I forgot to mention, that the Linux program must be executed as root. Otherwise it will not work, because it needs low level access to the serial port.
And the program must be compiled with optimization turned on.
The correct preprocessor command for GCC under Linux is "__linux__" - with two underscores and in small letters.
All commands in the instruction set are stated as ASCII characters. The return values and the parameters are in hex.
If someone got the circuit to work with Windows XP, please inform me. ;)