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    SubjectRe: I/O permissions for V86 tasks (Enhanced V86 Mode)?

    On Thu, 16 Jul 1998, Kendall Bennett wrote:

    > > And by the way, the answer to your previous question about IOPL is no,
    > > the Intel processor does not allow the V86 task to take notice of any IOPL
    > > so you are forced to do it via the bitmap.
    > Actually this brings up another issue. I recall reading in Dr Dobb's
    > (I think) about the undocumented Pentium Enhanced V86 mode. One of
    > the features of this if I remember correctly is that you can enable
    > IOPL priveledges for V86 tasks rather than having to use a full 8Kb
    > I/O bitmap.
    > Does anyone know any details of this, and if this has been looked
    > into for the V86 mode support in Linux? I am going to hunt through my
    > old issues to see if I can find the articles in question, and if the
    > info is enough I will try to hack the support for this into my kernel
    > sources. This would be the ideal solution if it would work, since
    > then we don't have to do anything special to get full I/O access for
    > V86 tasks (and I am sure this will make the DOSEMU crowd happier).

    It won't work, this IOPL thing ony affects the interrupt flag
    (CLI/STI/PUSHF/POPF/IRET) and the interrupt redirection bitmap which
    allows int n instructions to be executed without being reflected from
    protected mode. But the I/O bitmap itself is not affected, it would be
    far too dangerous to allow access to any I/O port.


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