On Sat, Mar 11, 2023 at 02:34 PM, Richard Cornwell wrote:
[...]
So
there would be no difference between say DOS/360 on a 16MB system and DOS/VS
on a 16MB system other then the overhead of paging hardware.
Agreed. Actually, DOS/VS (pre-VSE) would stop paging at just a bit over 8MB.
Pre-VSE, DOS/VS included real storage in the virtual address area but did not use it for virtual applications. See p.3-4 of DOS/VS System Management Guide Release 34, GC33-5371-6, Apr 1977, for a discussion and p. 1-9 for a diagram.
On a 16MB DOS/VS system, all partitions would run in real address space, so no paging. At 8mb, partitions would run either as real partitions (no paging) or as virtual partitions (8mb virtual address space backed by nearly 8MB real storage).
One could conceive of a workload on an 8MB system with several large real mode partitions reducing the page pool for a single 8MB virtual partition to the point where the virtual partition would thrash, but that's a bit contrived.
And this point is also a bit contrived, as 16MB is a small system today, and an 8mb DOS/VS system at the time would be unthinkably unlikely. But the point does illustrate an early design of virtual and real address spaces that we would think quite odd today. IBM did too; DOS/VSE separated the virtual and real address spaces (pre-AF; p.29, Introduction to DOS/VSE, GC33-5370-6, Jan 1979).
Steve
