On Monday, January 28, 2002, at 05:10 PM, Greg Shenaut wrote:
> In message <[EMAIL PROTECTED]>, "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 [EMAIL PROTECTED] with "unsubscribe freebsd-hackers" in the body of the message
