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Xref: sserve comp.unix.ultrix:24331 comp.unix.bsd:15785 comp.sys.dec:24634 comp.sys.dec.micro:3872 Path: sserve!newshost.anu.edu.au!munnari.oz.au!news.hawaii.edu!ames!agate!howland.reston.ans.net!news2.near.net!satisfied.elf.com!rpi!wilsonj From: wilsonj@alum01.its.rpi.edu (John Wilson) Newsgroups: comp.unix.ultrix,comp.unix.bsd,comp.sys.dec,comp.sys.dec.micro Subject: Re: UNIX (Ultrix, BSD?) for DEC Micro PDP-11? Date: 21 Dec 1994 23:25:16 GMT Organization: Rensselaer Polytechnic Institute, Troy NY Lines: 46 Message-ID: <3dadgs$ita@usenet.rpi.edu> References: <taubman.787470030@spot.Colorado.EDU> <3d4872$r81@topaz.sensor.com> <3d4r7k$70d@usenet.rpi.edu> <3d9v5o$opt@topaz.sensor.com> NNTP-Posting-Host: alum01.its.rpi.edu In article <3d9v5o$opt@topaz.sensor.com>, Ron Natalie <ron@topaz.sensor.com> wrote: >John Wilson (wilsonj@alum01.its.rpi.edu) wrote: >: The PDP-11 >: has variable sized pages -- sure, you could just define them all to be >: 4KW but then you'd eat through your PDP's tiny memory in no time > >Actually, it doesn't have variable sized pages. It has exactly 8 >per address space corresponding to the 8 8K spaces. You just didn't >need to point them at a full 8K's worth of memory. Right. 8 variable-sized pages. I didn't say a variable *number* of pages. The fact that a <4KW page still has to rattle around in 4KW of address space doesn't have anything to do with the point I was making, which was that memory fragmentation is a problem on PDP-11s, while it isn't on systems like the PDP10 that just have a zillion 512-word pages, forexample, since you can always whip up a seemingly contiguous area with the page tables w/o having to slide stuff around. So I'd question whether he newer UNIXes (written for machines with similar VM structures, like the VAX and 386) have decent (or any) defragging code. >The PDP-11 is really not much worse restartability wise than many other >platforms that support UNIX. You mean, "top-of-the-line PDP-11." Machines like the 11/23, 11/24, 11/34, 11/60 etc. that don't have MMR1 mechanized (but are otherwise perfectly usable, SOB/XOR/EIS, optional FPP, not toys like the 11/05) can easily get you in situations where instructions *can't* be restarted. Like if the faulting instruction is "MOV -(R2),-(R2)", and R2 crosses a page boundary in one of the autodecs, you don't know which ref it was so you don't know whether to add 2 or 4 to R2 after loading the page. Cheap fixes like promising that the C compiler will never generate confusing instructions like that and not allowing assembly language are too disgusting for words. >Most of the early UNIXes couldn't page either (they were done on the >PDP-11). This accounts for the combination swap/page architecture >that persists in UNIX today. If it persists then great. My point was that porting a paging-only OS back to the PDP-11 would be hell, obviously the old UNIXes did it but things have changed since then. The old UNIXes *already* ran on the 11 so porting them to the 11 ought to be pretty straightforward! :-) John Wilson