As the 6600 neared 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 denser packing of components and, in turn, a higher clock speed.
Transistors in general were also getting somewhat faster as the production processes and quality improved.
These sorts of improvements might be expected to make a machine twice as fast, perhaps as much as five times.
However, as with the 6600 design, Cray set himself the goal of producing a machine with ten times the performance.

One of the reasons the 6600 was so much faster than its contemporaries is that it had multiple functional units that could operate in parallel.
For instance, the machine could perform an addition of two numbers while simultaneously multiplying two others.
However, any given instruction had to complete its trip through the unit before the next could be fed into it, which caused a bottleneck when the scheduler system ran out of instructions.
Adding more functional units would not improve performance unless the scheduler was also greatly improved, especially in terms of allowing it to have more memory, so it could look through more instructions for ones that could be fed into the parallel units.
That appeared to be a major problem.

In order to solve this problem, Cray turned to the concept of an instruction pipeline.
Each functional unit consisted of several sections that operated in turn, for instance, an addition unit might have circuitry dedicated to retrieving the operands from memory, then the actual math unit, and finally another to send the results back to memory.
At any given instance only one part of the unit was active, while the rest waited their turn.
A pipeline improves on this by feeding in the next instruction before the first has completed, using up that idle time.
For instance, while one instruction is being added together, the operands for the next add instruction can be fetched.
That way, as soon as the current instruction completes and moves to the output circuitry, the operands for the next addition are already waiting to be added.
In this way each functional unit works 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.
To achieve the rest of the goal, the machine would have to run at a faster speed, now possible using new transistor designs.
However, there is a physical limit to performance because of the time it takes signals to move between parts of the machine, which in turn is defined by its physical size.
As always, Cray's design work spent considerable effort on this problem and thus allow higher operating frequencies.
For the 7600, each circuit module actually consisted of up to six printed circuit boards, each one stuffed with subminiature resistors, diodes, and transistors.
The six boards were stacked up and then interconnected along their edges, making 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 this system was mechanical, and therefore prone to failure, the 7600 was redesigned into a large "C" sha
As the 6600 neared 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 denser packing of components and, in turn, a higher clock speed. Transistors in general were also getting somewhat faster as the production processes and quality improved. These sorts of improvements might be expected to make a machine twice as fast, perhaps as much as five times. However, as with the 6600 design, Cray set himself the goal of producing a machine with ten times the performance.
One of the reasons the 6600 was so much faster than its contemporaries is that it had multiple functional units that could operate in parallel. For instance, the machine could perform an addition of two numbers while simultaneously multiplying two others. However, any given instruction had to complete its trip through the unit before the next could be fed into it, which caused a bottleneck when the scheduler system ran out of instructions. Adding more functional units would not improve performance unless the scheduler was also greatly improved, especially in terms of allowing it to have more memory, so it could look through more instructions for ones that could be fed into the parallel units. That appeared to be a major problem.
In order to solve this problem, Cray turned to the concept of an instruction pipeline. Each functional unit consisted of several sections that operated in turn, for instance, an addition unit might have circuitry dedicated to retrieving the operands from memory, then the actual math unit, and finally another to send the results back to memory. At any given instance only one part of the unit was active, while the rest waited their turn. A pipeline improves on this by feeding in the next instruction before the first has completed, using up that idle time. For instance, while one instruction is being added together, the operands for the next add instruction can be fetched. That way, as soon as the current instruction completes and moves to the output circuitry, the operands for the next addition are already waiting to be added. In this way each functional unit works 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. To achieve the rest of the goal, the machine would have to run at a faster speed, now possible using new transistor designs. However, there is a physical limit to performance because of the time it takes signals to move between parts of the machine, which in turn is defined by its physical size. As always, Cray's design work spent considerable effort on this problem and thus allow higher operating frequencies. For the 7600, each circuit module actually consisted of up to six printed circuit boards, each one stuffed with subminiature resistors, diodes, and transistors. The six boards were stacked up and then interconnected along their edges, making 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 this system was mechanical, and therefore prone to failure, the 7600 was redesigned into a large "C" sha
CDC 7600