Microcomputer with internal selection of on-chip or off-chip access

DETAILED DESCRIPTION In accordance with an embodiment of the invention, a microcomputer device contains a CPU with an arithmetic/logic unit and data/address registers on a single semiconductor integrated circuit having on-chip macrocode and microcode storage.
In the normal single-chip operation mode, a macrocode word is fetched from on-chip ROM and stored in an instruction register in the CPU, then a sequence of microcode words is fetched from the microcode store based on this macrocode word.
In another mode, such as used for self-test employing a check-code, the device loads macrocode from external into on-chip RAM then executes from RAM to perform the desired routine, switching modes of operation.