To subject data to execution of algorithms, such as a check sum function to certify maintainability and an encryption function to enable security by snooping the transaction interleft via a bus when the data is sent via the transaction.

An algorithmic snoop device 28 executes the algorithms to execute the functions to the data by snooping the interleft transaction on the common bus 42 when sending data via the transaction. Such functions include the check sum function to certify the maintainability and the encryption function to enable the security. Plural algorithmic entries 46 and 48 and an algorithm engine 44 exist in the algorithmic snoop device 28. As a result, the 'add-on' of the execution of the algorithm as many as desired on inevitably arising memory transfer is made possible.