| 摘要 |
<p>PURPOSE:To prevent a ROM to be protected from being subjected to dead copy easily by a ROM writer, by providing a trap area which never executes access as long as this system is being operated normally, in the ROM to be protected. CONSTITUTION:When taking a dead copy of a ROM 10 by a ROM writer, the ROM writer reads out the contents of the ROM 10 in order from the head address, therefore, a trap area is always brought to access, as well. When the trap area is brought to access, a trap code is outputted from a key ROM 15. A decoder 20 decodes a signal which has been outputted from the ROM 15 and discriminates only the trap code, and in case when the decoded signal is the trap code, a signal is outputted to a timer circuit 25. The circuit 25 is connected to a terminal CE of the ROM 10 through an OR circuit 30, the terminal CE is held in a holding state as the timer operates, and a state which does not react with access from the outside for a prescribed time is generated.</p> |
