Date: Thu, 23 Apr 1998 09:19:30 -0700 (PDT) From: patl@phoenix.volant.org To: Jonathan Lemon <jlemon@americantv.com> Cc: freebsd-hackers@FreeBSD.ORG Subject: VM architecture (was Re: Protected mode instructions which reduce to noop.) Message-ID: <ML-3.3.893348370.3628.patl@asimov> In-Reply-To: <19980423103837.32719@right.PCS>
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> What VM type architecture were you referring to? >From the context, I assumed that he was reffering to IBM's mainframe system. (Originally VM/370 IIRC.) It is a smallish OK kernel that simple provides multiple virtual machines that run other OSes. (Or other copies of itself. Back in the late 70s the recursion depth was around five or six before something failed.) The sub-OSes had access to real or virtual disks, virtual card readers, card punches, and printers, terminals, etc. The basic mechanism is to run the sub-OSes without kernel privilege and to trap on Priveleged Instruction exceptions. The instruction, registers, and memory would then be examined and the requested operation translated as necessary and performed. Or the exception would be passed back up to the client OS, as appropriate. There was also a mechanism, using a privileged diagnostic instruction, for a VM-aware OS to directly request the services of the VM kernel. CMS was the user interface to VM. It was an OS that actually required VM underneath since it didn't attempt to perform any of the functions that VM could provide for it. The standard mode of operation had each on-line user in a separate virtual machine running their own copy of CMS. There was also usually one copy of DOS (no, not much like PC-DOS), OS1, or OS2 (no, not OS/2) running batch jobs in it's own virtual machine. OS1 and OS2 could be configured to know that they were running under VM and delegate some of their operations for better system-wide efficiency. (One of the standard techniques was to tell OS1 that it had a full complement of physical memory and that it shouldn't page. This was a big win even when it was the only thing running on a VM system because VM's paging algorythms were better than OS1's.) -Pat To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-hackers" in the body of the message
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