The assembler language supported by High Level Assembler has functional extensions to the languages supported by Assembler H Version 2 and DOS/VSE Assembler. High Level Assembler uses the same language syntax, function, operation, and structure as these earlier assemblers. The functions provided by the Assembler H Version 2 macro facility are all provided by High Level Assembler.

Migration from Assembler H Version 2 or DOS/VSE Assembler to High Level Assembler requires an analysis of existing assembler language programs to ensure that they do not contain macro instructions with names that conflict with the High Level Assembler symbolic operation codes, or SET symbols with names that conflict with the names of High Level Assembler system variable symbols.

With the exception of these possible conflicts, and with appropriate High Level Assembler option values, assembler language source programs written for Assembler H Version 2 or DOS/VSE Assembler, that assemble without warning or error diagnostic messages, should assemble correctly using High Level Assembler.

The set of machine instructions that you can use in an assembler source program depend upon which operation code table you use for the assembly.