from Hacker News

Ask HN: If Unix is written in C, how did they run C before Unix?

by v7engine on 4/4/23, 4:10 PM with 48 comments

• by ksherlock on 4/4/23, 4:21 PM

  V1: (PDP-7 Unix) Kernel is written in assembly. C does not exist yet.

  V2: (PDP-11 Unix) Kernel is written in assembly. C compiler is written in assembly.

  V3: Kernel is written in assembly. C compiler is written in C.

  V4: Kernel is written in C. C compiler is written in C.

  https://www.tuhs.org/cgi-bin/utree.pl?file=PDP7-Unix

  https://www.tuhs.org/cgi-bin/utree.pl?file=V2

  https://www.tuhs.org/cgi-bin/utree.pl?file=V3

  https://www.tuhs.org/cgi-bin/utree.pl?file=V4

• by tetha on 4/4/23, 5:26 PM

  We had a few pretty cool courses in university touching this thing. It depends a bit on the work you want to do, but:

  - Pick a language that's simple enough. A subset of ML would be good, but if you want to complete it, I'd recommend a simple LISP. This is your new language. This is C.

  - Use a language you know and like to implement a compiler for this language. This is your bootstrap language. Compile this for example into C--, ASM or LLVM, depending on what you know. This is the target language. As a recommendation, keep this compiler as simple as possible so you have a reference for the next step. For C, both the bootstrap and the target language were ASM.

  - And now iterate on extending the stdlib your language has, until you can implement a compiler for your new language in your new language. Again, keep this compiler simple without optimization or passes, just generate the most trivial machine code possible. This usually takes a bit of back-and-forth. You'll need some function evaluation first, some expression evaluation first (this is where a lisp can be an advantage, as those are the same), then you need function definitions, then you need filesystem interactions and so on. You kinda discover what you need as you implement.

  - Once you have all of that, (i) compile the compiler for the new language in your bootstrap language and (ii) compile the compiler for the new language using the result of (i). If you want to verify the results, compile the compiler again with the output of (ii) and check if (ii) and (iii) are different.

  - Your new language is now self-hosted.

  This was fun, because it was accompanied with other courses like how processor microcode implements processor instructions, how different kinds of assembly is mapped onto processor instructions, and then how higher level languages are compiled down into assembly. All of this across 4-6 semesters resulted in a pretty good understanding how Java ends up being flip-flop operations.

  EDIT - got target & bootstrap mixed up in first part.

• by anon25783 on 4/4/23, 4:48 PM

  They didn't. There was no C before Unix. The first Unix was written in Assembly, and was used to develop the first C compiler.

• by athorax on 4/4/23, 5:13 PM

  Computerphile on youtube has a pretty good series on bootstrapping a compiler:

  https://www.youtube.com/watch?v=lJf2i87jgFA&list=RDLVlJf2i87...

• by starkparker on 4/4/23, 4:17 PM

  see Dennis Ritchie's The Development of the C Language: https://www.bell-labs.com/usr/dmr/www/chist.html

  pre-C Unix was written in assembly

• by AnimalMuppet on 4/4/23, 4:19 PM

  The start of C and the start of Unix are approximately at the same time. In that time, there was a stepwise process. The first Unix (IIRC, someone can correct me) was written in assembly. Also written at that time was a simple compiled language. That compiled language was used to write a more complicated compiler. There was another such cycle (I think) before there was a C compiler - and even then, it wasn't the full C, but a subset. Unix was then re-written in C.

• by jzellis on 4/4/23, 4:55 PM

  Redneck dog meme: If we wuz descended from wolves, how are there still wolves?

  (I'm just teasing, as you were.)

• by hawski on 4/4/23, 5:51 PM

  Bootstrapping is a fun thing. You already have some good answers.

  Now imagine how most things around you were made, how higher tech was made with lower tech. How they made high precision tools, when there were only lower precision tools available? For example: how to make a 0.001 mm precise caliper when all you have is a 0.1 mm one? There were a lot of challenges like that and we still get to new ones. I just wonder what general term is used for things like that.

• by abraxas on 4/4/23, 4:44 PM

  C does not depend on Unix in any way. Hell, 8 bit micros like Commodore 64 and ZX Spectrum had C compilers.

• by JohnFen on 4/4/23, 4:48 PM

  You don't need an operating system to run a program. Nothing about C requires Unix (or any other OS).

• by MarkusWandel on 4/4/23, 5:08 PM

  Any self-hosting language compiler has to start from somewhere.

  Generally it's an incremental process where the compiler for an early/subset version of the language is written in another, existing language (absent one, may be assembly code).

  Once it's possible to rewrite the compiler in its own subset language, it becomes self-hosting. Then you can add a feature to the language, and once it works, enhance the compiler to use it, and so on.

  Eventually the language and compiler go hand-in hand: The only way to compile it is with a compiler, and the only way to compile the compiler is with itself. This leads to interesting thought experiments such as:

  https://www.cs.cmu.edu/~rdriley/487/papers/Thompson_1984_Ref...

• by js2 on 4/4/23, 4:45 PM

  As the question has already been answered, I'll just add that the general principle is known as self-hosting:

  https://en.wikipedia.org/wiki/Self-hosting_(compilers)

• by coliveira on 4/4/23, 5:03 PM

  The question is malformed because there is no need for Unix to run C programs. In theory they could have created C without an OS. However, in practice C was written to rewrite Unix in a high level language (previously it was assembly).

• by snvzz on 4/4/23, 5:28 PM

  They ran it in UNICS.

  UNIX is just an UNICS rewrite in C, and was done by the authors of UNICS and C.

• by HeckFeck on 4/4/23, 5:14 PM

  A related question: was an Operating System first known as a Time Sharing System? I've seen most of what I've known as an OS in this era termed that, also I read that the term OS was coined for CP/M.

• by simonblack on 4/4/23, 10:10 PM

  Lots of C implementations don't/didn't use Unix as the underlying OS.

  There were Cs for MSDOS, Cs for CP/M, even Cs for Windows, etc, etc.

• by remram on 4/4/23, 5:15 PM

  You don't need an OS written in a language to use that language, as evidenced by Python/JavaScript/BASIC/Go/...

• by bitwize on 4/4/23, 4:45 PM

  As I understand it, the first versions of Unix were written in PDP-7 assembly. Thompson and Ritchie's C work took place on that.

• by moralestapia on 4/4/23, 5:54 PM

  >If UNIX is written ...

  Doesn't that imply already that C precludes UNIX? The question doesn't make sense.

• by _dain_ on 4/4/23, 4:45 PM

  you've always been able to run C on bare metal without an OS