Computer apparatus and method for replacing programs between memories by employing protection mechanism with write enable generation scheme

摘要

A program replacement device is incorporated in a computer system having a CPU and a program memory in which first programs to be executed by the CPU are stored. The program replacement device includes a RAM employed as the program memory and having a write enable terminal, and a protection circuit interposed between the write enable terminal of the program memory and a write signal outputted from the CPU. The protection circuit includes a push switch for instructing program replacement with respect to the program memory, a pulse generating circuit for generating a pulse signal having a given time interval in response to an instruction from the push switch, and a gate circuit for making, only when the pulse signal is generated, the write signal outputted from the CPU valid with respect to the write enable terminal of the program memory. The program replacement device further includes an interruption generating circuit for generating in the CPU an interruption required to write second programs in the program memory in response to the instruction from the instruction section, thereby replacing the first programs by the second programs.