An SMBus message handler, an integrated circuit and a method for controlling an SMBus are disclosed which identifies starting address of a program being stored in a memory. Instructions of the program are fetched one after another into a finite-state machine which controls the data transfer between an SMBus interface and a register set in compliance with the instruction present in the finite-state machine. Further, an SMBus test device and a method for controlling a testing system are described which check as to whether a key is input from a second interface. Upon inputting of a key it is mapped to a sequence of instructions for controlling devices connected to the SMBus or transferring data or receiving data from the devices connected to the SMBus.