This guide provides information on using the IBM High Level Assembler for various operating systems, including z/OS, z/VM, z/VSE, and Linux on z Systems. It covers topics such as controlling assembly with options, providing user exits, external functions, assembly error diagnostics, and programs on different operating systems. The guide also includes appendices on cross-system portability considerations, output formats, user interface macros, translation tables, error messages, and the HLASM services interface.