In order to ensure memory properties of an operating system, it is important to verify the implementation of its heap manager. In the case of an existing operating system, this is a difficult task because the heap manager is usually written in a low-level language that makes use of pointers, and it is usually not written with verification in mind. In this paper, our main contribution is the formal verification of the heap manager of an existing embedded operating system, namely Topsy. For this purpose, we develop in the Coq proof assistant a library for separation logic, an extension of Hoare logic to deal with pointers. Using this library, we were able to verify the C source code of the Topsy heap manager, and to find and correct bugs.