In a one-chip high density arithmetic control unit capable of prefetching user's instructions from main memory, an arithmetic logic unit (ALU) subtracts the contents of a location counter holding the address of the next instruction to be executed, from the contents of a memory address register holding an address into which data will be written. The difference is fed through the gates connected to the ALU for determining whether or not prefetched instructions will have to be refetched. An address matching mechanism provided outside the one-chip arithmetic control unit includes a comparator for comparing memory addresses to a preset execution stop address. The output signal of the comparator is stored in a memory section which is provided to correspond to the prefetched instruction buffer, and when an instruction stored in the prefetched instruction buffer is transferred to an instruction register, the signal stored in the corresponding memory section is also read out and used to determine whether to stop execution. Further, whenever access is made to main memory a signal indicating whether the access is legal is externally generated and may be stored in a second memory section. Like the address matching signal, this signal is read out when the corresponding instruction from the prefeteched instruction buffer is transferred to the instruction register. An illegal address interruption is produced when this signal indicates that the address was illegal.