The physical device driver for the IBM AT bus machines by default, supports two ASYNC ports, COM1 and COM2, each on separate levels. ASYNC ports with the following base I/O addresses are recognized by the physical device driver:

• 3F8H (must generate a level 4 interrupt)
  
• 2F8H (must generate a level 3 interrupt)

COM3 and COM4 are supported by the command line parameters for COM.SYS. COM3 and COM4 are supported through parameters on the DEVICE= statement in the CONFIG.SYS file. These parameters specify the desired base I/O address and interrupt levels. The physical ASYNC device driver for the AT- bus machine interfaces directly to the hardware and supports:

• IBM AT-bus serial/parallel adapter based on the NS 16450 Universal Asynchronous Receiver Transmitter (UART) device

• Other compatible adapters based on the UART architecture (NS 16550, NS 16550A)