| 摘要 |
PURPOSE:To protect a program stored in an internal ROM by invalidating the instruction code supplied from outside when an access is attained to the ROM. CONSTITUTION:A multiplexer switch control signal is outputted to a signal line 8 to switch a multiplexer to an internal ROM from an external memory when a mode switch control circuit 7 detects that a mode working with the use of an external memory only is changed to a mode working with the internal ROM only. While a control signal is outputted to an instruction pre-reading buffer 5 via a signal line 9 to set a specific instruction like a NOP instruction, etc. Thus the contents of all buffers 5 are set at said specific codes by the signals supplied through the line 9 and pre-read out of the external memory prior to an instruction. Then the instruction code stored in the buffer 5 is invalidated. |
