Archive of UserLand's first discussion group, started October 5, 1998.
Re: Run Linux and Windows at the same time
Author: Bruce Hoult Posted: 5/4/1999; 4:15:19 PM Topic: Run Linux and Windows at the same time Msg #: 5654 (In response to 5647) Prev/Next: 5653 / 5655
Vmware does the same sort of thing that Sheepshaver on Be for PPC does with the macos. It creates a machine interface for a copy of an operating system that runs natively on that hardware. It so happens that that machine interface is running in another OS.That's true, except that the 68k family (from the 68010 on) and PowerPC family were carefully designed so that you can implement virtual machines. This requires, in particular, that you run the hosted OS in user mode but it ,thinks that it is running supervisor mode and it has no way to find out otherwise. This means, for example, that the instruction for getting the processor status register has to be privileged so that when the hosted OS does this (from user mode) it traps and the real OS can adjust the values to show that the hosted OS is in supervisor mode (even though it isn't really).
There are numerous other examples, involving for example, manipulation of the virtual memory page tables, where the hosted OS has to believe it is manipulating the real hardware but it isn't.
The x86 family does not have support for virtual machines. In general the instructions for finding out the true state of the machine are not priviledged on x86.
This is why VMWare is such a good hack. On the 68k or PPC it would be mere standard engineering.
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