Experiences with 386i serial ports under DOS

[Guy Harris]

>Remember that the DOS interrupt handlers aren't really talking to the
>hardware directly - they're going through Unix!

Are they? The 80386 has an "I/O Permission Bit Map" that, I think, lets an
OS allow non-privileged programs (including DOS programs running in a DOS
emulation) directly manipulate *some* devices, while not allowing them to
manipulate *all* devices.  I don't know whether the implementation of
VP/ix on the '386i uses this.  I also don't know whether it allows
non-privileged DOS programs manipulate the serial ports.

However, this *still* doesn't necessarily mean "real-time" interrupt
handling will work well; just because the program can directly bang on the
device with I/O instructions doesn't mean it'll instantly see interrupts
from the device (for instance, what if the interrupt handling code is
paged out, assuming DOS tasks are paged?).