Rita Zhang wrote:... May I know what do you mean by 'The LNKLST' can only have 255 entents in total' ? i don't reach it , would you please help explain it? Thanks advance.
Every data set has at least one extent (not entent). An "extent" is just the data area on disk. A data area called a DEB is built by the system when it opens a data set. Most of the DEB is a map of the space allocated for the data set - or for a concatenated PDS all of the space allocated for the data sets. This space map is used by I/O to ensure a program does not use data areas allocated to other data sets. The space map has a maximum size of 255 extents; the number of entries in the space map is stored in a data area called DEBNMEXT. DEBNMEXT is defined as 1 byte, so the maximum value it can contain is X'FF', or 255. It has been this way for nearly 50 years; I can't imagine how many IBM and user programs would have to be changed if DEBNMEXT is expanded. A user program might use DEBNMEXT if it uses the space map to determine the number of tracks in an opened data set; this is much easier than trying to use the space maps in the Format 1 and Format 3 DSCBs for the data set, especially since OPEN has already done the arithmetic, which is kind of complicated.