Limiting the UCB scan

If you want to limit the UCB scan to a specific device class, you must provide the address of a one-byte field containing the hexadecimal code for that class. These fields are defined in the UCBDVCLS (or UCBTBYT3) bit string in the UCB. Figure 1 lists the valid device class specifications with their UCB definitions. For example, to restrict the search to tapes, set the byte containing the device class equal to the constant UCB3TAPE. If you use the UCB definitions in your program, you must include the UCB mapping macro (IEFUCBOB). To scan all of the UCBs in the system, provide the address of a one-byte field containing X‘00’.