| 摘要 |
<p>PURPOSE:To disable a 3rd party to read instructions out of an internal ROM in external bus mode by selectively controlling the output of information, read out of a read-only memory, to an internal bus on the basis of 1-bit information stored in a nonvolatile memory. CONSTITUTION:When information read out of an internal ROM matrix 21 is data in external bus mode, 1-bit output which is '0' is outputted from the nonvolatile memory 22 and supplied to a control circuit 27. Further, when the read information is an instruction, bit information which is '1' is outputted from the nonvolatile memory 22 and supplied to the control circuit 27. Consequently, when the read information is the data, the control circuit 27 controls a bus buffer 26 so as to output the data to the data bus as it is. Further, when the read information is the instruction, on the other hand, the control circuit 27 controls the bus buffer 26 so as to output fixed data by the data bus 8.</p> |
