How to set the UART's Divisor Register in NT using API

[Roy Wang]

I have a question about how to change the Divisor Latch in the serial port. In win98, _inp() and _outp() can work, but in NT it don't work. The source is as following:
unsigned char result;
unsigned short ppport = 0x2EB;

result = _inp( ppport );
_outp( ppport, (result | 0x80));
_outp( 0x2E8, 21 );
_outp( 0x2E9, 0x00 );
_outp( 0x2EB, (result & 0x7F));

Roy Wang

This is (nearly) not possible using NT. Direct hardware access, and by all
means, hardware access of a device already controlled by a device driver, is
not supported on NT. There _is_ a way doing this (see DrDobb's Journal, the
undocumented NT corner, for an article about a device driver named Giveio.sys),
but this really isn't feasible due to several reasons :

1) Programming the divider latch register of the UART means setting a new
baudrate - the COMM API of NT (and Win95) allows for that.

2) You might not know the I/O address of the serial port in use; NT does not
always use the direct COM1 = 0x3f8, COM2 = 0x2f8, COM3 = 0x3e8,
COM4 = 0x2e8 mapping; there may be more than four serial ports
(I've seen NT machines using 12 serial ports).

3) You might have serial ports using some completely different hardware;
that is, no 16550 at all - consider a USB to serial adapter.
OK, this can't yet be used with NT, but some bright sunny day, Redmond
might decide to ship Windows 2000 (formerly known as NT 4.0).

Post a follow-up if you have any further questions; detail what you intend to do.