From mboxrd@z Thu Jan 1 00:00:00 1970 From: bakul@bitblocks.com (Bakul Shah) Date: Wed, 25 Apr 2018 16:01:37 -0700 Subject: [TUHS] /dev/drum In-Reply-To: References: <8225C5DB-27BD-464E-930A-522C30C20EBD@tfeb.org> <25A1FED0-4F8B-408F-B27B-5728C649D8BE@collantes.us> <7wfu3nuqeb.fsf@junk.nocrew.org> <24637D2D-4865-4E45-821B-529CAEEA5589@ronnatalie.com> Message-ID: <22516FB0-C89C-4727-B171-BEB84B27DD7C@bitblocks.com> This is what we did at Fortune Systems for our 68k based v7 system. There was an external “mmu” which added a base value to a 16 bit virtual address to compute a physical address. And compared against a limit. There were four base,limit pairs that you had to rewrite to context switch: Text, data, spare and stack. At a minimum the system shipped with 256KB so you could have a number of processes memory resident. You swapped out a complete segment when you ran out of space. I imagine other 16bit word size machines of that era used similar schemes. > On Apr 25, 2018, at 1:29 PM, Paul Winalski wrote: > > Some PDP-11 models had a virtual addressing feature called PLAS > (Program Logical Address Space). The PDP-11 had 16-bit addressing, > allowing for at most 64K per process. To take advantage of physical > memory larger than 64K, PLAS allowed multiple 64K virtual address > spaces to be mapped to the larger physical memory. Sort of the > reverse of the usual virtual addressing scheme, where there is more > virtual memory than physical memory.