In a computer system (FIG. 1), targets (13) comprising peripheral devices (18) and target controllers (17) are connected to the host system by means of a device bus (12), illustratively the standard Small Computer System Interface (SCSI) bus. The device bus is connected to the host computer's central control (9) by means of a host adapter (10) and bus (11). The host adapter is controlled from the central control by a host adapter driver (23), while the targets are controlled by target drivers (22). Host adapter driver 23 provides to target drivers a standardized interface (24) for communicating with the targets through the host adapter driver, which interface is optimized to support a simple yet flexible target driver design, and which makes target drivers independent of differences in structure and operation of host computers and hence portable between computers having the same device buses.