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. Greg Shenaut To Unsubscribe: send mail to [EMAIL PROTECTED] with "unsubscribe freebsd-hackers" in the body of the message
