Actual Addresses

When the system returns the actual address of a block on a direct access volume to your program, it is in the form MBBCCHHR, in which the characters represent the following values:

M: 1-byte binary number specifying the relative extent number. Each extent is a set of consecutive tracks allocated for the data set.
BBCCHH: Three 2-byte binary numbers specifying the cell (bin), cylinder, and head number for the block (its track address). The cylinder and head numbers are recorded in the count area for each block. All DASDs require that the bin number (BB) be zero.
R: 1-byte binary number specifying the relative block number on the track. The block number is also recorded in the count area.

If your program stores actual addresses in your data set, and you refer to those addresses, the data set must be treated as unmovable. Data sets that are unmovable cannot reside on system-managed volumes.

If you store actual addresses in another data set, those addresses become nonvalid if the first data set is moved or migrated. Although you can mark the data set with the unmovable attribute in DSORG, that prevents the data set from being SMS managed.