> The APM spec says (regarding calling the 32-bit APM BIOS routines):
> - "The code segment descriptors must specify protection level 0, and
> the APM BIOS routines must be invoked with CPL=0 so they can execute
> privileged instructions such as port inputs and outputs."
>
> OK, this is an easy one: any kernel code will be running at
> protection level 0, right?
Yes.
> - "The caller must supply a stack large enough for use by the APM BIOS
> and potential interrupt handlers... The APM BIOS 32-bit protected mode
> interface must be called with a 32-bit stack."
>
> I'm *assuming* I don't have to worry about this. Is it even possible
> to change the kernel stack size? [interrupts are disabled before the
> apm routines are called, but I'm not ruling out some silly recursive
> routine in the BIOS.]
What constitutes "large enough"? There should be a figure, otherwise
the spec is broken.
> - "When the APM BIOS routines are called in protected mode, the current
> I/O permission bit map must allow access to the I/O ports the BIOS may
> need to access in the process of performing the selected function."
>
> This goes with the first question. Is there a special I/O port
> permissions map I need to fiddle with, or can I assume that kernel
> code will be running with all permissions wide open?
This is a nonsense requirement, which worries me. If CPL <= IOPL the
I/O permissions bitmap is never consulted, and given the CPL 0
requirement, CPL is always <= IOPL, it should make no difference at all.
-hpa
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