| 摘要 |
In order to transform enciphered instructions stored in an external memory into the original ones while a microcomputer executes the same, the instructions have been enciphered and stored in the memory such as to be logically partitioned into a plurality of groups. The enciphered instructions are read out of the memory for decipherment. A first instruction located at an initial storage address of a group is initially deciphered using a first value. Thereafter, the following instructions in the group are sequentially deciphered using a second value which is determined by transforming an immediately preceding instruction. If the microcomputer executes a branch instruction, the execution control is transferred to a branched address. Subsequently, a first instruction located at the group, which includes the branched address, is deciphered using the first value. Thereafter, the instruction(s) following the first instruction are deciphered, using the second value, until deciphering an instruction stored at the branched address. <IMAGE> |
