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The full 48-bit result is normally generated, but only the least significant 24-bit result is actually returned. To get the upper 24-bits of the result (which the compiler ignores) it is required to pull thre result directly from the mach register, as follows.

void Multiply24Bit ( int24 VdCmd, int24 VqCmd,

                     int24 ResultHi,

                     int24 ResultLo )

{

    if ( IsHostServiceRequestEvent(7) )

    {

         // Lower 24-bit result is in the register

        ResultLo = VdCmd * VqCmd;

        // Access the upper 24-bits of the result

        // directly from the register

        // (There is no 48-bit data type)

        register_mach mach;

        ResultHi = mach;

        NOP();

    }

    else {} // Error recovery code ...

}