In part the design was made for show effect. Reduction in wire length had only a minor influence, especially as it's only present in one dimension.

Cray 1 at the Deutsches Museum in Munich (Image taken from Wikipedia)

Mainframe circuit boards are connected via a hand wired backplane. In theory, arranging them in a circular fashion may save about 20% (*1) in horizontal wire length, but none in vertical (*2). Even more importantly, in backplane design it's rarely about making all connections as short as possible, but making them of defined length. Even more importantly, for wire groups where equal length is mandatory (*2). Looking at the above picture, we see that the wires aren't straight. The same can already be seen in its predecessors and successors (all the way to the 960) at CDC.

The design decision was much influenced by public reaction to a mock-up of the CDC 8600 with a quite similar design. In this multiprocessor setup, each CPU was meant to be one wedge-shaped unit allowing easy removal as a whole block before being opened (*3). While the machine was never finished, the positive reaction to the unconventional design might have stuck and transferred to the Cray-1, despite no longer being a CPU cluster.

Bottom line: it could have been built in a straight fashion; rolling it up was a great way to make it stand out - nerds do have aesthetics :)

Another, much more relevant, effect of this setup is cooling. This was in fact the main reason, even before compactness, why this setup was originally considered for the CDC 8600.

*1 - For a maximum value, the inner diameter as well as the medium can be used - this will give some upper limit. The outer diameter is about 1.45 m, while the inner is ca. 0.75 m. Thus the inner circumference is 2.35 m, while the median is ~3.45. So the upper limit of shortening in horizontal direction is about ~32%. In reality this comes to less than 20%, as

*2 - Maximum speed is only possible if all bits of a word arrive at the same time. The very same can be seen with modern PC mainboards, usually between CPU and RAM.

*3 - Design goal for the 8600 was extreme compact miniaturization, moving away from classic mainframe design. That's one of the reasons why it didn't work out.

In part the design was made for show effect. Reduction in wire length had only a minor influence, especially as it's only present in one dimension.

Cray 1 at the Deutsches Museum in Munich (Image taken from Wikipedia)

Mainframe circuit boards are connected via a hand wired backplane. In theory, arranging them in a circular fashion may save about 20% (*1) in horizontal wire length, but none in vertical (*2). In reality, and since Cray did put the boards horizontal, while not using the inner room for wiring, there were ZERO savings compared to a traditional setup.

(Taken from the CHM's Cray-1 Logic Column cutaway page)

Even more importantly, in backplane design it's not about making all connections as short as possible, but making them of defined length. Even more importantly, for wire groups where equal length is mandatory (*2). Looking at the above picture, we see that the wires aren't straight. The same can already be seen in its predecessors and successors (all the way to the 960) at CDC.

The design decision was much influenced by public reaction to a mock-up of the CDC 8600 with a quite similar design. In this multiprocessor setup, each CPU was meant to be one wedge-shaped unit allowing easy removal as a whole block before being opened (*3). While the machine was never finished, the positive reaction to the unconventional design might have stuck and transferred to the Cray-1, despite no longer being a CPU cluster.

Bottom line: It could have been built in a straight fashion; rolling it up was a great way to make it stand out - nerds do have aesthetics :).

Another, much more relevant, effect of this setup is cooling. This was in fact the main reason, even before compactness, why this setup was originally considered for the CDC 8600.

*1 - For a maximum value, the inner diameter as well as the medium can be used - this will give some upper limit. The outer diameter is about 1.45 m, while the inner is ca. 0.75 m. Thus the inner circumference is 2.35 m, while the median is ~3.45. So the upper limit of shortening in horizontal direction is about ~32%. In reality this would come to less than 20% - except it had zero effect on the real Cray :)

*2 - Maximum speed is only possible if all bits of a word arrive at the same time. The very same can be seen with modern PC mainboards, usually between CPU and RAM.

*3 - Design goal for the 8600 was extreme compact miniaturization, moving away from classic mainframe design. That's one of the reasons why it didn't work out.

In part the design was made for show effect. Reduction in wire length had only a minor influence, especially as it's only present in one dimension.

Cray 1 at the Deutsches Museum in Munich (Image taken from Wikipedia)

Mainframe circuit boards are connected via a hand wired backplane. In theory aranging them in a circular fashion may save about 20% (*1) in horizontal wire length could, but none in vertical (*2). Even more important, in backplane design it's rarely about making all connection the shortest possible, but making them of defined length. Even more important for wire groups where equal length is mandatory (*2). Looking at above picture shows that the wires aren't straight. The same can already be seen at it's predecessors and follow up designs (all the way to the 960) at CDC.

The design decision was much influenced by public reaction to a mock up of the CDC 8600 with a quite similar design. In this multiprocessor setup each CPU was meant to be one wedge shaped unit allowing easy removal as a whole block before being opened (*3). While the machine was never finished, the positive reaction to the unconventional design might have stuck and transferred to the Cray-1, despite no longer being a CPU cluster.

Bottom line, it could have been building a straight fashion, rolling it up was a great way to make it stand out - Nerds do have aesthetics :)

Another, way more relevant effect of this setup is cooling. This was in fact the main reason, even before compactness, why this setup was original considered for the CDC 8600.

*1 - For a maximum value the inner diameter as well as the medium can be used - this will give some upper limit. The outer diameter is about 1.45 m, while the inner is ca. 0.75 m. Thus the inner circumference is 2,35 m, while the median is ~3,45. So the upper limit of shortening in horizontal direction is about ~32%. In reality this comes to less than 20%, as

*2 - Maximum speed is only possible if all bits of a word arrive at the same time. The very same can be seen with modern PC mainboards, usually between CPU and RAM.

*3 - Design goal for the 8600 was extreme compact miniaturization, moving away from classic mainframe design. One of the reasons why it didn't work out.

In part the design was made for show effect. Reduction in wire length had only a minor influence, especially as it's only present in one dimension.

Cray 1 at the Deutsches Museum in Munich (Image taken from Wikipedia)

Mainframe circuit boards are connected via a hand wired backplane. In theory, arranging them in a circular fashion may save about 20% (*1) in horizontal wire length, but none in vertical (*2). Even more importantly, in backplane design it's rarely about making all connections as short as possible, but making them of defined length. Even more importantly, for wire groups where equal length is mandatory (*2). Looking at the above picture, we see that the wires aren't straight. The same can already be seen in its predecessors and successors (all the way to the 960) at CDC.

The design decision was much influenced by public reaction to a mock-up of the CDC 8600 with a quite similar design. In this multiprocessor setup, each CPU was meant to be one wedge-shaped unit allowing easy removal as a whole block before being opened (*3). While the machine was never finished, the positive reaction to the unconventional design might have stuck and transferred to the Cray-1, despite no longer being a CPU cluster.

Bottom line: it could have been built in a straight fashion; rolling it up was a great way to make it stand out - nerds do have aesthetics :)

Another, much more relevant, effect of this setup is cooling. This was in fact the main reason, even before compactness, why this setup was originally considered for the CDC 8600.

*1 - For a maximum value, the inner diameter as well as the medium can be used - this will give some upper limit. The outer diameter is about 1.45 m, while the inner is ca. 0.75 m. Thus the inner circumference is 2.35 m, while the median is ~3.45. So the upper limit of shortening in horizontal direction is about ~32%. In reality this comes to less than 20%, as

*2 - Maximum speed is only possible if all bits of a word arrive at the same time. The very same can be seen with modern PC mainboards, usually between CPU and RAM.

*3 - Design goal for the 8600 was extreme compact miniaturization, moving away from classic mainframe design. That's one of the reasons why it didn't work out.