Meaning of SYSTEM.INI's MaxCOMPort Switch (82792)

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A system may have more than four COM ports. IBM originally provided space for four COM ports in the BIOS port table in its first PC, so four is the limit for a default configuration for MS-DOS. To support more than four ports, a special MS-DOS device driver is necessary. Addressing the additional COM ports is the responsibility of this device driver. Most MS-DOS communications applications talk directly to the port hardware, so some of them do support more than four COM ports without requiring a special MS-DOS device driver.

The Windows communications driver (COMM.DRV) also assumes a limit of four COM ports. It is possible for a COM driver to support more, but the interface routines in USER.EXE limits the maximum to nine. There is no method to configure COMM.DRV to support more; however, with the Device Development Kit (DDK) sources, it is possible to build a new COM driver that can support more by changing a single equate.

The Virtual Communications Device (VCD) for 386 enhanced mode can be configured to support more than four COM ports; the extra ports are addressable only from MS-DOS applications unless COMM.DRV is replaced. The SYSTEM.INI MaxCOMPort= switch in the [386Enh] section can specify a higher number. This switch assumes that the COM ports are each addressed through separate I/O ports. There are some multiport adapters that have all of the COM ports share a common set of I/O ports, and then an index value is written to it that indicates which COM port is being addressed. These multiplexed cards are not supported by VCD.