Disk Addresses
BDAM accepts disk addresses as either actual device addresses, as relative track addresses, or as relative block numbers.
Device addresses are eight byte fields in the form MBBCCHHR. All subfields are binary numbers.
- M is the extent number for files with more than one extent — non-contiguous files.
- BB is zero. Previously this indicated the bin on an IBM 2321 Data Cell.
- CC is the absolute cylinder number on the device.
- HH is the track (head) number.
- R is the record number on the track. Record zero contained control information for the track and could not be used.
Specifying device addresses would make the dataset unmovable.
Relative track addresses are three byte fields in the form TTR. All subfields are binary numbers.
- TT is the relative track number in the file, starting with zero.
- R is the record number on the track.
Relative block numbers are four byte (fullword) binary numbers indicating the block number in the file. The first block is block 0. This form of addressing can only be used with fixed length blocks (RECFM=F).
Read more about this topic: Basic Direct Access Method
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