Volume layout and record formats on CKD Devices

This appendix gives information on volume layout and record formats for CKD devices. It includes:

  • An example of a 3380 volume map
  • Track contents after initialization
  • IPL bootstrap record contents
  • How track associations are recorded

The track is the smallest directly addressable space on a device. Tracks typically occupy a full 360-degree rotation of the disk medium; however, a track can correspond to either fractional or multiple device rotations.

Each track has a starting point, called an index (see Figure 1). After its index point, each track on a CKD device begins with a home address that identifies the track's address and defines its operational status.
Figure 1. CKD track and record formats
CKD track and record formats are described in the text that follows.

The track descriptor record is always the first record on the track following the home address. It is record zero (R0). If a flag in the home address area indicates the track is defective, then the count area of record zero contains the track address of the track that is to be used as an alternate. The count area of record 0 on the alternate track contains the track address of the defective, primary track.

Following the track descriptor record (record 0), one or more user records can be written on the track. These records are typically numbered in sequence. Each of these physical records contains a count area, an optional key area, and a data area, each of which is separated by a gap. Checking information is added to each area when it is written and is used later for detecting and correcting data errors.

Count Area contains the ID of the data area that follows. The record ID is specified by a value expressed as five bytes (CCHHR). In addition to the record ID, the count area specifies the length in bytes of the key and data areas of the record.

Key Area is an optional portion of the record. It can be used by the programmer to identify the information in the data area of the record. Note that there is no key area in the standard record zero.

Data Area contains data that has been organized and arranged by the programmer.