| 摘要 |
PURPOSE:To protect programs stored in an internal ROM by inhibiting the write to a memory space, an I/O space, or a register space while an instruction, which is read out from an external memory, stored in a buffer for the change of the mode is executed. CONSTITUTION:When it is detected by a mode switching control circuit 7 that the mode of operation where only an external memory 2 is used is switched to the mode of operation where only an internal ROM 1 is used, a multiplexer switching control signal line 8 is made active to switch a multiplexer 4 from the side of the external memory 2 to the side of the internal ROM 1. While all instructions which are stored in an instruction advance red buffer 5 and are read out from the external memory 2 are executed, a control signal line 9 is set to the low level to inhibit the output of the signal of an instruction decoder 6 to a write permission signal line 10. After execution of all instructions read out from the external memory 2, the control signal line 9 is set to the high level, and the write permission signal line 10 is set to the outputtable state for instructions read out from the internal ROM 1. |
