Each 32-bit word of RAM could contain either a program instruction or data. In a program instruction, bits 0–12 represented the memory address of the operand to be used, and bits 13–15 specified the operation to be executed, such as storing a number in memory; the remaining 16 bits were unused. The Baby's 0-operand instruction set|single operand architecture meant that the second operand of any operation was implicit: the accumulator or the program counter (instruction address); program instructions specified only the address of the data in memory.