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The avionics system in launch vehicles consists of multiple hardware and software subsystems which exchange essential data for their operation through a communication bus. The 1553B protocol is a very reliable communication standard, widely followed in mission critical systems. An exclusive protocol controller chip is often used to carry out the protocol functions and unburden the main processor. The development of the Real time OS (RTOS) software that acts as a supervisor software in the launch vehicle onboard computer is a challenging task. In this work, we elaborate on the design and development of the embedded RTOS software. It is responsible for various vital functions like handling power-on, interfacing with the 1553B protocol controller, scheduling the different tasks, handling of hardware and software faults and servicing interrupts. The details of the testing and the results of simulations are also explained. The functioning of the software was found to be normal and as expected in the tests.