| 摘要 |
PURPOSE:To prevent the destruction of the memory contents due to the program runaway, etc., by storing a memory write inhibiting command received from a CPU into a register and inhibiting the write to a memory as log as the inhibiting command is stored. CONSTITUTION:A CPU 101 transfers the desired programs and data to be protected by a secondary memory 102 to a memory 103 and sets the value of a register 105 at '1'. In a write mode to the memory 103, the output of a NOR 106 is kept at '1'. Thus the output of an AND 107 is equal to '14b and the output of an OR 108 is set at '1' although a decoder 104 is equal to '0' (the memory 103 selected). Therefore the memory 103 is not selected and not written. As a result, the output of the register 105 is not equal to '0' as long as the register 105 is not reset. Thus the CPU 101 is unable to set the output of the register 105 at '0' despite the runaway of the CPU 101. In such a constitution, the write protection is secured for the memory 103. |
