PURPOSE: To improve the versatility of a circuit board, by taking a measure capable of changing an external location value used to make access through an external bus, a buffer for mediating the data transfer between an internal bus and the external bus.

CONSTITUTION: A gate 13 for a microprocessor 1 to read an external location value set by an external jumper 8 through an internal bus 2 in response to a request from a diagnostic program in a ROM 5 is provided. By providing this gate 13, the diagnostic program can obtain the external location value set by the external jumper 8, by instructing the microprocessor 1 when a buffer 4 needs to be accessed through an external bus 3 during the diagnosis of the buffer 4.