TurboIMAGE/XL Database Management System Reference Manual (30391-90012)

Chapter 10 481
Internal Structures and Techniques
Data Set Internal Structures
The first two bytes of the 10-byte link in the media record of each secondary entry are
always zero. The remaining eight bytes consist of two pointers bi-directionally linking the
secondary entries of the synonym chain to each other. As with detail chains, the first
member of this chain of secondary entries contains a zero backward pointer, and the last
member of the chain contains a zero forward pointer.
Blocks and Bit Maps
Each group of media records involved in a single MPE file record is a block. The first few
halfwords of each block contain a bit map employed by TurboIMAGE/XL to indicate
whether the corresponding media record is full or empty. There is one bit for each record in
the block. The bits occur in the bit map in the same order that the records occur in the
block. The bit map occupies as many integral halfwords as are required to contain one bit
for each record in the block. If a bit is zero, the corresponding record is empty. If a bit is
one, the record contains a data entry preceded by the associated structure information.
The format of a block is illustrated in Figure 10-4. The sample block contains four records
and the third record contains no entry.
Figure 10-4. Block with Blocking Factor of Four