CDC 7600 - Design

Design

After the 6600 started to near production quality, Cray lost interest in it and turned to designing its replacement. Making a machine "somewhat" faster would not be too difficult in the late 1960s; the introduction of integrated circuits allowed for denser packing of components, and in turn a high clock speed. Transistors in general were also getting somewhat faster as the production processes and quality improved. However these sorts of improvements might be expected to make a machine twice as fast, perhaps as much as five times, but not the tenfold increase he demanded. Likewise the 6600 already had a hard time filling its existing ten functional units, so simply adding more parallelism wouldn't help all that much.

In order to solve this problem, Cray turned to the concept of an instruction pipeline. While the 6600 could work on several instructions at once, it had to wait for any one to complete its trip through a functional unit before moving on to the next. For some period of time, the majority of the circuitry in any one unit was not being used. A pipeline improves on this by feeding in the next instruction before the first has completed, thereby having each unit effectively work in "parallel", as well as the machine as a whole. The improvement in performance generally depends on the number of steps the unit takes to complete, for instance, the 6600's multiply unit took 10 cycles to complete an instruction, so by pipelining the units it could be expected to gain about 10 times the speed.

Things are never that simple, however. Pipelining requires that the unit's internals can be effectively separated to the point where each step of the operation is running on completely separate circuitry. This is rarely achievable in the real world. Nevertheless, the use of pipelining on the 7600 improved performance over the 6600 by a factor of about 3.

As always, Cray's design also focussed on packaging to reduce size, shorten signal paths, and thereby increase operating frequency. For the 7600 each circuit module actually consisted of up to six PC boards, each one stuffed with subminiature resistors, diodes, and transistors. The six boards were stacked up and then interconnected along their edges, making for a very compact, but basically unrepairable module.

However the same dense packing also led to the machine's biggest problem—heat. For the 7600, Cray once again turned to his refrigeration engineer, Dean Roush, formerly of the Amana company. Roush added an aluminum plate to the back of each side of the cordwood stack, which were in turn cooled by a liquid freon system running through the core of the machine. Since the system was mechanical and therefore prone to failure, the 7600 was redesigned into a large "C" shape to allow access to the modules on either side of the cooling piping by walking into the inside of the C and opening the cabinet.