From owner-freebsd-hackers Mon Jan 28 17: 9:14 2002 Delivered-To: freebsd-hackers@freebsd.org Received: from bogslab.ucdavis.edu (bogslab.ucdavis.edu [169.237.68.34]) by hub.freebsd.org (Postfix) with ESMTP id DD4E637B404 for ; Mon, 28 Jan 2002 17:09:11 -0800 (PST) Received: from thistle.bogs.org (thistle.bogs.org [198.137.203.61]) by bogslab.ucdavis.edu (8.9.3/8.9.3) with ESMTP id RAA07508 for ; Mon, 28 Jan 2002 17:09:04 -0800 (PST) (envelope-from greg@bogslab.ucdavis.edu) Received: from thistle.bogs.org (localhost [127.0.0.1]) by thistle.bogs.org (8.11.3/8.11.3) with ESMTP id g0T1A5W42274 for ; Mon, 28 Jan 2002 17:10:06 -0800 (PST) (envelope-from greg@thistle.bogs.org) Message-Id: <200201290110.g0T1A5W42274@thistle.bogs.org> To: hackers@FreeBSD.ORG X-To: "Justin C.Walker" X-Sender: owner-freebsd-hackers@FreeBSD.ORG Subject: Re: OS Textbook FreeBSD Appendix In-reply-to: Your message of "Mon, 28 Jan 2002 16:07:14 PST." <2652E782-144C-11D6-B323-00306544D642@mac.com> Reply-To: gkshenaut@ucdavis.edu Date: Mon, 28 Jan 2002 17:10:05 -0800 From: Greg Shenaut Sender: owner-freebsd-hackers@FreeBSD.ORG Precedence: bulk List-ID: List-Archive: (Web Archive) List-Help: (List Instructions) List-Subscribe: List-Unsubscribe: X-Loop: FreeBSD.ORG 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. Greg Shenaut To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-hackers" in the body of the message