1. A single chip microcomputer formed on a semiconductor substrate including a first processor, said first processor comprising: a) memory means for storing microinstructions; b) a processing circuit, having an arithmetic unit, which operates in response to said microinstructions stored in said memory means; c) a plurality of address registers for storing addresses for effecting access to said memory means to read out microinstructions for operating said processing circuit; d) a plurality of status registers each of which hold status information, including at least either one of zero information and carry information, generated as a result of processing performed in said processing circuit; and e) selecting means, including a programmable logic array, for selecting a pair of registers, consisting of one address register and one status register, from said plurality of address registers and said plurality of status registers, in response to information written into said programmable logic array from outside of said microcomputer, to supply an address to said memory means and status information to said arithmetic unit in said processing circuit.