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The IBM VM is based on the use of the "Diagnose" instruction, which is not really an instruction at all. It is an instruction operation code that actually generates an invalid instruction interrupt. The 'invalid' instruction is examined by software (more of the VM OS) to determine what you really meant to do, and it does it for you. This is the heart of IBMs VM. Historically, "Diagnose" instructions were a 'secret' used in "CE Only Code" (Customer Enginner, that did hardware support mainly) used for system diagnostics without the customers OS or software running. Intelegent use of the Diag instruction allowed some OSes (like VM, MVS, etc) to realize they were running as a 'guest' or not as the main OS on the machine, and let the 'first level' VM take care of tables like those used for memory virtualization, etc. I am not sure what an Intel based (style) processor does when it hits an 'invalid' instruction or how VMWare implements it. Does this help, or am I just reminising about former jobs? Sorry if it was only reminising. ... Jack > Actually, the 370 instruction set was never written with VM in mind either. > Each 370 had a different internal architechture. Most of the 370s had > support for VM in the microcode. Additionally, production OSs like VS1 and > DOS(370 DOS not PC DOS) had additional handles for VM. > There are some interesting issues: > Let's say what heppens when an application page faults. Does the VM allow > the OS to select another app or does the VM place the entire OS on a page > wait. > On 22 Jul 2002 at 16:48, Nathan Meyers wrote: > > I hope my answer didn't read like an attempt at a VM tutorial :-). > > > > As I understand it, the Intel architecture was never designed to support > > VMs the way systems like VM370 did, and the VMWare folks had to be pretty > > clever about making it work. > > -- > Jerry Feldman <gaf at blu.org> > Associate Director > Boston Linux and Unix user group > http://www.blu.org PGP key id:C5061EA9 > PGP Key fingerprint:053C 73EC 3AC1 5C44 3E14 9245 FB00 3ED5 C506 1EA9 > > _______________________________________________ > Discuss mailing list > Discuss at blu.org > http://www.blu.org/mailman/listinfo/discuss > > -- Coats Closet - Faith, Family, Fun, Fortune - http://www.coats.org
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