Why do you need z/VSE SDL entries ?
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The System Directory List (SDL) is the directory of all phases (programs) loaded into the SVA (Shared Virtual Area - 24-bit) or SVA (31-bit). There is one SDL entry per SVA phase. An entry holds e.g. the phase name and the entry point of the phase. The SDL is allocated in the SVA (24 bit).
The size of the SDL depends on the number of (system) phases to be loaded automatically into the SVA and the number of phases defined by the IPL SVA SDL parameter. It's a static table. The value of the SDL parameter need to be increased if additional vendor, user phases or phases of installed products are to be loaded into the SVA.
IPL SVA PSIZE defines additional SVA size (24-bit and 31-bit) for SVA-eligible phases to be loaded with the SET SDL JCL command.
The PHASE statement provides the linkage editor with a phase name, the load address and an indication, if the phase is SVA-eligible (SVA or SVAPFIX parameter).
Products may recommend to load phases into the SVA e.g. to reduce partition virtual storage or if the want to share information between partitions. Those kind of phases need to be re-entrant.
The SET SDL JCL command can be issued at any time after IPL. A fresh copy of an existing SVA phase is loaded each time a SET SDL command for that phase is issued. That is the space of a prior copy is not freed. The new copy is used by the system dependent on the implementation of the caller. If the caller saves the entry point only once, the original copy is used (an IPL is required to get to the new copy). If the caller always retrieved the address before the call, the latest copy is used.
The LIBDEF (define sublibrary chain) statement defines, which sublibraries are to be searched e.g. for phases. The SDL can also be specified explicitly in the operand list.
More information is in the z/VSE System Control Statements or z/VSE Guide to System Functions books. As always these boos are on our z/VS