which covers COBOL history, TSO commands, and DB2. You can also find a Mainframes Refresher Part 2 for advanced topics. Academia.edu : Hosts the Mainframes Refresher Part - 1
Unfortunately, we can't provide a direct link to the PDF due to copyright restrictions. However, we can suggest a few options to help you find the document: mainframe refresher by muthu pdf link
If you're looking for a mainframe refresher, here are some general topics and resources that might be helpful: which covers COBOL history, TSO commands, and DB2
: Online transaction processing, commands, and common abend codes. which covers COBOL history