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  The elements of an assembly-language subroutine are easy to understand if
  they are laid out in the order they occur. As you may recall, the logical
  organization was fully explained in Chapter 8, where we described an
  interface routine as five nested parts:

    Level 1: General assembler overhead
      Level 2: Subroutine assembler overhead
        Level 3: Entry code
          Level 4: Get parameter data from caller
            Level 5: Invoke ROM BIOS service
          Level 4: Pass results back to caller
        Level 3: Exit code
      Level 2: Finishing subroutine assembler overhead
    Level 1: Finishing general assembler overhead

  You can follow this basic organization for most interface routines written
  for system services or conventional assembly-language subroutines, but be
  aware that the actual coding will vary with every programming language.