Each PAM page of a conventional K-SAM file has a 16-byte PAM key, located outside the page, in which DMS management information is kept.
In a K-SAM file in which n PAM pages have been grouped together using either the operand BLKSIZE=(STD,n) in the FILE or FCB macro or the operand BUFFER-LENGTH=*STD( SIZE=n) in the ADD-FILE-LINK command to form a logical block, this block has the following structure:
All n * 2048 bytes of the logical block are available for user data, since the DMS management information is stored in the 16-byte PAM key. For SAM files, the PAM key has the following structure:
where:
CFID | Coded File IDentification: an identifier which enables DMS to determine, by comparing it with a corresponding CFID in the catalog entry, whether the data in the PAM page belongs to the file or whether the page is reserved (for this file) but does not yet contain data. |
VN | Version number: This number permits a partial backup of a file by means of ARCHIVE or HSMS. This is a save run in which only those PAM pages of a file which have been modified since the last partial or incremental backup are saved. |
LHP | Logical Half-Page number: this is used for consistency checking for files and for decompression when a file is recovered by ARCHIVE or HSMS from a backup copy. |
BBN | Binary block number. |
DL | Length of the usable data in the buffer. |
nv | Bytes of the PAM key not used by SAM. |