1. A semiconductor memory comprising: a normal decoder; a fixed spare decoder for relieving the normal decoder; a fixed memory disposed in association with the fixed spare decoder; a mapping spare decoder for relieving the normal decoder; and a mapping memory disposed independent of the mapping spare decoder, and for being recorded with mapping data, wherein the normal decoder is replaced with the fixed spare decoder when a fail address for selecting the normal decoder is registered in the fixed memory, and the normal decoder is replaced with the mapping spare decoder when the fail address is registered in the mapping memory and also when the mapping data selects the mapping spare decoder.