1AESS used memory with 26-bit words, of which two were for parity checking. The initial version had 32 Kilowords of core mats. Later versions used semiconductor memory. Program stores were arranged to feed two words (52 bits) at a time to the CPU via the Program Store Bus, while Call Stores only gave one word at a time via the Call Store Bus. 1A Program Stores were writable and not fully duplicated but were backed up by the File Stores. They were provided in multiplicity of N+2, i.e. as many as were needed for the size of the office, plus two hot standby units to be loaded from disk as needed.