The IPL process, dynamic activate, or the HCD report function call the UIM with a request to build DFTs. The UIM builds the device feature parameters and calls the DFT build routine to finally build the device features tables.

A UIM must call the DFT build routine once for each DFT to be built. A DFT must be built for each device number defined to an operating system in the IODF whether that device number represents a device or an exposure of a multiple exposure device. For a parallel access volume, a DFT must be built for the base device number and each of its alias device numbers.

A multiple exposure device is a device that can be allocated by a single device number but can be accessed by multiple device numbers from the system.

A parallel access volume can handle multiple, concurrent I/O requests to a single volume from the same system. Each base and alias unit control block (UCB) for a parallel access volume is represented by a device number in HCD. You specify the base device number for allocation. The I/O request identifies the base UCB and the system uses the base UCB or one of its alias UCBs depending on availability.

A DFT must be built for each device number defined in the I/O device internal text record (IODV).

To call the DFT build routine within a UIM, use a BALR instruction in 31-bit addressing mode. Use standard register save area conventions. The UCADFTP field in the UCA contains the address of the DFT build routine.

DFT Build Routine Input Parameters

A UIM provides the input to the DFT build routine in the device features parameters. The device features parameters reside in the UIM and are mapped by CBDZDFP.

Registers on Entry to the DFT Build Routine

Register 0

Undefined

Register 1

Pointer to a two-word parameter list

• Word 1 - Address of the UCA
• Word 2 - Address of the device features parameters

Register 2-12

Undefined

Register 13

Address of an 18-word save area

Register 14

Return address

Register 15

The DFT build routine entry point address

Registers on Exit from the DFT Build Routine

Register 0-14

Restored

Register 15

Return code

Return Codes

RC 0

No errors detected, DFT built

RC 8

Error detected, no DFT built, message written