iX V6.5 (30391-90011)

Chapter 10 475

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