1. A microcomputer system comprising: (a.) a microcomputer device formed in a single integrated circuit and having data terminals: (b.) memory means having an input and output, and input/output peripheral means having an input and an output, both said memory means and said peripheral means being external to the microcomputer device: (c.) external data bus means coupled to the data terminals of the microcomputer device, coupled to said input and output of the memory means, and coupled to said input and output of the input/output peripheral means; (d.) the microcomputer device comprising: an arithmetic/logic unit having data input and data output; an internal data memory including a memory cell array; said data memory having an address input, and having data input/output means coupled to said cell array; internal data bus means coupled to the data input and data output of the arithmetic/logic unit, and coupled to the address input and the data input/output means of the data memory; an internal program memory having an address input and having an instruction output, the program memory being permanently-programmed and storing instruction words; program address means connected to the address input of the program memory means; control means having an input, said control means generating controls in response to instruction words, the controls defining operation of the arithmetic/logic unit and transfer to and from the internal bus means; internal program bus means separate from the internal data bus means; said program bus means being coupled to said instruction output, to said input of the control means, and to said program address means; bus interchange means for transferring instruction words from the program bus means to the internal data bus means without applying such instruction words to said input of the control means, and means to transfer said instruction words to said input of the control means, and means to transfer said instruction words from the internal data bus means to the external data bus means via said data terminals, to provide a test mode of operation.