A mainframe structure includes a housing having a detachable front cover, and a circuit module accommodated in the housing. The circuit module includes a main circuit board affixed to the back side of the detachable front cover in vertical and carrying first and second electrical connectors, a chip (or chips) and memory devices, a functional circuit board horizontally mounted in the housing at the bottom side and having a first connection port connected to the first electrical connector of the main circuit board, and a display circuit board vertically mounted in the housing at the top side and having a second connection port connected to the second electrical connector of the main circuit board. The detachable design of the circuit module minimizes the sizes of the main circuit board and facilitates maintenance of the main circuit board.