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). Ahh, memories. _______________________________________________ Discuss mailing list Discuss@blu.org http://lists.blu.org/mailman/listinfo/discuss
