Hardware Platform
A Model 67 was not available for building CP-40, so a custom virtual memory device based on associative memory (the "CAT box") was designed and built for CSC. It involved both hardware and microcode changes to a specially modified System/360 Model 40. These changes gave the unit the technology needed for full virtualization of the System/360 hardware. This modified Model 40 influenced the design of the forthcoming Model 67, which was intended to meet the needs of the same community of time-sharing users (notably MIT's Project MAC and Bell Laboratories – though both of these sites became notable IBM sales failures).
Three distinct virtual memory systems were implemented by IBM during this period:
- The "Blaauw Box" (named for Gerry Blaauw), part of the original design of the S/360-67
- The "CAT Box" (Cambridge Address Translator), added to CSC's S/360-40 to run CP-40
- The "DAT Box" (Dynamic Address Translation), announced as an addition to the S/370 series in 1972
These systems were all different, but bore a family resemblance. CP-40's CAT box was a key milestone. Pugh cites an IEEE paper about the CP-40 virtual memory hardware, and states that it was "unique in that it included a parallel-search register bank to speed dynamic address translation. With funds supplied by Cambridge, IBM engineer...built a 64-register associative memory and integrated it into a 360/40. The one-of-a-kind result was shipped to Cambridge early in 1966."
It is important to note that, although virtualization support was an explicit goal for CSC's modified Model 40, this was not apparently the case for the original Model 67 design. The fact that virtualization capabilities were ultimately implemented in the -67, and thus enabled the success of CP-67/CMS, speaks to the tenacity and persuasivness of the CSC team.
Read more about this topic: IBM CP-40
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