To reduce the size of a memory map by reading respective time stamps, individually determining whether or not respective time stamps are advanced, and individually advancing the respective time stamps in response to the decision means.

This system waits for an update cycle to start (step 5) and updates respective time stamps after a timer for an update cycle ends. Random numbers are generated for the respective time stamps (step 10) and then the time stamps are read out of their locations in the memory (step 20). To advance time stamps, a minimum run length of 0's of the random numbers is determine by using an index table and after the random numbers are compared with the index table (step 30), it is determined whether or not the time stamps are advanced (step 40). When the random numbers match the minimum number of 0's being advanced, the time stamp is advanced.