ColeSoft Marketing produces z/XDC - a comprehensive and powerful debugging/diagnostic tool for z/OS-based Assembler and C languages.  z/XDC allows you to work on programs ranging from simple applications to complex, critical, bet-your-business solutions that run in batch regions, multiple address spaces, whether APF-authorized or not.

In one sentence:  "Using z/XDC, a knowledgeable programmer may more easily find and fix bugs in arbitrary object code within any csect within any module running under any Request Block under any Task Control Block (or SRB) in any address space on the system including much of the internals of the OS itself."