On 07/26/2015 08:09 PM, Peter Easthope wrote:
On Sunday, July 26, 2015 at 10:32:05 AM UTC-7, M Philbrook wrote:
if Bios has control of it, it simply flips a bit
in a device manager chip on the board.

OK, that would be in evidence when the Bios interface
allows disabling.

From that point, the OS does not see that serial port.

There must be a customary way for the OS to read the state
of the bit. Otherwise communication software would have
to allow for obscure hardware variations. Can software
flip the bit? Ie. can a port can be enabled and disabled
when the system is running?

Depends what you mean by software.
The ancient vt1211 superi/o chip does its configuration via port writes.
You may need to be in kernel context to be allowed to do that(or ioperm()).
You may have to browse datasheets, to find out if others are similar
or if there's a de-facto standard.
Having a look at intels LPC bus specs may be necessary, too.
(they're available online)
USB serial ports are a different story.

Fortunately I'm only interested in RS-232 style serial ports,
for now at least.

Thanks! ... Peter E.