## 1 Introduction  
Embedded-system design nowadays plays a key role in the development of industry, science and technology, and many areas of the economy [1, 2]. Embedded systems are special-purpose computer systems that integrate a hardware component (a microprocessor or microcontroller with the necessary peripheral environment) with software, to perform a specific function in a larger electronic or mechanical system [3]. Over the past several years, there has been a great deal of research work focused on creating and improving various approaches and tools for teaching as well as supporting the process of designing and simulating embedded systems. The following can be distinguished: an integrated approach to teaching of embedded systems [4], the use of the Linux system [5], a motivation-driven approach [6], a teaching process with graphical-system-design methodologies [7], application-oriented teaching [8], teaching based on outcomes-based education [9], and many others [10, 11, 12, 13, 14]. An important part of the development of the tools to support the design of embedded systems are the various types of simulation tools, virtual platforms, and virtual labs [15, 16, 17, 18]. Virtual platforms of embedded systems are the key to software's modern and rapid development process [19, 20, 21]. The programming process, using those platforms, can start in the early-project stage, in parallel to hardware development [22], so in the very early stage of an embedded project. There are many solutions in the field of virtual platforms [23]. They can be oriented on precise system emulation [24, 25] or try to imitate the system behavior of embedded systems through their simulation [26]. Virtual simulators can imitate a whole system [27; 28] or can reproduce behavior at the hardware level [29]. A mixed approach is also common [30], to prepare an efficient virtual environment for embedded-system programming [18]. Software modeling for embedded systems using such virtual platforms can reduce the time to market and allow for performing extensive unit tests [31] that could be troublesome in real-device [32] design. For didactic purposes in the time of the COVID-19 pandemic, such virtual platforms are invaluable at any teaching level [33; 34; 35]. There are many solutions in the area of virtual platforms and simulators [19; 20; 21]. To name a few, we have hardware emulators, such as QEMU or IDEs, with hardware simulation support (e.g., Keil \(\upmu\)Vision (Richardson, TX, USA), Microchip Studio (Chandler, AZ, USA)). There are system-level simulators that can reproduce the behavior of different software architectures (GFX simulator (New York, NY, USA), Qt simulator (Espoo, Finland)) or a whole embedded operating system such as FreeRTOS. In case of developing a highly customized embedded system, with a so-called in-house system (which is a common situation) and a very specific hardware setup, we cannot expect that a simulator will be available. Both the software and the virtual hardware platform can be unavailable for either emulation or simulation purposes. In our previous paper [36], a simple and effective algorithm for the precise embedded-system peripherals' visualization used in virtual simulators was presented. The true motivation for this paper as well as for the accompanying work was the activity of our ambitious students. With every new class of microprocessor techniques or embedded-software-development labs, based on TMSLAB or RPILAB systems, the same question arises: are virtual simulators of the embedded systems available? The paper tries to answer that question and satisfy the needs of students by giving them the tools for developing programs without direct hardware. The first ideas and main concepts were created some time before the COVID-19 pandemic, but the real need for virtual simulators arose during the lockdown, to allow students continue their labs. In this article, two custom virtual-simulator solutions for highly customized embedded systems are presented.