The following figure illustrates the one-way chain of available blocks.
In The following figure the following structures use a one-way linked list to keep track of empty blocks:
-
Compact data set
-
Index random set
-
Index sequential set
-
Ordered data set
-
Random data set
-
Unordered data set
-
Unordered list
The available chain begins in word 0 of block 0 and passes through word 0 of each available block. When there are no more available blocks, word 0 points immediately beyond the end of file. That is, it points to the next block which is to be allocated when the file is extended.
Word 1 always points immediately beyond the end of file. It is advanced each time the file is extended. A limit error (LIMITERROR) occurs on a data set if extending the data set file would cause the file to exceed the maximum number of areas specified by the user. Once a limit error has occurred, all attempts to add new records result in limit errors.
