Migration considerations for MSC exit routines
Exit routines that use information from the MSC logical and physical blocks, such as the TM and MSC Message Routing and Control User exit routine (DFSMSCE0) and the Message Control/Error exit routine (DFSCMUX0), must be changed to either use callable services or use the FIND/SCAN function of the DFSCBTS macro to find the logical link blocks (LLBs) and the physical link control blocks (LCBs).
The exit routines can no longer use the MSCDLLB and MSCDLCB fields in the MSCD block.
Exit routines that access remote logical terminal blocks (RCNTs) must be updated to use the FIND and SCAN control block callable services to access the RCNTs. In IMS 13 and earlier, exit routines could access RCNTs by referencing the RCNTs in the SCDRCNTM field of the IMS system contents directory (SCD). In IMS 15, exit routines can no longer use the SCDRCNTM field to access RCNTs. For more information about the FIND and SCAN control block callable services, see IMS Callable Control Block Services requests.
User exit routines that reference MSC control blocks must run in 31-bit mode.
User exits in the IMS nucleus that reference MSC DDM or command modules with a VCON will need to be changed. Examples include the TM and MSC Message Routing and Control User Exit routine (DFSMSCE0) and the Message Control/Error exit routine (DFSCMUX0). MSC DDM modules are removed from the nucleus and now reside in 31-bit storage. Their addresses can be accessed from pointers in other control blocks.
The IMS nucleus no longer contains any MSC modules. The MSC modules that were in the nucleus in earlier releases of IMS are now in 31-bit storage. Their 31-bit addresses are stored in the MSC blocks. As a result of this change, MSC control blocks can no longer be sequentially scanned or their addresses calculated by multiplying the block length by the link number. The new pools (MSCP and MSCL) must be scanned using the DFSCBTS macro or FIND/SCAN control block callable services.
User exit routines that reference MSC modules in the nucleus must be changed to reference MSC modules in 31-bit storage, whose addresses are now saved in the MSCD.
The LLB is the main control block for MSC logical links and the LCB is the main control blocks for MSC physical links. The pointers within the MSC blocks that are used to tie the blocks together have not changed in IMS 15.