Free space element (FSE)

An FSE describes each area of free space in a CI or block that is 8 or more bytes in length.

IMS automatically generates and maintains FSEs. FSEs occupy the first 8 bytes of the area that is free space. FSEs consist of three fields:

• Free space chain pointer (CP) field. This field contains, in bytes, the offset from the beginning of this CI or block to the next FSE in the CI or block. This field is 2 bytes long. The CP field is set to zero if this is the last FSE in the block or CI.
• Available length (AL) field. This field contains, in bytes, the length of the free space identified by this FSE. The value in this field includes the length of the FSE itself. The AL field is 2 bytes long.
• Task ID (ID) field. This field contains the task ID of the program that freed the space identified by the FSE. The task ID allows a given program to free and reuse the same space during a given scheduling without contending for that space with other programs. The ID field is 4 bytes long.

An FSE is shown in the following figure.

Figure 1. An FSE