAY-1 uses only three basic (16-pin flat-pack) ECL IC types: a 16 x 4 register IC (cycle time 6 ns), a 1024 x 1 memory IC (cycle time 50 ns), and a logic IC containing two NAND gates, one 5-input and one 4-input. The ICs are mounted on 5-layer 6" by 8" printed circuit boards, and two such boards are mounted back-to-back against a central copper plate to form a module. The copper plates carry the heat from the ICs to the cold bars which form the vertical walls of columns and into which up to 72 modules can be slotted. The cold bars are aluminium blocks containing stainless steel tubes through which flows the Freon coolant. The complete machine consists of twelve columns arranged in a 270 deg. arc, thus giving the machine its now famous C-shaped horizontal cross-section.

The X-MP uses the same constructional technology as the CRAY-1, but the 16-gate array integrated circuits used to implement the logic are not only faster but also denser than the circuitry used in the CRAY-1. Thus an X-MP/4 occupies the same space as a CRAY-1, an X-MP/2 occupies eight columns in a 180deg. arc and an X-MP/1 six columns in a 135deg. arc. Memory in the X-MP/4 is constructed of ECL circuits (cycle time 38 ns); in the X-MP/2 and X-MP/1 MOS circuits are used (cycle time 76 ns). The Y-MP moves away from the C-shape and is in fact Y-shaped with the processors occupying the stem and the SSD and Input-Output system occupying arcs which form the arms of the Y.

The CRAY-2 uses the same logic circuits as the X-MP, but mounted in three-dimensional pluggable modules. Each module contains eight printed circuit boards, each holding an 8 x 12 array of integrated circuits. (The largest CRAY-2 contains approximately 240,000 ICs, nearly 75,000 of which are memory.) Up to 24 modules can be contained in a column and the complete system contains 14 columns in a 300deg. arc. The power supplies are integrated into the bottom of each column, rather than forming a `seat' around the base as they do on the CRAY-1 and X-MP machines, and the cooling technology is also different.

The CRAY-2 uses the same logic circuits as the X-MP, but mounted in three-dimensional pluggable modules. Each module contains eight printed circuit boards, each holding an 8 x 12 array of integrated circuits. (The largest CRAY-2 contains approximately 240,000 ICs, nearly 75,000 of which are memory.) Up to 24 modules can be contained in a column and the complete system contains 14 columns in a 300deg. arc. The power supplies are integrated into the bottom of each column, rather than forming a `seat' around the base as they do on the CRAY-1 and X-MP machines, and the cooling technology is also different.

Cray has always maintained that the most important consideration in building a supercomputer is to use the fastest available component technology, and this inevitably means that heat dissipation is a problem. In fact, building the first cold bar was one of the most significant problems encountered in developing the CRAY-1. In the CRAY-2 the move to bring the heat-generating circuits and the coolant into ever closer proximity is taken to its logical extreme through the use of liquid immersion cooling. Here the coolant flows through the module circuit boards (at a velocity of one inch per second), and is in direct contact with the integrated circuits and the power supplies.