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[Discuss] How do Linux guys back up Windows?

On 12/28/2012 10:56 AM, Jack Coats wrote:
> VM370 was pretty good.  Of course it was not built as an OS but as a
> hardware emulator.  (IBM initially built it because they couldn't afford a
> mainframe for every development group, hardware and software, to use.)
> I was a sysprog for several years at a big company where we had thousands
> of users logged in and mainly active running anything from CMS to MVS under
> VM. (CMS was a single user simple OS that most interactive users used.  We
> often ran entire VM systems second level to do production work especially
> for disaster recovery testing, testing new releases of VM or other systems,
> or on rare occasion for better computing security.
> VM took about 5% of the hardware 'bandwidth' to run the VM.  But like was
> mentioned it was very efficient at virtual memory.  To test, the most VM I
> ran under VM was 6 levels deep.  Even with a nice size mainframe, it was
> terribly slow at that many levels of VM running under each other.
> But like was mentioned for VS1, most OSes could detect that they were
> running 'second level' (or further) and passed back up to the top level VM
> to actually handle the paging/swapping (and there was a difference back
> then).
> VMs were not new with IBM either. I think it was Burroughs that used them
> and published on it way before IBMs VM came about.
> Oh, and to let the secret out, the way IBM did VM was to use a 'diagnose'
> instruction which was officially an un-authorized machine language
> instruction so it generated an interrupt and an interrupt service routine
> took over (just like would happen when there was a I/O operation end or a
> hardware timer go off, or violating virtual memory boundries).  The
> interrupt service routing looked at the instruction and the byte or two
> after it in the 'user memory' to determine if it was a 'real diagnose' or
> 'a user problem', and took appropriate action.  The diagnose was initially
> an instruction used by the IBM CE (customer engineer) in their hardware
> diagnostic programs, but the software guys got in on the act too.
> Oh on each mainframe we typically ran around 2000 CMS users and a MVS or
> two for batch or to take care of SNA networking.  RSCS was another virtual
> system that ran the network communications.  VM did it's own spooling and
> driving of printers and such directly, but it could let guest operating
> systems like MVS or VS1 deal with it too.
> And yes, we had a source code license, and we did no occasion read the
> source code. There were many places in the source that the documentation
> was not 'kept up' or did not describe what the code really did, so we still
> had to be able to read the assembler.  Most of VM had been rewritten from
> assembler to SPL (System Programming Language, similar to PL/1) by the time
> I got into the arena in the early to mid 80's.
> My first UNIX was running Amdahl's UTS under VM.  A different animal when
> working on block mode terminals.  We did bring up UTS on first level a time
> or two for play, and it did scream.  In those years we did a lot of seismic
> processing on Perkin-Elmer machines (small IBM 360 type clones), and they
> ran their own unix-like system but with a more MVS style JES JCL (job
> control language) flavor.
> One consultant we had on VM over the years was on the initial HASP
> development team in Clear Lake City at NASA.  NASA had a lot of big iron
> mainframes in those days, but even their budget couldn't support what IBM
> told them they needed, so NASA said they needed a way to share input and
> output peripherals (mainly card readers and line printers in those days),
> so IBM put together a small team to make it happen.  In just a few months
> they did come out with the first cut of HASP (Houston Automatic Spooling
> Program).  It basically ran all the cards to disk, then fed them to
> computers (not just 1) on demand.  And took the 'printer output' as
> generated on each computer and saved it to disk till the job on the other
> computer was done, then put it in a print queue.  Saved a lot of hardware
> and IBMs collective butt at NASA.  IBM was almost thrown out over it, but
> HASP saved the day, and went on to be a big cash cow and saved mainframes
> for many years.  It eventually was 'virtualized' and rolled into the basics
> of JES in its flavors (Job Entry System that also handled spooling
> printouts, and job control networking).
IBM actually developed it on the IBM 360 where they had some contracts
that required virtual memory and neither OS/MFT nor OS/MVT was Virtual
Memory capable (it was called CP/360 for the high end 360 systems). But,
yes, it was a hardware emulator and had some very good benefits. We sold
our 370/135 to an insurance company in San Antonio who had some ancient
applications that would not run on a 370 native.

I don't know if Burroughs had a VM, but they did have virtual memory
before IBM. Burroughs had an operating system, MCP, that was very

Jerry Feldman <gaf at>
Boston Linux and Unix
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