Currently, the design of platforms based on microcontrollers is a very promising direction due to their reduction in cost and increase in the performance of circuits. Microcontrollers are installed in industrial equipment, smartphones and audio players, video equipment, and much more. The paper provides the theoretical foundations for working with microcontrollers of the STM32 series, their purpose, system architecture, software model, as well as the features of the ARM Cortex M3 core. 32-bit core has many advantages, but the main one is its versatility. The structure of a software and hardware complex for studying and programming microcontrollers located on a remote server using the step-by-step debugging mode is described. The paper also discusses the features of writing programs for microcontrollers of the STM32 series. The virtual laboratory is a platform for the visual study of microcontrollers and the Assembler programming language. The system allows you to program, flash and debug microcontrollers without purchasing them. The user can test a code on a real device and view memory areas, registers and other debugging information in real time, as well as watch all changes in peripheral devices using video broadcast. The GDB GUI web interface allows you not to install additional applications, but to debug the program right in the browser. All kinds of sensors and indicators can be connected to the system, with which the user can work. Examples of programs written in the Assembler programming language are given to demonstrate the work of a virtual laboratory. The proposed platform can be integrated into a training course for learning the Assembler programming language.