You can use Mic-1 MMV's IJVM Assembler to practice writing assembly language programs, which you can then test on the simulator. Read the JAS Language Specification for an explanation of the structure of an IJVM program.
You may want to create a copy of echo.jas or add.jas and modify it to suit your needs.
If you modify your JAS program, you can reassemble and reload the current IJVM program by using Assemble/Load | Current JAS Assemble/Load.