steve@basser.oz (Stephen Russell) (09/27/87)
In article <819@sugar.UUCP> peter@sugar.UUCP (Peter da Silva) writes: >In article <1745@ncr-sd>, greg@ncr-sd (Greg Noel) writes: >> In article <8490@think.UUCP> rlk@THINK.COM writes: >> >.... The PDP11 didn't have virtual memory either, >> >if my memory serves me. .... >> >> At the risk of re-opening an old debate, the PDP-11 \does/ have virtual >> memory. It's just that, for various technical reasons, the original Unix >> implementation for it chose to use swapping instead of paging as its virtual >> memory technique. > >And neither did any other operating system for the PDP-11 (RSX, RSTS, RT-11), >probably because it didn't in fact have the capability of supporting VM. Well, you're both right. SOME PDP-11's do provide support for VM, if your main criteria is the ability to restart an instruction after a page fault. The MMR1 register (017777574) tracks the effects of auto-incs and decs of registers, but is not present in many versions of the PDP-11. However, given the presence of MMR1 and MMR2 (PC of faulting instruction), you can implement VM quite easily. However, the main limitations for VM are a small number (8 or 16) of large (8K) pages, and no hardware reference bit (but neither does a VAX :-). Also, your VM on a PDP-11 is usually smaller than your physical memory, which is contrary to normal expectations. >Why do you think DEC developed the Virtual Address Extension (VAX) in the >first place? > To break the 128K boundary? I suspect they really wanted a 32 bitter, which almost forces you to adopt full VM.
pcm@ogcvax.UUCP (Phil Miller) (09/29/87)
When I went to read my comp.os.minix articles today, 4 of the 7 articles were about virtual memory on the PDP-11. MUST this vacuous debate continue within comp.os.minix? Flames to me directly, please -- enough comp.os.minix space has already been wasted on this topic.