Date: Mon, 28 Jan 2002 17:55:03 -0800 From: Justin C.Walker <justin@mac.com> To: hackers@FreeBSD.ORG Subject: Re: OS Textbook FreeBSD Appendix Message-ID: <35CB1CEE-145B-11D6-B323-00306544D642@mac.com> In-Reply-To: <200201290110.g0T1A5W42274@thistle.bogs.org>
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On Monday, January 28, 2002, at 05:10 PM, Greg Shenaut wrote:
> In message <2652E782-144C-11D6-B323-00306544D642@mac.com>, "Justin
> C.Walker" cleopede:
>>>> I've took a brief look on Unix presentation and was wondering, why
>>>> author says that "...most Unix systems have not permitted shared
>>>> memory because the PDP-11 hardware did not encourage it..."?
>
>>> where'd they get this? that's an odd statement. Shared memory was
>>> used all the time on Unix on -11s, that's the whole point of the
>>> shared text a.out format. Of course shared read-only text is not
>>> exactly the standard shared memory, but at the same time it shows
>>> feasibility. The address space was so small though that other
>>> mechanisms were used.
>
>> I'd guess that the point deals with the use of "shared memory" between
>> processes for the purposes of sharing data. Given the granularity of
>> the PDP-11 "VM" hardware, it seemed like a bad tradeoff, and wasn't
>> considered useful until long after the PDP-11 went to the Boston
>> Computer Museum, where it sipped tea and complained about the Red Sox.
>
> Well, on PDP11s, which I used for V6, V7, and 2.8 & 2.9 BSD, you
> could share text memory, as has already been stated, and IIRC you
> could also share data memory after a vfork (once vfork became
> available on 2.9). It seems to me that I actually used the vfork
> memory sharing trick for some kind of primitive multithreaded
> program at one point. I think the limitation was that you couldn't
> map a small piece of memory & share it among processes, only all
> text or all data, but I admit my memory is almost gone, and I don't
> remember PDP/11 architecture all that well either.
You're correct; that's what I meant by the 'granularity' of the
hardware. You had to share a fairly hefty chunk of memory, so (except
for vfork-like-things), it put too much of a constraint on the use of
the sharing.
Regards,
Justin
--
Justin C. Walker, Curmudgeon-At-Large *
Institute for General Semantics | Men are from Earth.
| Women are from Earth.
| Deal with it.
*--------------------------------------*-------------------------------*
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