The device-class driver receives OS/2 request packets from the kernel. It is responsible for mapping the received request to a generic IOCtl request to be passed to the SCSI device driver. When a request from the kernel results in sending a Transfer SCB command to the SCSI driver, the device-class driver allocates the SCB chain header and formats the SCB and the SCB chain header. The TSB also must be allocated. When a request from the kernel results in multiple Transfer SCBs, the device-class driver chains the SCBs and sends only one Transfer SCB command to the SCSI driver. This achieves better performance and guarantees that requests are processed sequentially.

The device-class driver calls the SCSI driver to send the request to the device. The SCSI driver returns to the device-class driver after the request is completed. When a Transfer SCB request completes with an error, the SCSI driver performs a Request Sense command to the device to obtain sense data. The sense data is passed back to the caller in the data buffer area of the generic IOCtl request packet. The device-class driver might take some error-recovery steps at this point or return to the kernel, passing the return code from the device.