[TUHS] Article on 'not meant to understand this'

Angelo Papenhoff aap at papnet.eu
Mon Jan 16 21:26:16 AEST 2017


On 16/01/17, Tony Finch wrote:
> Warren Toomey <wkt at tuhs.org> wrote:
> 
> > http://thenewstack.io/not-expected-understand-explainer/
> 
> Rob Pike observed on Twitter:
> 
> https://twitter.com/rob_pike/status/820777895689732096
> 
> > The article misses an important fact: A few years later we understood it
> > well and could do it much simpler.
> 
> https://twitter.com/rob_pike/status/820777988924981254
> 
> > We are always learning, and that comment was as much a note to the
> > author as to the reader. Now, stack switching is almost trivial.
> 
> https://twitter.com/rob_pike/status/820778110253613056
> 
> > A similar thing happened a generation earlier figuring out subroutine
> > (function) calls. Whole books were written. Now it's an instruction.

The author also actually missed the part that we're not supposed to
understand. I'll just paste what I posted on hackernews:

I don't think it's the context switching in general that we're not
expected to understand, since it's pretty much the same in v7 but the
comment is gone. Dmr actually explained the problem on his website
(https://www.bell-labs.com/usr/dmr/www/odd.html). savu is used to save
the current call stack, retu is used to switch to a saved call stack.
The problem is that the function which did the savu was not necessarily
the same as the function that does the retu, so after retu the function
could have the call stack of a different function. As dmr explained,
this worked with the PDP-11 compiler but not with the interdata
compiler. In V7 the stack switching was moved into separate functions,
save and resume. save retured 0 and resume returned 1 so that an if
statement could be used to check if the return from save was actually
that of resume after the stack switch (the same trick as that of fork).
This way the code that was to be executed after a stack switch was in
the same function and stack frame as the one that did the save (as
opposed to swtch).

Note that Lions doesn't explain this either, he assumed that the
difficulty was with with u_rsav and u_ssav, but those are still in v7
with the comment gone (he probably wasn't that wrong though, it really
is confusing, but it's just not what the comment refers to)

aap


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