## Input/Output Device Driver Reference

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### AT Bus Adapter Support

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)