Accessing the COM3: or COM4: Port

MS-DOS                 | WINDOWS

The information in this article applies to:

• Microsoft C for MS-DOS, versions 6.0, 6.0a, and 6.0ax
• Microsoft C/C++ for MS-DOS, version 7.0
• Microsoft Visual C++ for Windows, versions 1.0 and 1.5

If MS-DOS, the ROM BIOS, and the hardware all support COM3: and COM4: as valid devices, then the MS-DOS MODE command may be used to redirect communication to access the ports from within Microsoft C. For example, to redirect COM3: to COM1:, give the following command:

   mode COM3:=COM1:

Versions of MS-DOS earlier than MS-DOS version 3.3 do not recognize the COM3: and COM4: ports. Versions beginning with MS-DOS version 3.3 allow some systems to access the ports, but the ROM BIOS also must support the extra communications port hardware.

The serial port hardware must also allow setting of IRQ and memory addresses to allow multiple serial ports. In some cases only two ports can actually be used, even though more are physically available. Frequently a choice has to be made between COM1: and COM3: and between COM2: and COM4: since control lines and addresses are shared. A manufacturer's hardware reference is usually needed to determine the exact settings for a given situation.