| 摘要 |
PURPOSE:To prevent the data of the external storage device of a CPU from being unfairly read. CONSTITUTION:In an external storage device 2, programs are stored by being divided into plural cipher data systems. An address control circuit 15 outputs a selection signal 13 only when the head address of each cipher data system is read, outputs the head address of the cipher data system where a branch address is included by a branch signal 18, successively reads cipher data from the external storage device 2 when an external address bus 21 and an internal address bus 16 do not match, outputs BUSY signals 20 till the matching is recovered and the reading operation of a CPU 3 is retained. A signal selection circuit 11 outputs an intial value 12 to a storage circuit 14 only when a selection signal 13 is inputted. The data for which an exclusive OR is taken with the storage circuit 14 and which is decoded is inputted in the CPU, and is inputted in a conversion table 8 for decoding next cipher data. |
