COMPUTER HAVING A PROTECTION DEVICE TO SELECTIVELY BLOCK INCORRECT CONTROL SIGNALS

In a computer having a program including a first type of instruction and a second type of instruction, a program memory has a first area for storing the first type of instruction and a second area for storing the second type of instruction. An address code is supplied from a program counter to the program memory, which thereby produces an instruction code stored at the addressed memory location. A control unit is responsive to the instruction code from the program memory for producing a control signal for controlling the operation of the computer. A protection circuit is provided for preventing execution of the first type of instruction when the address code output from the program counter does not designate any memory location in the first area.