1. ls: list file and directory.
2. cd [directory]: enter the directory.
3. cd : enter the root.
4. incom [file name]: insert component
5. rmcom [component's name]: remove component
6. swap [file name] [component's name]: swap new_file to old component
7. lscom: list component
8. run: run application

As shown in Figure 1, the main framework of Turning OS is divided into three major parts: components, kernel symbol tables, and managers that provide kernel services. In the design of components, components are classified into three categories:

1. components determined at system build time, for example, file system FAT16 and FAT32 which are classified as file system components and the file system supported in the operating system is selected according to the requirements of the device.
2. components that can be inserted, removed, and swapped, for instance, device driver of GPIO which is classified as a hardware device driver component.
3. components that only support swap operation, for examples,scheduler which is classified as a kernel service component. Turning OS supports component swapping operation, so it will save time in restarting and relocating applications and components. In lightweight operating systems, usually only one scheduling algorithm is used at a time. Therefore, the system swaps scheduler component according to application requirement and runs one scheduling algorithm at a time in system can save memory and resources in embedded systems with limited resources.

Figure 1:

The implementation of Turning OS is done on a single-board computer Raspbarry-Pi3 model B+ for its popularity and uses aarch64-linux-gnu-gcc package as building tool, and Ubuntu16.04.3 is choosen as the development environment. In term of the implementation, in addition to basic kernel services required in the test cases, such as shell, mutex, and fread/fwrite, the most important parts in the component operation functionality are the component manager and the kernel symbol table:

1. The Component Manager is implemented as a background service. It is responsible for insert, remove or swap operations on components, and updates the component table and kernel symbol table. The operations of updating the component table and kernel symbol table are completed in the uninterrupted component manager service. Threrefore, it ensures that the component is installed or removed before the next use.
2. The Kernel Symbol Table is implemented taking as example the Loadable Kernel Module (LKM) of Linux. When inserting a component, the kernel symbol table serves as both an interface and relocation tool. After checking whether the functions of component fully meet the kernel API requirement, the system will relocate those functions and load components into the memory before the address of the component is inserted into the kernel symbol table. Kernel symbol table is the interface when a component is used by other applications or components.

For demonstration and verification purposes, POC (proof of concept) tests are conducted for each type of the components, file system component, hardware driver component, and scheduler component. The results can verify the feasibility of those components design and implementation. The scenarios of testing FAT32/16 file system components are shown in Figure 2. To begin with, the file system component is selected at system build time and an SD card is formatted with four partitions of different type of devices such as P1: FAT32/46MB reserved for boot, P2: FAT16/3GB, P3: FAT32/3GB, P4: FAT16/3GB. The operating steps are done as follows:

1. Ubuntu16.04 writes a Read.txt file. The content is 2050 groups of {"Belly", 17} and {"Louis", 33} with 32 bytes per group. The total size is greater than 64KB.
2. Turning OS reads the Read.txt file. If the contents are all {"Belly", 17} and {"Louis", 33}, the shell prints "Success". Otherwise, shell will print different part.
3. Turning OS writes the Read.txt file from group 2049 to group 2051 of the data {"Tina", 25} and {"Sunny", 31}.
4. Ubuntu16.04 reads the Read.txt file. If the contents are all {"Belly", 17} and {"Louis", 33}, the shell prints "Success". Otherwise, shell will print different part.

Figure 2:

For the results, Figure 3 shows that Turning OS uses FAT32 file system component to read the FAT32 partition (P3) in SD card, and then overwrite it from group 2049 to group 2052 as described by step (b) and step (c) above. The result of testing with FAT16 file system component shown in Figure 4.

Figure 3:

Figure 4:

The result of testing FAT16/32 file system components is shown in Figure 5. The different part of Read.txt is the data from 2049 group to 2051 group. The 2052 group do not exist because the file pointer stops at 65664th byte position.

Figure 5:

To be continue....