How an Accident of Hardware Design Encouraged Open Source
Subject:   ASCII on the PDP-8
Date:   2007-03-05 22:00:53
From:   Marc.Ramsey
I spent a lot of time building complex systems using various models of PDP-8, PDP-15 (18 bit!), and PDP-10 (36 bit). I don't remember ever resorting to packing ASCII characters in the fashion described. Instead, characters were almost immediately transliterated into SIXBIT code, which allowed for packing 2 characters in a 12 bit word, 3 in 18, and 6 in 36 bits. One lost control characters, lower case letters, and a few other symbols, but given the limited capability of the output devices at the time, this was hardly an issue. Most of the assemblers, compilers, and interpreters we used at the time limited themselves to using characters from the SIXBIT set.

The PDP-11 was the first computer I used ASCII on to any great extent. The endian issue was already being dealt with by ARPAnet protocols in 1975, when I was involved in interfacing the first PDP-11s to that network.

If you really want to know how open source software came about, ask Richard Stallman about EMACS and James Gosling...

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  • ASCII on the PDP-8
    2007-03-05 23:14:28  Mark_Rosenthal [View]

    There were a few different encoding schemes we used when I worked in DEC's Small Systems Group. It wasn't necessary to explain all of them in the article to make the point that on non-byte-addressable machines it was more cumbersome to write software to manipulate text than on byte-addressable machines.

    When we could live with only uppercase letters, digits and a very few punctuation characters, we used a 6-bit encoding scheme called RAD50. But when we needed a fuller set of punctuation characters or lowercase letters, we used an 8-bit encoding which was 7-bit ASCII plus parity.

    As for Gosling Emacs, I heard stories back then that Gosling had incorporated other programmers' contributions (including Stallman's) into his version of Emacs, and then sold it to Unipress who treated it as proprietary and violated the programmer's tradition of sharing code. Since Stallman hadn't included a copyright in the code he contributed, it was in the public domain so Unipress could claim proprietary rights to it. This is part of what made Stallman decide to copyright his later code and invent a license that would require recipients to behave better than Gosling and Unipress had done.