Comment by nivertech

2 hours ago

> Q. How do you port an OS from one language to a new one, pre-LLM?

A. you reduce impedance mismatch, but making the new language using similar concepts and mechanisms

--

You're right, this table is just a coincidence, probably derived from the standard math notation, and used in many PLs like FORTRAN and Python ;)

  PDP-11        C
  
  INC R         ++i
  DEC R         --i
  ADD src, dst  dst += src
  SUB src, dst  dst -= src

  (R)+          *p++
  -(R)          *--p
  X(R)          p[x]
  @(R)+         **pp++
  @X(R)         *p[x]

  BR label      goto label  ; near jump
  JMP label     goto label  ; far jump

3 comments

nivertech

Reply
uecker  2 hours ago

I do not disagree with the point that some superficial aspects are inspired by the PDP-11, I disagree with that this has anything to do with the fundamentals of computing architecture.

It would also completely contradict the whole idea that everything today simulates the PDP-11 design because of C, as few architectures have this auto-in/decrement addressing modes despite C having native syntax.

rramadass  1 hour ago

You are being somewhat disingenuous with your comment ;-)

PDP-11 was hugely influential in the later designs of various Hardware, Software, OS, Languages etc. See for example; Dave Cheney's What Have We Learned from the PDP-11? - https://dave.cheney.net/2017/12/04/what-have-we-learned-from... The conclusion from the article;

While its development was sometimes chaotic, and not without its flaws, the PDP-11 is at the intersection of many threads of history.

Hardware, software, programming languages, operating systems, have all been influenced by the PDP-11. I wager there is not a single person in this room who cannot trace the lineage of the language they work with, the computer they use, or the operating system it runs, back to the PDP-11.

And that is worth celebrating.

While the PDP-11 instruction set was certainly influential in the design of the "C Abstract Machine" the latter was generalized to accommodate other architectures extent at that time (eg. Honeywell 6000, IBM System/370) with enough flexibility that you can implement a C compiler for almost any architecture you can think of. That is its strength.

David Chisnall's criticisms in his C Is Not a Low-level Language: Your computer is not a fast PDP-11 (https://queue.acm.org/doi/10.1145/3212477.3212479) has to do mainly with the fact that the abstract machine was serial execution with no concept of memory protection/models. But this very flexibility is what makes C easily portable to dinky little MCUs which do not have those features while allowing the programmer to explicitly program those using libraries on more complex processors with lots of parallel cores, mmus etc.

Thus a single thread runs on a "C abstract machine" on a core (i.e. the bare minimum) and it is up to the programmer to manage interactions between the threads on various cores. We have lost nothing but perhaps burdening the programmer with more knowledge of hardware complexity which was an acceptable tradeoff then. Note also that there already exists various extensions to C to handle parallel programming directly eg. "Concurrent C" by Narain Gehani et al. The industry however chose to settle on external libraries and optional thread support in C11 again keeping with its minimality and flexibility mantras.