From owner-freebsd-hackers Mon Jan 28 17:55:17 2002 Delivered-To: freebsd-hackers@freebsd.org Received: from rwcrmhc51.attbi.com (rwcrmhc51.attbi.com [204.127.198.38]) by hub.freebsd.org (Postfix) with ESMTP id 4766537B402 for ; Mon, 28 Jan 2002 17:55:05 -0800 (PST) Received: from grinch ([12.234.224.67]) by rwcrmhc51.attbi.com (InterMail vM.4.01.03.27 201-229-121-127-20010626) with ESMTP id <20020129015504.CAYO26243.rwcrmhc51.attbi.com@grinch> for ; Tue, 29 Jan 2002 01:55:04 +0000 Date: Mon, 28 Jan 2002 17:55:03 -0800 Subject: Re: OS Textbook FreeBSD Appendix Content-Type: text/plain; charset=US-ASCII; format=flowed Mime-Version: 1.0 (Apple Message framework v475) From: Justin C.Walker To: hackers@FreeBSD.ORG Content-Transfer-Encoding: 7bit In-Reply-To: <200201290110.g0T1A5W42274@thistle.bogs.org> Message-Id: <35CB1CEE-145B-11D6-B323-00306544D642@mac.com> X-Mailer: Apple Mail (2.475) 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 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. *--------------------------------------*-------------------------------* To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-hackers" in the body of the message