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  As we've mentioned before, the wisest approach to programming the PC
  family is to write nearly all your programs in a high-level language (such
  as BASIC, Pascal, or C) and when necessary use the DOS or ROM BIOS
  services for whatever the high-level languages don't provide. On occasion,
  you may also want to create your own assembly-language routines to perform
  specialized tasks not available through your programming language or
  system services.

  When creating programs within the confines of a single programming
  language, you really don't need to know anything more about a language
  than what you can find in the manuals that come with it. However, if you
  need to break out of the bounds of a single language to access DOS or ROM
  BIOS routines, or perhaps to tie into a program that's written in a
  different language, you'll need to dig deeper into the technical aspects
  of both DOS (to learn how to link programs together) and the programming
  languages (to learn the requirements for program interfaces, which let the
  different languages communicate with each other).

  This chapter presents some overall considerations that apply to the
  advanced use of most programming languages. We'll start by describing the
  structure of the executable programs generated by compilers and
  assemblers. Later we'll consider the details of combining separate program
  modules into a unified program.