Insertion and deletion of segments

The terminal-related dynamic MSDB database accepts ISRT and DLET calls, and the other MSDB databases do not.

Actual physical insertion and deletion of segments do not occur in the dynamic database. Rather, a segment is assigned to an LTERM from a pool of free segments by an ISRT call. The DLET call releases the segment back to the free segment pool.

The figure in Combination of binary and direct access methods shows a layout of the four MSDBs and the control blocks and tables necessary to access them. The Extended Communications Node Table (ECNT) is located by a pointer from the Extended System Contents Directory (ESCD), which in turn is located by a pointer from the System Contents Directory (SCD). The ESCD contains first and last header pointers to the MSDB header queue. Each of the MSDB headers contains a pointer to the start of its respective database area.