Abstract: A host data processing system which includes a plurality of input/output devices operates under the control of an enhanced version of the UNIX operating system. The host system includes an emulator which runs as an application process for executing user emulated system (ES) application programs. The emulator includes a number of emulated system executive service components operating in shared memory and an interpreter, an emulator monitor call unit (EMCU) and a network terminal driver (NTD) component operating in the host memory. Configuration command means are provided for initially configuring a host terminal to operate in a plurality of asynchronous driver (ATD) modes implemented by the NTD component. When a terminal has been configured by a user to run applications written for the ATD driver, it sets a mode indication which causes the NTD component to respond to user applications as an ATD driver and determine if each ATD request can be carried out with functionality included within the NTD component.