Talk:Seymour Cray/Archives/2017

"Imprecise interrupts"
I believe the statements below that appear in the current version of the biography are incorrect. When other companies (namely IBM) attempted to create machines with similar performance, they stumbled (Stretch/IBM 7030). Indeed, the 6600 solved a critical design problem — "imprecise interrupts"[8] — that was largely responsible for IBM's failure. He did this by replacing I/O interrupts with a polled request issued by one of 10 so-called peripheral processors, ... Namely,   Interrupts in the IBM Stretch were precise.  I believe that the recovery time for mispredicted arithmetic branches and the store delays were worse performance problems for Stretch than interrupts.  Exceptions in the CDC 6600, such as exponent overflow, were imprecise.  I believe that Cray and Thornton were likely inspired by the UNIVAC LARC in off-loading I/O from the CDC 6600 central processor to the PPUs; see Thornton's 1980 paper in the Annals of the History of Computing entitled "The CDC 6600 Project". I believe that their decision about off-loading I/O was unrelated to whether exceptions should be precise or imprecise and that it was made so that the central processor could execute application code without the overhead of I/O interrupts. I believe that Cray saw this approach to I/O as fast and simple. (Note: the subsequent CDC 7600 used a different scheme for I/O that included I/O interrupts.)  Mark Smotherman 2620:103:A000:401:6DB6:6118:DEA6:248F (talk) 00:23, 28 July 2017 (UTC)

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