The A Series enables choice of memory size by removable memory cassette construction so that users can find the most economical memory size. From the smallest 16k byte memory cassette, A3NMCA-2, to the largest 1M byte, A4UMCA-128, 9 different memory cassette sizes are available. In addition, three additional E2 PROM type memory cassettes are provided for AnU users. Those memory cassette have EEPROM memory as non-volatile program storage in addition to the same size of SRAM memory.