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A 256K bit CMOS ROM with a speed-power product of 0.085 pJ/bit has been developed. The excellent speed-power product and the high packing density have been achieved by using n-well CMOS technology and a serial-parallel ROM cell structure. The concept and characteristics of a serial-parallel ROM cell structure are discussed and compared to conventional ROM cell structures. The serial-parallel ROM cell structure gives more flexibility for ROM matrix design. The chip size and memory cell size of the 256K CMOS ROM are 5.98/spl times/6.00 mm and 7.0/spl times/7.0 /spl mu/m, respectively. Access time is 370 ns. The power supply currents in active and quiescent modes are 12 mA and less than 0.1 /spl mu/A at +5 V, respectively.