Most requirements for coding vary depending on the part of DFSMS you customize. Be aware of these:

• The examples in this information are written in assembler language and you can also create exits in assembler.
• At entry to your exit routine, save all registers and restore them before you return to your calling routine. The section for each exit specifies any registers that you must return with special contents. For example, frequently you must supply a return code in register 15 upon returning to DFSMS processing.
• If you replace a module, make sure you thoroughly test it before you make it available.
• Your routine should be reentrant so it can handle concurrent requests.
• Keep an unmodified copy of any replaceable modules you modify.