- Find out the model z machine you are using.
- Go here: https://www.ibm.com/docs/en/systems-hardware/zsystems
- Click on your model.
- Choose Related Publications twice.
- Scroll to z/Architecture Principles of Operation
- Bookmark it.
You will also get a link for the z/Architecture Reference Summary that contains all the machine instruction formats, a listing of all instructions by mnemonic, and all sorts of details about the machine architecture.