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At the Tenth Hawaii International Conference on the System Sciences in 1977 Dennis Ritchie presented the paper The Unix Time-sharing System: A retrospective in which he states:

...a good case can be made that UNIX is in essence a modern implementation of MIT’s CTSS system. PDF

Why did he write that?

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    Presumably he wrote that because he meant that. However, that would be an opinion-based answer. How would one answer this question definitively?
    – Jon Custer
    Apr 6 at 13:34
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    Unfortunately Mr. Ritchie passed away in 2011 so there is no way to ask him if he meant what he said. So I think the only option is to accept it at face value. He was almost certainly familiar with CTSS and so I see no reason to doubt what he said in the presentation.
    – jwh20
    Apr 6 at 13:41
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    Taken in isolation, the quote seems "mysterious". But it is presented as part of a lengthy document, not in isolation.
    – Brian H
    Apr 6 at 14:01
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    I think the explanation is in the immediately-preceding sentence: Only a handful of its ideas are genuinely new.
    – another-dave
    Apr 6 at 22:23
  • Professor Corbato explains what Ritchie meant. CTSS was built out of polished ideas, not experiments. UNIX was the same way, Ritchie calls it "conservative" like CTSS. See youtube.com/watch?v=KZPYBDA6XVo
    – Acceptable Name
    Apr 7 at 12:37

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It seems highly likely that the context in the accompanying document will provide the best clue of DMR's meaning behind the quote. After all, this quote is just one line in the broad description of how Unix came to be, including its relationship with MIT CTSS.

Indeed, Unix and CTSS share many common traits, as DMR acknowledges in the document. But CTSS was a product of the 1960's, and was developed for the hardware at hand- which was IBM mainframe computers. Unix was a product of the 1970's, and was also developed for the hardware at hand. DMR goes on to make many references to the DEC PDP-11 in the document. This because, by the time of his writing, the Unix OS and the PDP-11 hardware had risen to influence in a symbiotic way. The whole document is providing context for Unix in relation to the computer hardware of this "new era".

"UNIX is a general-purpose, interactive time-sharing operating system primarily for the DEC PDP-11 series of computers, and recently for the Interdata 8/32."

Thus, it is very likely that the interpretation of "modern" in your quote simply means "for modern (contemporary) computer hardware". DMR had been witnessing the industry transition, as IBM mainframe hardware, originally designed for batch processing, gave way to minicomputers, more suited to interactive usage. Such "modern systems" benefited from a compatible OS (Unix) with "CTSS-like" abilities.

So, "modern" most likely just means an OS for the modern hardware, rather than for "aging" IBM mainframe hardware used for CTSS in the previous decade(s).

Note: The title of the document is "The Unix Time-Sharing System...". It is not a document about the Unix C code or even the OS design, as that is not a system. The word System implies an implementation of hardware and software, and this adds farther evidence that the word "modern" in the quote means for modern hardware.

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    UNIX block devices have a strategy. CTSS disk, drum and tape drivers have strategy modules. UNIX and CTSS both have PANIC and core dump. The terminal drivers have many similarities Erasing (killing) a line of input typed into a terminal on both CTSS and (vintage) UNIX (by default) is accomplished by typing the @ character. Likewise # erases a single character on both. The term canonical as in canonical terminal input mode (process erase and kill characters typed on a terminal) is used by both CTSS and UNIX. On UNIX the opposite of canonical mode is raw mode. UNIX and CTSS both have swapping.
    – Acceptable Name
    Apr 6 at 14:15
  • Have you ever watched any of these videos? youtube.com/playlist?list=PLT8Ms9NT2xpb6yBvtVpBXzUoIMAhDu0G9
    – Acceptable Name
    Apr 6 at 14:17
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    @AcceptableName I don't see your point, exactly. Are you wanting me to acknowledge the similarity of Unix with CTSS, after DMR already did that in the document you referenced?
    – Brian H
    Apr 6 at 14:21
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    @AcceptableName I dd not mean to imply that, and cannot see any text in my answer where that is so implied. Can you be specific?
    – Brian H
    Apr 6 at 14:25
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    @AcceptableName Yes, I can see the point you are making. I believe that the document is describing the Unix system_(it's in the Title). As a system, the word _implementation refers to software+hardware, and does not specifically mean the code.
    – Brian H
    Apr 6 at 15:08

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