| 摘要 |
<p>PURPOSE:To detect runaway even if a system enters into a routine for clearing a monitor timer on account of the malfunction of a program counter by taking the difference of addresses when respective instructions are read by a subtracter and comparing whether the difference is within an appropriate range or not by a comparator when the instruction is not a branch instruction. CONSTITUTION:The address of an address bus 10 at that time is latched into an address storage latch 5 by an address fetch signal following the generation of an operand code fetch signal 4. Then, data is fetched into an instruction decoder 2 from a data bus 1, and a branch instruction judgement circuit 3 judges data. When the instruction is not the branch instruction, the maximum count number of the program counter of the instruction is set in an instruction length holding circuit 7. The similar operation is executed by the next instruction, and the value of the address storage latch 5 is shifted to an address storage latch 6. The address storage latches 5 and 6 execute subtraction, and the result and the value of the instruction length holding circuit 7 are compared the comparator 9. When a subtraction result is larger, it is taken as runaway and the system is reset.</p> |
