Microcomputer with test instruction memory

The invention relates to a microcomputer and to a method of testing such a microcomputer. The microcomputer has an instruction memory interface that applies instructions to an instruction execution unit. In a normal state instructions are obtained from instruction memory under control of a normal program counter. In a test state a same source of an instruction is used cyclically to apply a same instruction information from a test instruction memory. Normal addressing is suppressed in the test state, so that the same instruction is executed repeatedly independent of normal program flow.