## Physical Device Driver Reference

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### 17h - Remarks

On entry the request packet header unit field identifies a physical disk number (based on 0) instead of a logical unit number. The device driver returns a bit map of which logical units exist on the physical drive. The physical drive relates to the partitionable hard disks reported to the system by way of the PARTITIONABLE HARD DISKS command. It is possible that no logical units exist on a given physical disk because it has not yet been initialized.

The physical device driver must perform the following actions:

Set the 4-byte bit mask to indicate which logical units that it owns exist on the physical partitionable hard disk for which the information is being requested.

Set the status WORD in the request packet header.

The bit mask is set up as follows:

0 The logical unit does not exist
1 The logical unit exists

The first logical unit that the physical device driver supports is the low- order bit of the first byte. The bits are used from right-to-left, starting at the low-order bit of each following byte. It is possible that all the bits will be 0. As an example, a block device driver supports five units spread over the two diskette drives and one partitionable hard disk in a system. Unit 0 and unit 1 map to the diskette drives. Unit 2, 3, and 4 map to the hard disk. For the device command, this physical device driver sets the 4-byte bit map to:

        '0000 0000 0000 0000 0000 0000 0001 1100'  binary

    or

        '00 00 00 1C'  hex