Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Path: utzoo!utgpu!water!watmath!clyde!burl!codas!killer!jfh From: jfh@killer.UUCP Newsgroups: comp.arch,comp.unix.wizards,comp.os.minix Subject: Re: Free Software Foundation (was: Re: Mach, the new standard?) Message-ID: <1713@killer.UUCP> Date: Fri, 2-Oct-87 17:44:05 EDT Article-I.D.: killer.1713 Posted: Fri Oct 2 17:44:05 1987 Date-Received: Sun, 4-Oct-87 02:07:08 EDT References: <8490@think.UUCP> <1745@ncr-sd.SanDiego.NCR.COM> <819@sugar.UUCP> Organization: Big "D" Home for Wayward Hackers Lines: 71 Xref: utgpu comp.arch:2318 comp.unix.wizards:4311 comp.os.minix:1691 Summary: Sorry, Pete, PDP's had VM. 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. > Why do you think DEC developed the Virtual Address Extension (VAX) in the > first place? > > > Yes, it's a nit, but the PDP-11 is a fine machine, and deserves to be > > remembered correctly. > > As a great little non-virtual system. Nothing wromng with that. Sometimes > virtual memory means virtual performance, as a good many PDP-11 fans have > pointed out. You can run way more users on and get way better real-time response > from a PDP 11/70 than any VAX you care to name. > -- > -- Peter da Silva `-_-' ...!hoptoad!academ!uhnix1!sugar!peter > -- 'U` Have you hugged your wolf today? > -- Disclaimer: These aren't mere opinions... these are *values*. No, the registers and micro-code were present in _certain_ PDP-11's to handle virtual memory. I believe that certain micro-Vaxen _didn't_ actually have enough support to handle virtual memory as well as, say, a PDP-11/70. All that is required to support virtual memory is the ability to generate a CPU trap/fault on a reference to a non-existence/non-resident page of memory, and then figure out how to restart the intruction so that it completes as if the page were resident. Might sound a bit simple, but for all it's greatness, the 68000 and 808[86] can't handle virtual memory because in the general case, an instruction can not be restarted. The PDP-11's of the larger type (44/45/70/73/(34?)) could support virtual memory if the memory management hardware had been installed. When a non-resident segment (8 8KB segments for instruction and 8 more for data, except the 34, which didn't support separted I&D) was referenced, a trap was generated and the address of the instruction and PSW were stacked. The operating system then needed to simulate the rest of the instruction, if the failing cycle was a write. In the case of a read, you could make the segment valid (load the MMU's and what not) if no registers had been modified during the instruction, or attempt to simulate the instruction. Whew! Needless to say, I'd much rather swap than go through that mess. Anyway, Unix on 11's did do some neat things. I seem to recall that a trap on a write into a deeper stack address caused the system to backup that instruction and expand the stack. This is the suggested use for the direction bit in the MMU, according to my old '45 manual. If I remember from the days of wierd gone by, we had a C compiler that on entry to a function generated a jump past the jsr pc,cret which was a jump back to the instruction after the first jump. I suppose this `feature' could have been used to demand page text in, had anyone bothered. Anyhow, go read a _modern_ (or even old, like 11/45) manual and you too will become a believer... - John. -- John F. Haugh II HECI Exploration Co. Inc. UUCP: ...!ihnp4!killer!jfh 11910 Greenville Ave, Suite 600 "Don't Have an Oil Well?" Dallas, TX. 75243 " ... Then Buy One!" (214) 231-0993