The book describes the DOSSES Task Management Language, the operators of the EDITOR and LIBRARIAN system programs, which allows you to store both ready-made programs and parts of them in the machine memory, build complexes of programs, including overlay programs, from relatively simple program modules developed by various programmers. The book also describes high-level languages ​ ​ that can be used in the Disk Operating System, the most widely used computer on the younger EU models. The presentation of these languages ​ ​ is carried out in comparison with the ALGOL-60 language using the Normal form of Backus. A method of debugging programs using diagnostic messages of translators generated by the system both at the stage of program translation and at the stage of their execution is described. The book contains examples to help assimilate the material. It can serve as a guide for a programmer who is familiar with any algorithmic language or has some experience in programming on computers of other systems, while mastering the skills of working with EC computers. The book will also be useful for software developers of ISS, ACS and other software systems based on the Unified System computer.