Abstract: Method and apparatus for producing mask-configured integrated circuits which are pin, logic, and timing compatible substitutes for user-configured logic arrays, without the need for logic or timing simulations of the mask-configured circuit design. Scan testing networks of test blocks and modified flip flops are included in the mask-configured substitutes to test functionality. Logic compatibility to the user-configured logic array (an FPGA) is preserved by clustering together in the mask-configured integrated circuit (a gate array) all of the logic gates which perform the functions of a particular FPGA logic block. Moreover, only those FPGA logic gates or functions which are used are replicated in the gate array, to conserve chip area. Test blocks are inserted in the gate array only where needed, i.e. at the output of any function generator that has connections external to the configurable logic block, and all flip flops are modified to also function as test blocks in a test mode.